Coolant Temperature Sensor

I have a 99 540iT and had the coolant system replaced at about 80k and now have about 150k on the car. My wife comes home last week and says the car is running very rough. It throws the code for replacing the camshaft position sensor so I got one and proceeded to replace the driver side bank. Once finished the key turned as normal but the starter did not engage. I did notice the coolant guage pegging. So I got back under the hood thinking I would replace the old CPS and see if it would crank. After moving the air box to get to the CPS I noticed a sensor connector laying freely under the area where the CPS goes in. Upon further investigation I see that half of the coolant temperature sensor is still in the connector and the rest is in the thermostat unit. I noticed that there appears the option of buying the whole thermostat unit or one can get just the temp sensor. Is this common for the sensor to break and appear to turn to mush? and should I just replace the sensor and not the whole thermostat system? Could this bad connection have caused the limp mode and when completely broken lose caused the computer to not let the starter engage?

Originally Posted by cweo5

Is this common for the sensor to break and appear to turn to mush?

I have no experience with the engine mounted coolant temperature sensor - but you may find useful information about it here:
- How to replace the M54 dual temperature sensor (DTS, aka double temperature switch) in the M54 (1) (2) (3) (4) & how to make your own short DTS removal tool out of a 22mm or 7/8" box wrench (1)

Originally Posted by cweo5

and should I just replace the sensor and not the whole thermostat system?

The general consensus is that the weak cooling system is in the top three or four common repairs of the E39.

Certainly one of the major killers of engines.

So, if your entire cooling system hasn't been overhauled recently (in the last 60K to 90K miles), then yes, replace the entire system.
- List & picture of all the cooling system overhaul parts to replace (1) (2) (3)

I've not heard of the sensor breaking like that, nor inhibiting the starter. Did you perhaps bump the sensor whilst changing the CPS? Have you scanned for any more codes?

In any event, the sensor must be replaced.

BTW, you are almost due for another cooling system overhaul. Either do it as a preventative measure, or wait for a catastrophic failure; your choice.

Originally Posted by edjack

I've not heard of the sensor breaking like that

Since this is apparently a new failure mode for the BMW E39 cooling system, may I ask the OP to post a picture for us of the broken DTS in situ?

That way, we can reference the OP's picture in the future to others who come after us, long after we're gone.

I will get a picture up either tonight or in the morning. This is not the engine mounted DTS in the diagram of the six cylinder in the diagrams from bluebee. Instead it is the coolant temp sensor on what looks to be the thermostat housing. In looking up the part I have found two different options. One diagram of the part shows the whole thermostat housing with the sensor already in place. The other shows just the sensor with a rubber o-ring. Right where the break occurred there was a c-clip that I assume was meant to hold the sensor in place upon insertion into the thermostat housing. Pictures may help clear this up and may be beneficial to others. If the sensor sends no signal I was wondering if default may be overheat mode and is there a protective overheat mode that would not allow the engine to be started?

Gentlemen, these are the pictures of the broken sensor. I don't think I broke it while installing the new CPS. It looks as if it had been cracked or stressed for quite a while with all the gunk that built up between the halves. It looks as if the copper wires were all that was holding it together for a while. I hope these pictures help. This is the first time I have posted pics. BTW I also found out that apparently the sensor was built into the thermostat unit up through 09/98 production. Mine is 12/98 and thus has the separate CTS. If this thought is wrong please let me know. I am going to try to by a new BMW CTS from the stealer tomorrow. I'll post the results of the installation.

Thanks. I'll add these to the pictorial failures thread:
- Pictorial look at typical E39 cooling system failure modes (1)

These look like nothing on my M54 engine ... so I hope those with experience on yours can help further.

Do you know if there is an auto shut down that doesn't allow the starter to engage while the tempaerature is high?

That is a heater (not a sensor) that is used to control the t-stat which will make the engine run hotter or cooler depending on different operating conditions.
There are 5 fuses in the back side of the DME box, one of them supplies power to the heater and other circuits.
There have been instances where the heater has shorted out, burned up the wiring, and blown the fuse.
Usually the burnt wiring is inside the "wire box" that is above the passenger side valve cover.

Thanks for the information. You may have saved me the frustration of getting the wrong part. I can't find a part number for this. I looked on the diagram on realoem and it doesn't show it. It should be right on the thermostat housing. Can someone please tell me what this part number is. Also thanks JimLev. It makes sense that this is a heat element. I was trying to figure out why a sensor would get so hot that it breaks down. Is this a common part to fail? Additionally, any advice on how to best get the rest out of the housing? My last resort will be trying to remove the thermostat housing and push it out from the inside.

You can't buy it as a separate part, you have to buy the t-stat to get the heater and housing.
RealOEM shows the t-stat and it's housing as a separate part, but only a p/n for the t-stat
If you click on the pic tab for #6 you will see it.
Did you find a bad fuse?

http://www.realoem.com/bmw/showparts...35&hg=11&fg=35

http://www.realoem.com/bmw/showphoto.do?id=11531436386

My coolant sensor on my lower rad hose snapped off just like that one.

Thanks for the help. I will buy the assembly and install and I'll also check the fuses you mentioned.

Here's the fuse block you are looking for.
If the fuse is blown I'd take the cover off the black plastic box above the valve cover just to make sure the wires aren't melted and possibly shorted out.

Originally Posted by cweo5

Gentlemen, these are the pictures of the broken sensor. I don't think I broke it while installing the new CPS. It looks as if it had been cracked or stressed for quite a while with all the gunk that built up between the halves. It looks as if the copper wires were all that was holding it together for a while. I hope these pictures help. This is the first time I have posted pics. BTW I also found out that apparently the sensor was built into the thermostat unit up through 09/98 production. Mine is 12/98 and thus has the separate CTS. If this thought is wrong please let me know. I am going to try to by a new BMW CTS from the stealer tomorrow. I'll post the results of the installation.

Originally Posted by bcbp14

My coolant sensor on my lower rad hose snapped off just like that one.

This same issue just happened on my wife's Touring. I was able to get the broken/snapped sensor body out of the hose by screwing a small screw into the sensor body. I used the small hole which is right near the alignment notch on the sensor body that faces the rear of the car and threaded in a screw a couple of turns. Then using 2 flat heads to release the broken clips and I had my wife use plier to grab the screw and pop the broken sensor body out of the hose

put in the new FAE Temp Sensor but I noticed the O-ring it came with was black, not green like I see on the old sensor and in many pics here at the Fest


posting this because when this happened I came to the net to search around for advice on how to fix and couldn't find any. Didn't want to have to get a new hose (just put it in a few months ago but hadn't read online that re-using the temp sensor can cause leaks and to put a new O-ring in with it). Hopefully this post helps someone in the future

Originally Posted by JimLev

Here's the fuse block you are looking for

That's a nice picture to add to the pictorial thread on fuse locations!
- Where to find E39 fuse boxes (1) (2) (WDS) (WDS.ru)