Still having coolant level sensor problems

[flyfishvt]

If the jumper wires prevent the OBC warning then your issue is with the sensors.

[26kick]

What ended up happening here? I just replaced my sensor (Febi brand - but made in China. What?) and I got a low coolant warning once out of about 5 times I've driven the car. Grr.

[JDStrickland]

I have the BMW sensor from the local dealership, and I get the low coolant error when I shut the engine off. The sensor must be polled on some rotating basis because if the sensor is missing then the message comes up instantly with engine start, but I get no message at any point during a trip, and when I press the CHECK button there is no message even though I specifically request the computer tell me what it thinks is wrong. I can press the CHECK button and get a CONTROL CHECK OKAY report, then turn the engine off and get the message that the coolant is low.

When I turn the engine off, then I get the message. I can turn the engine off while driving and get the meessage, then restart the engine and drive some more and not get the message on the next Engine Off cycle. Such a random thing, I can't find it. I've replaced the sensor twice, once with one from one of the sponsors here and once with trhe real deal from my local dealership. The real deal is the part that's in my car now.

The coolant level is always good.

One thing I have not done yet is to use a jeweler's screwdriver to close the female pins inside of the connector. If the connector is unplugged, the message is immediate and constant, so I would expect a poor connection to at least show up when specifically asked.

RELATED BUT DIFFERENT
I had my car heat up when stopped, and then cool down when moving. I replaced the fan clutch to cure the problem.

[modestasjari]

The float has a magnet in it. The switch is inside the bottom of the probe. Its called a "reed switch". it is normally open. When the float/magnet drops it closes the switch and triggers the warning on the OBC. On the float switches I use if the magnet is upside down the switch won't work correctly.

i was looking for information about this problem for sereval months.
i have replaced rad and coolant tank with new coolant level sensor on my 316 e46 .bit the light does not go away.
coolant level is correct at all time

ok i have tried this: i open coolant tank cap ,coolant level is correct and then i remove coolant level sensor on the bottom of the tank but no drop of coolant came out.

is this normal?

thanks

[ewrjontan]

Another bump to an old thread, has anyone figured this out? I replaced most of my cooling parts last June including genuine BMW level sensor. It was fine up until maybe spring of this year. Whether my coolant is proper level or low, my sensor will go off then go away. Usually at the start and end of my drives. However, sometimes I never get the warning messages.

[JDStrickland]

Holy cow, you said three different things.

LOW COOLANT at start of trip, your coolant is low and the error message is true.
LOW COOLANT at end of trip and the coolant level is good, the error message is false.

No error AND the coolant is low, the low coolant sensor has failed.

If you get a low coolant message at the end of a trip but the coolant level is good when you wait for the temp to fall enough that you can open the radiator cap, then I think you need an expansion tank. When I had this trouble, I discovered that the tank was broken on the inside. Where the top hose connects to the radiator tank, there is a small hose that goes across the top of the radiator and connects to the expansion tank. The nipple that the hose connects to goes inside of the tank and goes all of the way to the bottom of the tank. This tube was broken inside of my expansion tank. I got LOW COOLANT messages upon engine off, but when the engine was cold the coolant level was good.

PS
You should fix your Profile so we know what car we are trying to help you with. Nobody here gives a rat's ass that you have a GTI. We cannot fix a GTI -- actually, many of us can but the tips we have to help with your GTI will not apply to your BMW.

[Brian 328is]

I had similar coolant level sensor problems. I cut the wires, and installed male/female quick connectors, so I could test what the sensor was sending to the OBC. Yes, you can do this without cutting the wires, but it made measuring the variable resistance of the sensor, and jumpering the wires much easier.

Use an ohm meter, and test the sensor prior to installing.

[JDStrickland]

There is no "variable" to the sensor. It is a reed switch, it's either open or closed, nothing in the middle. It does not measure the volume of the system, it only tells you that the expansion tank is empty or not empty.

When the coolant level is sufficient, the sensor is closed. As the level falls, the sensor opens. The float that operates the reed switch only has to move about 1/4 inch to open the circuit, but because the switch is so low in the tank, the coolant level is far more than 1/4 inch low. The point of the 1/4 inch comment is that the float does not move very far to signal a low coolant condition.

[ewrjontan]

Expansion tank was replaced with a genuine BMW unit. Am I correct in assuming that the sensor will not go off until coolant is below the float (which is about halfway down from the line on the expansion tank)? Car is a 95 M3.

[Corellian Corve]

Bumping since i have the exact problems as others have described. Coolant level warning when car comes on, coolant level warning when I shut off.

When I hit the check button on the OBC (or leave it alone for a few seconds after driving) the warning goes away.

Going to replace the sender but it sounds like there may be some other underlying issue...

[JDStrickland]

Expansion tank was replaced with a genuine BMW unit. Am I correct in assuming that the sensor will not go off until coolant is below the float (which is about halfway down from the line on the expansion tank)? Car is a 95 M3.

Are you sure the low coolant sensor is half way up? Mine screws into the very bottom of the tank. The sensor is about 4 inches long, so the float will be raised that distance above the bottom. The sensor switch is held closed by the float, and when the level drops then the switch opens. If the coolant falls enough while driving to set the error (LOW COOLANT), then if the error is true then the level should still be low on the next engine start. If the coolant is low on engine start, the error message is immediate. If the coolant becomes low during a trip, the message appears upon engine off.

