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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 11-29-2010, 12:13 AM
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BMW E39 failed radiator & failed expansion tank pictorial autopsy, new questions aske

Below I describe an OEM Behr radiator autopsy (verified by date code); and an OEM Behr expansion tank autopsy in a 2002 BMW 525i, with 91K miles on the odometer.

OBSERVATIONS:
- The plastic side panels are held on merely by crimps & a rubber gasket.
- The aluminum of the radiator was in pristine condition.
- There was only a very slight amount of gray gunk in the radiator.
- It was very difficult to find any failure in the plastic side panels.
- An enterprising person could become rich by perfecting a radiator rebuilding procedure that replaces just the two side panels, either with plastic, or aluminum.
- The expansion tank failed at the nipple during removal.
- The Nissens radiator nipple bulb was much larger in diameter than the original Behr (and BMW replacement) radiator nipple (necessitating a larger hose clamp than expected).
- The radiator failed at the nipple during removal (the nipple didn't fail, the radiator tabs failed).
- The expansion tank failed long ago at the bobbing stick rod.
- Cutting the expansion tank in a lengthwise direction revealed inner workings.
- The expansion tank works off of a small disk magnet in the float.
- The float connection to the stick is a very weak point that breaks easily.

HISTORY:
As I was pulling into a parking lot at the mall last week, I noticed a few puffs of steam coming out of the drivers side of my hood. Glancing at the temperature gauge as I idled, I saw the needle shoot to the right from midpoint to about a quarter of an inch to the right of midpoint before I shut the engine down and opened the hood.

I do not remember seeing a "low coolant" warning or any other warning.

I saw liquid spurting from the drivers side, seemingly below the upper hose by a couple of inches, and of to the side, as if a seam split. When it cooled I filled the radiator with tap water and bought a few jugs of water at the mall to keep in the front seat, just in case.

I unlocked the cluster, by memory, with a few mis-attempts. The procedure was basically the following:
- Press the right cluster button for about ten seconds
- When the first test showed, press the left cluster button
- This shows the VIN last seven digits (IIRC)
- Add up the last five digits of my VIN (e.g., 30)
- Press the right cluster button until it showed the last test (LOCK ON)
- Press the left cluster button until it showed the VIN sum (30)
- Press the right cluster button
- This should unlock the cluster
- Now press the right cluster button to go to test number 07.
- This reads out the coolant in degrees centigrade.

At no point in the 15 miles home did the temperature go above 93C.

Once home, I proceeded to replace the entire cooling system, using parts previously stockpiled from OemBimmerparts' web site after my previous alternator and belt drive system emergency overhaul.
- One users' example of alternator failure (AAA emergency tow) (1) (2)

Below I will describe, pictorially, the radiator and expansion tank (aka surge tank) autopsy.

One observation worthy of note is that it should be possible to "rebuild" a radiator at vastly less expense merely by perfecting a method of re-crimping the side plastic panels.

Even better would be a method of crimping on stamped aluminum panels in place of the side plastic panels!


Last edited by bluebee; 11-29-2010 at 01:12 AM.
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  #2  
Old 11-29-2010, 12:15 AM
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Radiator autopsy

I first looked closely at the entire radiator in order to find where it had failed.

It was not easy to see any flaws, which was surprising to me since I had seen water leak out of the driver's side, below the hose, about midway or just above midway on the plastic side. (In hindsight, I should have looked harder while the engine was running; but I just assumed a major leak based on the volume of spray coming out.)

To help me find the flaws, I rubbed chalk over any imperfection I could find.


About the only place on the radiator I could find a flaw was this apparently slight crack on the PASSENGER side (note that it appeared that the driver side was where the leak lay, although there was some liquid at the expansion tank side of the radiator but it could have been thrown there by the fan).


I noticed the only thing holding the plastic side panels in place were an evenly spaced series of crimps. With a pry bar, I easily pushed up the soft aluminum crimps to free the passenger-side panel containing the alleged crack.


Soon the passenger-side plastic panel popped free, along with a rubber gasket along the edge.


I repeated the effort on the drivers side aluminum crimps, which, with experience, came off easier than I had expected.


