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Discussion Starter #1
Ok, hopefully that got your attention :)

As per my last thread (http://www.bimmerfest.com/forums/showthread.php?t=381905)
I've had issues with my thermostat- but not the normal issues. I've searched the forums and googled pretty extensively, and havent found anyone else who's suffered the same problem as this. So I'm going to put this out here for other E39 owners, as part of the list of M62 V8 cooling problems. :rolleyes:

Firstly, here are some scary photos of whats left of the electrical plug that sits on the top of the thermostat housing. Yes, it was actually on fire and would have caught other things on fire around the general area, because lots of wires run in that spot :yikes:

I couldnt believe this, and not thinking twice, I grabbed a pair of sidecutters and immediately cut the wires to this connector (R.I.P. wiring loom), because the wires were red hot, the wire insulation had burnt off, and these wires lead into the electrical box above the valve cover, which looks very important and very expensive... :eeps:

My next question was 'What is this plug on top of the thermostat housing, and what does it do?'Well, its the electrical 12V feed into the heating element inside the thermostat.

'So why is there a heating element inside my thermostat? Doesnt the coolant temp make the thermostat open/close like a normal one in my Toyota/Honda/Chevy?'
Well, yes and no.. The thermostat has a wax pellet inside it, like a normal thermostat does. You know the drill- coolant temp rises, wax melts & expands, pushing the thermostat open. Big woop. BUT the 12V heating element in a BMW thermostat can heat the wax pellet even if the coolant temp is low- forcing the thermostat to open independant of coolant temperature. It can even do this when the engine is at a very high temperature, to get the thermostat to open 100%, to get the most coolant circulating and to try to bring the temperature down and prevent boiling. The engine ECU decides when to activate this heating element, based on the signal it receives from the coolant temp sensor (and other engine sensors) so the heating element is turned on and off based on when the ECU thinks its needed. Thus giving a means to control when the thermostat opens, instead of relying on coolant temperature to melt the wax pellet, like a Toyota/Chev/Ford thermostat traditionally does.
Thats it in simple, layman terms. Its more complicated, but thats the gist of why the wires are going into the top of the thermostat.

So, why did my thermostat heating element electrical connector catch alight and why didnt a fuse blow, and stop it from happening?? :dunno:
I'll tell you why- two reasons:

1. The two wires that supply +12V into the heating element rubbed together for years. Eventually the insulation wore through, and they contacted each other and this caused a serious electrical short.
2. No fuse ever blew, because normally, the heating element inside the thermostat draws a lot of current to heat up. (I dont know if there is even a fuse in this circuit in case this type of incident happens) but normally it takes a lot of current to heat the element inside the thermostat. When the two wires touched, it may have mimicked the load of the heating element operating, and therefore no fuse blew, because the high electrical load conditions seemed 'normal'. So it continued to burn away. :mad:

Suppose this happens to your 535i or 540i (both have the M62 V8 engines, but the 535i is a Europe-only model, which I have) will this affect the car, and is it still driveable with these wires burnt off/cut off/inoperable? :dunno:

Short answer: Yes. Still driveable. But be careful. Dont drive it like you stole it.
Long answer: The engine may now run cooler in cold weather (around 95-96 degrees celcius instead of 110-113 degrees celcius) and engine idle may be rougher than usual during a cold start.
In hot weather, there is now the potential to overheat, as the ECU has no way of opening the thermostat 100% fully open. This restricts coolant flow during higher temperature driving conditions. So hot weather driving becomes perilous.

Get to the point already Matthew- how do I prevent this from happening to my 540i and catching my engine wiring on fire??

Go out to your car, locate the black electrical connector plug on top of the thermostat, and trace the wires. There are two of them, and they run from the plug, up into the electrical box that sits over your valve cover (the one with the oil filler cap on it). Check the wires thoroughly for any abrasions, cuts or pinches to the insulation. If anything looks suspect, take it to an automotive electrician or the dealer or fix the problem yourself.
This circuit is poorly fused (there may not even be a fuse in the circuit- I couldnt find one) and it could lead to an electrical fire in the engine compartment if the wires short out.
That was a long post. I'm going to get coffee :thumbup:
 

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I think you are one of the lucky ones, Others here have had that issue and it torched the entire front of the car. Glad you caught it when you did.
Max
 

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How often is the DME supplying power to the heating element? Is it used all the time to keep the normal operating temp around 108 C?
I'm guessing the whole reason for the heated thermostat is to be able to let the engine run hotter during light loads for more efficiency and to lower the temp under heavy load to avoid problems.
 