Soon I had both plastic side panels in my hand. Amazingly, the purported crack I saw from the outside did not appear to extend to the inside of the radiator. I was perplexed. In fact, there were NO CRACKS apparent looking at the inside of the radiator.

At this point I wondered if the failure was on the driver side hose itself, or in the expansion tank. However, I re-used the hose (that hose had been recently replaced with the alternator only a month ago due to my clumsy destruction of the plastic bleed screw during bleeding); and a later post will show no apparent cracks in the expansion tank.


Looking closely at the bottom of the drivers side panel, near the drain screw, I noticed a bit of gray clayish gunk. Not much. But a little gritty stuff around the screw itself.


I noticed the same very thin fine gray paste on the corresponding passenger side panel. (Any idea what this is? Is it aluminum dust?)


After looking the two panels over for fifteen minutes, I finally gave up trying to identify the flaw that caused the coolant leak. Whatever it was, it was minute if it existed in the radiator at all.


As it's important to be able to remove the nipple from the drivers side of the radiator, I took a look at how that nipple socket is constructed. This, in the future, may be useful because we don't yet have a cn90-patented technique for removing the nipple intact, so as to make a radiator shroud removal a dry affair, without the need to replace the expansion tank overflow hose radiator nipple hose clamp in the future.

It's hard to tell here, but it is worthy of note that this radiator nipple was "saved" during removal. This is a Behr nipple bought at the BMW dealership because the original nipple broke when it was removed during an alternator repair a few months back. While the new Behr nipple went in place at that time without problem, it was nearly impossible to remove that radiator nipple without destroying the two tabs on the radiator holding that nipple in place.

What we need is a better mechanism for removing this nipple intact, to facilitate shroud removals that do not require cutting off the original BMW hose clamps.
- WANTED: An ingenious method of removing radiator nipple

It is also important to note that the new Nissens nipple, which came with the new radiator, was much larger in diameter at the bulb end, necessitating a larger hose clamp than was initially expected, and further necessitating a trip to the auto parts store to find a larger solid hose clamp. The initial replacement hose clamp for the Behr replacement nipple was 11-13 mm solid band. The parts store was missing the size in between; they only had the 14-16mm size. The Nissens bulb end needed something in between (lesson learned). I had to grease and tug and pull and coerce the smaller clamp onto the larger Nissens bulb.


I also took a look at how the drain screw works. It appears to snap into place to lock in the closed position. If leverage is required, a small Phillips screwdriver can be inserted into the hole to turn the drain 90 degrees to open it up to the flow of coolant. This appeared to be working correctly.


Lastly, I took a long hard look at the radiator metal itself. I was shocked at how clean and pristine looking the inner flat tubes appeared to the naked eye. They were amazingly clean, considering the car was built in late 2001 and 91K miles had been driven on this OEM radiator (date code verified).


In the future, an enterprising individual who comes up with a method for re-crimping new plastic (or better yet, aluminum) side panels, will be a very rich person!
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Last edited by bluebee; 11-30-2010 at 04:15 AM.
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  #3  
Old 11-29-2010, 12:16 AM
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Expansion tank autopsy

Moving on to the expansion tank (aka surge tank) autopsy, it should be noted that I opted for a longitudinal autopsy because the only prior BMW E39 expansion tank autopsy I know of employed horizontal cuts. As such, that prior horizontal autopsy sliced directly into the level measurement pieces, complicating the analysis of the possible failure modes.

Vowing to not make the same mistake, and to further our knowledge, I bought a dremel tool and 200 dremel bits so that I could slice into the expansion tank without destroying the delicate level-sensing mechanism inside.


As luck would have it, I broke the only bits that cut into the expansion tank and had to return to the store to pick up just a set of the desired cutting tool.

Note: The "D" and the "M" are the markings for the distal and medial tubes respectively (the hoses are similarly marked so as to avoid confusion in replacement).


I carefully measured by eye a longitudinal slit that would bisect the expansion tank without damaging the coolant level sensing mechanism within. The second set of dremel tool bits easily cut into the expansion tank along that desired line.


At some points the plastic was thicker than others and melting occurred, but that was of no major consequence. Notice, by the way, that the level sensing bobber stick is outside the expansion tank. This is due to a failure previously noted about six months ago in this thread:
- My radiator floating level indicator seems to be MIA (where does it go?)


Soon I had the expansion tank longitudinally girdled. Notice the broken expansion tank nipple. Even though this Behr expansion tank nipple is reinforced with a tube of metal, this broke off when I removed the hose clamp from it in order to remove it from the car.


Once girdled, the expansion tank easily cracked open, revealing that I had cut it at the perfect junction of the inner bulkhead between the level-sensing compartment and the overflow relief compartment.


Placing the electrical sensor at approximately the position it would lie, I moved the float to the "full" position, with the broken-off bobber stick also placed in the correct OEM position on the side of the float.

It is important to notice the steel band of metal which I used to prove that the disc in the center of the float is slightly magnetic. It is clearly magnetic, but only slightly so. The white part of the float itself is made of a light plastic material and which is not magnetic.


Here I placed the float and bobber stick in approximately the coolant "empty" position. Notice that the magnetic disc now encircles the electrical sensor at the bottom of the expansion tank.


Here I've placed the float in the "too full" position. Notice also that there is an incomplete bulkhead on the other half of the expansion tank (much as was seen in the Titanic's six front watertight bulkheads which were only watertight horizontally, not vertically). At the moment, I surmise this thin plastic bulkhead (incomplete at the top) apparently separates the overflow operation of the expansion tank with the level-sensing operation.


In my tentative summary, I assume the following operational explanation (which I invite others to help flesh out in detail).

Surmised operation:
- The expansion tank has at least three different operations, namely visual level sensing, electrical level sensing, and overflow operation (if there are more, let me know).

- The electrical level sensing is apparently triggered when a float containing a slightly magnetic disc with a hole in the center, envelopes the tip of the electrical sensor. This apparently triggers the "low coolant" warning.

- That same float also contains a loosely glued on bobber stick, which indicates visually the coolant level. A typical failure is for that stick to disengage from its float. This disables visual level sensing; but it does not seem to have any effect on electrical sensing unless the stick itself causes the float to hang up in operation.

- In practice, it is very difficult to remove an old expansion tank overflow hose clamp without breaking the nipple, even though that expansion tank nipple is metal reinforced against that happenstance. If you're going to remove the expansion tank hose, you may as well assume you're going to replace the expansion tank.
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Last edited by bluebee; 11-30-2010 at 04:17 AM.
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  #4  
Old 03-27-2013, 12:19 PM
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Quote:
Originally Posted by bluebee View Post
Here I've placed the float in the "too full" position.
Notice also that there is an incomplete bulkhead on the other half of the expansion tank (much as was seen in the Titanic's six front watertight bulkheads which were only watertight horizontally, not vertically).
At the moment, I surmise this thin plastic bulkhead (incomplete at the top) apparently separates the overflow operation of the expansion tank with the level-sensing operation.


In my tentative summary, I assume the following operational explanation (which I invite others to help flesh out in detail).

Surmised operation:
- The expansion tank has at least three different operations, namely visual level sensing, electrical level sensing, and overflow operation (if there are more, let me know).

- The electrical level sensing is apparently triggered when a float containing a slightly magnetic disc with a hole in the center, envelopes the tip of the electrical sensor.
This apparently triggers the "low coolant" warning.

- That same float also contains a loosely glued on bobber stick, which indicates visually the coolant level. A typical failure is for that stick to disengage from its float. This disables visual level sensing; but it does not seem to have any effect on electrical sensing unless the stick itself causes the float to hang up in operation.

- In practice, it is very difficult to remove an old expansion tank overflow hose clamp without breaking the nipple, even though that expansion tank nipple is metal reinforced against that happenstance.
If you're going to remove the expansion tank hose, you may as well assume you're going to replace the expansion tank.
I recently have checked my Reservoir, and while looking inside, I noticed that the plastic wall for the overflow section within the reservoir was cracked vertically.