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Finally an explanation! These fires have been mentioned before, but no explanation of the cause. Seems this happens frequently enough I am surprised BMW never had a product campaign, and improved the insulation on those wires. If I knew what tool I needed to pop the pins back out the back of that plastic plug, I would slide some pieces of heat shrink tubing over the wires to beef up the insulation.
 

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Discussion Starter #8
Hey guys.
Yep, apparently this is one of the main causes of E39 engine compartment fires, as a few of you have stated above. It doesnt seem to be a fluke, as others on other boards have also reported the same problems with these heating element wires. My educated guess is that there is NO fuse in this circuit.

The main reason this feature is built into the thermostat (in a nutshell), is to get the engine to run a hotter temp during partial load, so it improves efficiency and burns the combustion mixture more completely, thus being more "environmentally friendly".

Unfortunately, the wires are routed through the big electrical box on top of the valve cover, which will burn easily. This could explain a lot of mysterious engine compartment fires.

The engine ECU does NOT supply voltage through these wires all the time. Only when it wants to open the thermostat 'more' manually.

I'm getting my wiring loom fixed next tuesday, and getting a new thermostat housing (you think the photos of the electrical plug is scary, you should see the scorch marks on the top of the aluminum thermostat housing!!!)
And I will be leaving this connector unplugged until I can sort out a suitable sized fuse to run inline. It was scary, and it makes me glad I keep the fire extinguisher in the back seat.

** This affects ALL model year E39 535i & 540i cars as well as ALL model year E38 735i and 740i cars (basically all that use an M62 V8 engine.)

:)
 

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Great info

I don't drive a 540,but this is great follow up information. Yet another example of how valuable this forum and all its members are!!! I'm proud to be a part of it:thumbup:
 

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:yikes: whoa that crazy shi!t, im gone check my 540i wires when i get a chance.

can you take a picture of the engine bay for more accurate location where did u find this wire.




awesome write up :thumbup:
 

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Discussion Starter #11
Taking a picture is a little difficult in-place, as the thermostat housing is hidden directly below the corrugated throttlebody/MAF pipe. But when I change the thermostat, I'll get some pictures of the scorching...

Maybe the moderators could add this thread to one of the E39 archives? It might be a good idea for future owners to check their thermostat heater wiring as quick preventative 'maintenance'...
 

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I wonder if the shorting of the wires is caused by a mechanical chafing, or simply from the wire gauge and insulation being too thin for the high current loads? Seems these wires should have had an outer protective jacket over them like most of the other wire runs in the engine compartment.
 

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Discussion Starter #13
This guy also had the same problem, and his car burnt to the ground:
http://www.bimmerfest.com/forums/showthread.php?t=250344

Enough googling seems to convince me that this is a frequent failure, as a lot of owners have had their cars do this, and quite a few have burnt down. I was just talking to a BMW club member here where I live, whose 540i caught fire while driving down the middle of a shopping precinct, and it took a fire truck to put the car out, at which point there was nothing left of it. I think I remember seeing it on the local news a few years ago come to think of it. (I admit I didnt own a BMW back then, so I didnt think much of it when I saw it on the news...)

Seems like its mechanical rubbing together of the two wires that does it. The insulation on each wire is very thin (about 0.020" thick), and definitely not what I would put on high load wires that are so close to each other and draw so much current.

I'm not going to reconnect this plug until I can get a measurement of the current draw of the heating element, and work out what size fuse to run in these two wires. I was pretty lucky I caught this, but maybe next time I wont be so lucky...
 

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Any idea why this problem does not affect the E39 V6s? I would assume the V6 and V8 use same/similar circuit for the thermostat, unless the circuit for the V8 is designed to draw more current.
 