I wonder if the crack prevents the Reservoir to function correctly...?
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Last edited by Jason5driver; 03-27-2013 at 01:18 PM.
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  #5  
Old 03-27-2013, 12:47 PM
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Quote:
Originally Posted by Jason5driver View Post
I wonder if the crack prevents the Reservoir to function correctly...?
To date, NOBODY has been able (to my knowledge) to explain HOW that second black section works, in the expansion tank (mine is a 2002 M54).

However, it does have a bunch of holes in it, so, I would think (guessing here) that a crack would not be meaningful from the standpoint of leaking fluid from one chamber to another.

However, just the fact that a crack exists in the bulkhead would be cause for concern, in that I would wonder HOW it got there.

Interestingly, you'll notice MINE (below) has a crack in the black bulkhead; but I have no idea if I caused that crack when I cut it open.
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Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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Old 03-27-2013, 01:13 PM
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For the crosslinked record, JimLev kindly posted an expansion tank autopsy here:
> E39 (1997 - 2003) > 540 Expansion Tank issue. Bluebee U were looking for this info

Quote:
Originally Posted by JimLev View Post
I posted this same info over on Roadfly, unless you have an account there you can't view the pics.

540 Expansion tank problem and dissection

A few weeks ago I helped a friend change his lower oil pan gasket on his 2000 540ia.
He mentioned that when driving on the highway and then needing to slow down because of traffic the heat would sometimes go cold and the temp gauge would start to rise. Throwing the car into neutral and giving it gas would bring the temp gauge back down and the heat would return.
After the oil pan gasket job was done I took the cap off the expansion tank. Looking inside the level didn't look right.
The left side of the tank was almost empty, it should be 1/2 full. This is the section that fills the waterpump/engine.
The section of the tank that the stick is in was full to the top part that the stick traverses thru. Didn't look right to me.
We dumped more coolant in even though the stick showed it was overfilled.
He ordered a new expansion tank. The previous tank was installed in '08 and was not an OEM BMW tank.
Last Sat. we installed the new tank and all is well.
We then cut the tank open to find out what the problem was. This tank seems way more complicated than it needs to be...must be the German way of doing things?
There are 4 sections inside the tank which should all be connected to each other thru openings in the lower wall partitions.
3 of them were connected, but the important section wasn't.
If you purchase a non-BMW tank pour a little water into the tank before you install it. It should run out the bottom hose connection that goes to the waterpump.
Anyone ever run into this problem before?

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Quote:
Originally Posted by bluebee View Post
Thanks JimLev, for the expansion tank autopsy, and, for the picture of the passageway that was never cut out at the factory.
This information can be helpful for this thread:
- Does anyone have a BMW description of HOW the E39 M54 expansion tank actually works?

So that this information is found easily in the future, I will cross reference this thread to the existing expansion tank autopsy threads - so that we can get a better handle on how the thing works.
- Behr radiator and Behr expansion (aka surge) tank autopsy (1) (2) & request for another autopsy to understand the function of that second (black) chamber & pinned float (1)

It would be nice if someone can explain with an exploded picture HOW the expansion tanks works (at least the flow of fluid, with arrows going in and out).

I, for one, still don't understand at all how the "black" section works!
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__________________
Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need

Last edited by bluebee; 03-27-2013 at 01:24 PM.
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  #7  
Old 03-27-2013, 01:27 PM
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Quote:
Originally Posted by bluebee View Post
To date, NOBODY has been able (to my knowledge) to explain HOW that second black section works, in the expansion tank (mine is a 2002 M54).

However, it does have a bunch of holes in it, so, I would think (guessing here) that a crack would not be meaningful from the standpoint of leaking fluid from one chamber to another.

However, just the fact that a crack exists in the bulkhead would be cause for concern, in that I would wonder HOW it got there.

Interestingly, you'll notice MINE (below) has a crack in the black bulkhead; but I have no idea if I caused that crack when I cut it open.
Right.
Regardless, I still plan on replacing the Reservoir.

I think the vertical crack was caused when I accidentally dropped the reservoir when working/ replacing my power steering system...
See your modified picture, showing where the crack is on my Reservoir (running vertically, almost identical to the line shown on your reservoir):
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Last edited by Jason5driver; 03-27-2013 at 01:28 PM.
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Old 11-29-2010, 08:24 AM
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Quote:
Originally Posted by bluebee View Post
Notice the broken expansion tank nipple. Even though this Behr expansion tank nipple is reinforced with a tube of metal, this broke off when I removed the hose clamp from it in order to remove it from the car.
...
In practice, it is very difficult to remove an old expansion tank overflow hose clamp without breaking the nipple, even though that expansion tank nipple is metal reinforced against that happenstance. If you're going to remove the expansion tank hose, you may as well assume you're going to replace the expansion tank.
This was my experience as well. The radiator in my car was replaced in July; I bought the car in August. It was only while replacing all the other parts of the cooling system (from what I learned thanks to the many contributors to this forum) when I discovered that the plastic part of the expansion tank nipple had been broken previously - I assume from the work done in July - and glued back in place. Whatever glue was used had become brittle and essentially failed, so there was a coolant seep coming from the cracked plastic on the nipple. This piece broke despite, as BB notes, the metal reinforcement tube inside the outer plastic connector. This design definitely belongs on the list of head-scratchers...

Needless to say I had to replace the expansion tank. Having not purchased one (or the cap or the bleed screw - sold separately!) before the day I had set aside for service (a Saturday) and needing to get it done that day, I had to make the trip and pay stealer prices for what I needed - easily more than double what I could have sourced the parts for elsewhere. Ouch.

So yes, definitely assume you're going to have to replace the expansion tank. I think ordering one and sending it back if it isn't needed beats the alternative I had to use.
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For me, the e39 is the ... best balance of luxury ... performance ... good looks and class. Sort of the Catherine Deneuve of cars, if you get my drift.
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  #9  
Old 11-29-2010, 09:21 AM
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Quote:
Originally Posted by agent15 View Post
This piece broke despite, as BB notes, the metal reinforcement tube inside the outer plastic connector. This design definitely belongs on the list of head-scratchers...
I agree. My plastic nipple was so brittle that almost anything would have broken it off.

Because of that, and because people may wish to re-use an expansion tank if it's fairly new, it behooves us to come up with an OEM-hose-clamp-removal process that is the least forceful we can devise.

At first, I tried to "chop" off the crimped nub of the OEM hose clamp with snub-nosed wire cutters.


Almost instantly, as soon as a modicum of side force was applied, the nipple simply "twisted" off (it twisted because the plastic broke but the metal sleeve was still intact).


At this point, I realized I should have either dremeled off the hose clamp (so as to exert zero sidewise force) or I should have lifted up on the end of the hose clamp wrapped and locked end.


Here you see the pliers used to lift up on the end freeing successive lock pins in the OEM clamp. In hindsight, even this would have exerted too much sidewise force on the nipple, dooming it to failure.


Finally the OEM hose clamp was "peeled off". In the future, I would dremel off the hose clamp. I welcome a BETTER method from the peanut gallery if you can propose one.


BTW, I do realize that it's almost futile to try to "save" the expansion tank if the nipple is as brittle as mine was. However, I can imagine a situation where you have to "go back in" after doing recent work - and then, you would have a less-brittle nipple (my assumption anyway) and you'd want to "save" the expansion tank if you could.

So, if anyone has a less damaging way to remove the hose clamp, let us know!

BB
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Old 11-29-2010, 09:28 AM
edjack edjack is offline
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bluebee,

I'm not sure you can buy the radiator tanks separately, but, even if you could, the repair may not save you much money over the cost of a new rad.

The only lasting solution is the Zionsville rad, an expensive alternate. Some on these forums have opted for it.
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Old 11-29-2010, 06:22 PM
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540iman 540iman is offline
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Great post Blue. Steel band is likely a 400 series stainles steel. 300 series stainless, which is used for flatware, knives, pots and pans, etc. is austenetic stainless and is much more corrosion resistant, but is not magnetic at all. 400 series which is ferritic has some iron and therefore will corrode over time. Alloys like 409 (mufflers, tail pipes, Cats, etc.
430, 439, et al) are "slightly magnetic" and have decent magnetic properties for use as a sensor trigger. My plastic bobber must have separated from the float and possibly a re-bleed is necessary-why I'm not sure after all this time, but hey-if it works- who cares!