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Discussion Starter #15
Scott, no E39's were ever made with V6 engines- they are all inline 6 cylinder engines :)

I believe the 6 cylinder E39's all have different thermostat setups, and the wiring is different. It may be possible on a 6 cylinder car, but I have not found any instances of it happening. To truly confirm this, I need to know if 6 cylinder E39s have an electronic 'Characteristic-map thermostat' fitted to them. If they do, then yes, it is possible..

Update- there is supposed to be a fuse located in the ECU compartment (under the cabin microfilter in the engine bay) but obviously they are not rated properly to avoid these occurences from happening.

There are some people (in that link I just posted above) who are of the opinion that coolant is tracking up the electrical path out of the thermostat, and finding its way into the electrical connector and shorting it out. Mine did not do this, but it is also another possibility. So even if I fix this, there is no guarantee that it cant happen again, just beause it is a poorly designed part of every V8 E39. At this stage, I've been driving the car for a few days without the thermostat wires connected, and I see no difference in driveability and engine temperature is still being maintained fine.

In my case, the cons of reconnecting this plug once fixed, seem to outweigh the benefits of just leaving it unplugged. I know that seems counter-intuitive, but it seems the thermostat is capable of regulating engine coolant temperature on its own, without the heating element. It is running at a constant 96 degrees C, versus the 110 degrees C that a normal E39 should be running at. Running 14 degrees celcius lower shouldnt be an issue, as the engine ECU still has the means to retard/advance engine timing to prevent engine detonation, and the engine is not running so cold that increased wear would become a problem.

So I will try running without the thermostat heater connected for a while longer and see if there is any adverse outcome. I highly doubt it at this stage, based on what I have researched.
 

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ah

can't these wires be fused directly? Insert an inline fuse so if it overdraws current' or shorts out whatever the fuse pops and ...No problem? or?

this is spooky stuff
 

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Discussion Starter #17
^Well you could do that, or you could lower the amperage of the existing fuse in the ECU compartment...
I'm still looking at what I'm going to do for my car in the long term, because I dont want a repeat situation, and because I'm pretty sure my insurance company will be %^&*'s to deal with if I had to call them & say that the car spontaneously combusted in the driveway..
(Insurance companies sure can suck sometimes. I hate dealing with them at any time)
 

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Scott, no E39's were ever made with V6 engines- they are all inline 6 cylinder engines :)

I believe the 6 cylinder E39's all have different thermostat setups, and the wiring is different. It may be possible on a 6 cylinder car, but I have not found any instances of it happening. To truly confirm this, I need to know if 6 cylinder E39s have an electronic 'Characteristic-map thermostat' fitted to them. If they do, then yes, it is possible..

Update- there is supposed to be a fuse located in the ECU compartment (under the cabin microfilter in the engine bay) but obviously they are not rated properly to avoid these occurences from happening.

There are some people (in that link I just posted above) who are of the opinion that coolant is tracking up the electrical path out of the thermostat, and finding its way into the electrical connector and shorting it out. Mine did not do this, but it is also another possibility. So even if I fix this, there is no guarantee that it cant happen again, just beause it is a poorly designed part of every V8 E39. At this stage, I've been driving the car for a few days without the thermostat wires connected, and I see no difference in driveability and engine temperature is still being maintained fine.

In my case, the cons of reconnecting this plug once fixed, seem to outweigh the benefits of just leaving it unplugged. I know that seems counter-intuitive, but it seems the thermostat is capable of regulating engine coolant temperature on its own, without the heating element. It is running at a constant 96 degrees C, versus the 110 degrees C that a normal E39 should be running at. Running 14 degrees celcius lower shouldnt be an issue, as the engine ECU still has the means to retard/advance engine timing to prevent engine detonation, and the engine is not running so cold that increased wear would become a problem.

So I will try running without the thermostat heater connected for a while longer and see if there is any adverse outcome. I highly doubt it at this stage, based on what I have researched.
I'm guessing you have a CEL on with the thermostat unplugged?
 

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Discussion Starter #20
^Strangely enough no, there is no CEL light on the dash, even though the wires are disconnected.
Hooked the Peake code reader in, and there are no error codes for the electric thermostat control either.

Been driving it 4 days like this now, and nothing out of the ordinary to report. If I didnt personally know the wires were disconnected, I wouldnt know the difference in driving the car.
 
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