That would be the combo that explains my behavior. Sensor is still working. Bobber is just useless as an indicator. Must use sight and watch for DIC msgs.

Bill
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Old 11-29-2010, 06:44 PM
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dms540i dms540i is offline
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Great post Blubee, as always. I look forward to reading your posts. BMW should comp you a donor car just to drive around and do post mortems on their engineering flaws. Keep up the great work!
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Old 11-29-2010, 08:09 PM
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Quote:
Originally Posted by 540iman View Post
Steel band is likely a 400 series stainles steel. 300 series stainless, which is used for flatware, knives, pots and pans, etc.
Hi Bill,
I think I may have confused the description a bit.

The copper-colored bent "steel band" that I picked up off a cardboard box (it was a steel staple in a refrigerator box) was just a piece of steel that I used to prove that the circular disc in the center of the white float was slightly magnetic.

Here's a picture of the float mechanism showing the magnetic disc about in the center of that float. That magnetic disc, when it settles over the tip of the coolant level sensor, magnetically triggers "something" somehow.

How do you think the level sensor works when triggered magnetically?


Quote:
Originally Posted by 540iman View Post
My plastic bobber must have separated from the float
For the record, Bill is referring to this:
- Check Coolant warning-float 2" above filler neck???
- Check coolant light, coolant is tad low, but "bobber is full up
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Old 11-29-2010, 07:53 PM
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Quote:
Originally Posted by edjack View Post
I'm not sure you can buy the radiator tanks separately
I understand. I was just surprised how pristine my core radiator looked, and how flimsy the plastic was, and that the only thing in between was a little rubber gasket.

Just like with the headlight adjusters and abs control module, a little bit of non-BMW American ingenuity could outsmart the inherent built-in BMW material flaws.

So, I'm "suggesting" that a "fabricator" out there, might consider making the end plates out of aluminum, and they can then do a booming business "rebuilding" radiators by swapping out the plastic plates with aluminum ones.

Here's a picture of the side of the radiator core with the side plastic removed. There's only the crimp and a rubber gasket. That's it.

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  #15  
Old 11-29-2010, 03:20 AM
SeanPhang SeanPhang is offline
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great post(s)!
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  #16  
Old 11-29-2010, 04:31 AM
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You are an e39 CSI!
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  #17  
Old 11-29-2010, 06:35 AM
Patrick Patrick is offline
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I no longer own an E39 but I really do enjoy reading your threads! Keep up the good work bluebee!

When are you going to buy an E61?
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  #18  
Old 11-29-2010, 08:05 PM
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Blubee you are very adamant and meticulous in learning E39
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Old 11-30-2010, 08:52 AM
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Route 66 Route 66 is offline
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Great write up and a great explanation thanks for sharing.
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Old 12-08-2010, 10:32 AM
gaplayer gaplayer is offline
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BLUEBEE, you should consider writting repair manuals, you have alot of knowledge on BMW's and a great ability to write in a manner easy to comprehend!
anyway, reading this one, i saw your

Quote:
An enterprising person could become rich by perfecting a radiator rebuilding procedure that replaces just the two side panels, either with plastic, or aluminum.
I have a Jeep, grand cherokee as well, and have removed the radiator in it, removed the side tanks (plastic) rodded out my radiator and replaced the side tanks with the same rubber gasket, and have driven it 70k miles since without a problem.

im always looking for ways to make a dollar, but wanted to give you the link 1st,

http://www.allradiator.com/
you have to call to order, but they do have a look up feature to get your parts numbers of any part you need,
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  #21  
Old 12-08-2010, 05:06 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by gaplayer View Post
you should consider writting repair manuals
I'm currently out of a job - so - if any of the sponsors want to employ me to write DIYs for them - they should simply ask.

I'm willing to make their site the best on the planet for BMW DIYs if they only supply the car (if it's not my E39) and pay me by the article!

Quote:
Originally Posted by gaplayer View Post
My radiator is replaced already, so, too late for me.

However, the NEXT person on a budget who needs to replace their radiator should look 'em up! Thanks for the pointer!
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  #22  
Old 12-30-2010, 05:58 PM
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540iman 540iman is offline
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Hey Blue, sorry I don't keep up with all the posts and answer in more timely manner. "Magnetic" switches work in a couple different ways. When you were a child, did you ever wrap a wire around a nail and hook to a battery? You essentially can have a sensor whereby a metal rod passes through a coil and current is induced to flow in direct proportion to how far the metal rod penetrates the electric coil.

More simple (and likely) the way it works in the radiator expansion tank is a magnetized set of contacts are simply waiting for a piece of ferrous metal to get close enough to the magnet so that the magnet is attracted to the steel and it closes a set of contacts. The metal coud be covered in plastic to prevent rust or the steel could be like a 400 series stainless that has good magnetic properties and good anti-corrosion properties.

It looks like it's time for me to do a complete cooling system re-build as I have owned my 2000 since 2007 when it had 68K miles on it. That was when the radiator and expansion tank was replaced last. I'm at 130,000 miles now and have a slow coolant leak that I'm guessing is the water pump weep hole so will be doing entire deal and then likely selling to a guy at work. I will them drive the wife's 2002 540 and she may get a new SUV to replace her 2006 explorer. Having 3 carsand just 2 drivers is a luxury we really don't need and she insists on an SUV for the dogs....
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Old 12-30-2010, 06:20 PM
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bluebee bluebee is offline
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Looking up how the sensor works, I saw this description at
- BMW E39 Expansion Tank Failure Analysis
http://members.cox.net/rsm540i/E39ExpansionTank.htm
"I am fairly sure this sensor works as follows: the coolant level float in the tank contains a metallic "donut", and this sensor must use this (via magnetic field, Hall Effect or ???) to determine the proximity of the float to the sensor. When the coolant level gets low, the float gets closer and closer to the sensor, until the low-coolant alert is triggered..."

In case that members.cox.net article goes away, it's reproduced below. I tried to simply print it to a PDF file, but the file was 4MB which is 3MB too large for an attachment. So I just reproduced the screen shots to help others and for reference since he cut the tank open at a different angle than I did so what you see is another angle on the same theme.

Interestingly, he didn't go into any detail whatsoever of the second (hidden) chamber with the black float, nor does his picture show that entire section. Maybe it doesn't exist on the 540?









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Last edited by bluebee; 12-31-2010 at 08:46 PM.
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Old 01-10-2011, 09:48 PM
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bluebee bluebee is offline
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Interestingly, today, cn90 over here pointed me to an old thread over here that explained how the (E46) coolant level sensor worked as shown below ...

In post #29, bidman explains:
"I am afraid Critter7r's 13 years in parts and service at 2 BMW dealers did not serve him well on this one. There is no such thing as a heating element in the coolant level sensor. There is no pocket of air for heating or cooling and no readings are taken, it is a simple magnetic float switch that operates around the outside of the tube in which the sensor sits. This is fitted to ALL the E46 models."

Later, in post #32, valvtronicdude explains further:
"The coolant [level] temp sensor is a magnetic switch. if you remove the sensor no coolant comes out right it's a dry sensor. there is a float inside the jug with a magnet in it. when the coolant level is low enough the float sits down over the glass bulb of the sensor and the magnet of the float causes two contacts to touch each other inside the sensor and cause a complete circuit. The instrument cluster sees this complete circuit and then knows the coolant level is low."

The last post gives the switch a name:
"To provide enough warning time for coolant loss, the sensor needs to be located part way up the tank rather than at the bottom of the tank. By the time the sensor contact gets made at the bottom of the tank, all coolant has drained out.

Also for those interested, the proper term for magnetically activated switch/sensor is call a Reed switch/sensor
."
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  #25  
Old 12-09-2010, 11:44 AM
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ross1 ross1 is offline
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I'm pretty sure the low coolant warning only comes on at shut down or start-up, nice huh?
Lucky for you you caught it before harming the engine.
I wasn't aware there was a coolant temp history available. One of my cars came to me with an overheated engine (resulting from a failed radiator hose nipple and an oblivious driver), I'd like to know how hot it got. Hot enough to pull the headbolt threads from the block is all I know.
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Last edited by ross1; 12-09-2010 at 11:45 AM.
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