If the coolant becomes low while driving, then you will observe this as a temp control issue.

[Brian 328is]

I'm having this OBC error again.

If I jumper the wires, the error goes away.
The expansion tank is full of coolant.
The sensor reads electrically closed (0 ohms) with a voltmeter.

I may replace the expansion tank.
Has anyone tried this cheap eBay expansion tank? Good or bad?
http://www.ebay.com/itm/BMW-Coolant-...-/131005145350

I will also try bleeding the system again.

Brian

[Brian 328is]

I just fixed this.

I replaced the expansion tank (cheap one on eBay), but not sure if that was the cause

After replacing the tank, I started the car with the expansion cap off. I noticed that the coolant level in the tank dropped below the point where it would trigger an OBC error.

I typically would never see this happen.

I topped it off with more coolant, bled the system, and no more errors. Wow, I was dealing with that for a long time.

If you replace the expansion tank, do not forget the o-ring for the coolant level sensor. If you forget to install one, it will leak. Ask me how I know.

Hope this helps,

Brian

[JDStrickland]

CHEAP cooling system parts -- the expansion tank -- is folly. Just sayin'

[Andrewk79]

Why did you replace the expansion tank ? What made you suspect that ? I'm just curious, not criticising.

There are two o rings on an expansion tank. Unless the tank itself is cracked, all you ever need to replace are those two o rings if you suspect there are leaks. For good measure, when you reinstall the tank, you apply high temp silicone sealant liberally around the space where the o ring would fit into as well as the o ring itself. This is for the big o ring at the bottom of the expansion tank. The small one at the o ring, just apply the silicone sealant around the o ring alone.

As you mentioned, the o ring around the coolant level sensor needs to be checked and ideally replaced when this is being done.

O rings cost $1-$5 purchased piecemeal. The dimensions for your specific expansion tank can be found on realoem.com. Or you can buy the o ring kits for the expansion tanks through the usual online retailers.

Props on fixing a long running issue.

[shogun]

...... you apply high temp silicone sealant liberally around the space where the o ring would fit into as well as the o ring itself. This is for the big o ring at the bottom of the expansion tank. The small one at the o ring, just apply the silicone sealant around the o ring alone.

Silicone sealant? Silicone sealant is used to seal joints between two surfaces, it will harden after application.
I assume you mean silicone grease?
Silicone Grease is applied around O-rings and helps to protect the O-ring from damage by abrasion, pinching, cutting, also helps to seat the O-ring properly. Silicone grease should only be applied when the O-ring material is resistant against Silicone. If not sure, if it will attack o-ring of rubber and synthetic-rubber make, check corresponding chemical resistance charts.

[Andrewk79]

Silicone sealant? Silicone sealant is used to seal joints between two surfaces, it will harden after application.
I assume you mean silicone grease?

Hi Shogun,

I meant silicone sealant, not silicone grease. I've used silicone sealant very successfully on O rings joints I don't ever want to touch again. It hardens yes, but not like epoxy, so is suitable for such areas. Particularly when you are dealing with non oil areas ( more cleaning and preparation required for oil areas). And its not difficult to remove when needed as well.

I've also used sealant on the outside of gasket lines, and even on the outside of gasket lines that were leaking oil which stopped after I got the application technique correct after several tries.

Hi temp silicone sealant. I swear by it. You should have three things in your trunk's toolkit at all times. High temp silicone sealant (not the same as hg sealant which doesn't harden the same way), 3m self sealing tape that can be used to seal rubber stuff (for both coolant and vacuum), and pvc pipe tape that can also be used to reinforce, or even completely create, O rings, in a pinch, and of course its used to wrap bolts good and to seal against fluid leaks for bolts. All of them work, all of them hold against pressure and temperature, and all of them can be undone and removed without difficulty. ou And all of them are cheap...you are looking at $15 to them all, and they last months typically.

[JDStrickland]

Hi Shogun,

I meant silicone sealant, not silicone grease. I've used silicone sealant very successfully on O rings joints I don't ever want to touch again. It hardens yes, but not like epoxy, so is suitable for such areas. Particularly when you are dealing with non oil areas ( more cleaning and preparation required for oil areas). And its not difficult to remove when needed as well.

I've also used sealant on the outside of gasket lines, and even on the outside of gasket lines that were leaking oil which stopped after I got the application technique correct after several tries.

Hi temp silicone sealant. I swear by it. You should have three things in your trunk's toolkit at all times. High temp silicone sealant (not the same as hg sealant which doesn't harden the same way), 3m self sealing tape that can be used to seal rubber stuff (for both coolant and vacuum), and pvc pipe tape that can also be used to reinforce, or even completely create, O rings, in a pinch, and of course its used to wrap bolts good and to seal against fluid leaks for bolts. All of them work, all of them hold against pressure and temperature, and all of them can be undone and removed without difficulty. ou And all of them are cheap...you are looking at $15 to them all, and they last months typically.

I'm calling Bullshit.

You can swear by whatever you want, but for the rest of the universe, DO NOT USE SEALANT ON YOUR O-RINGS. There is nothing on the car that you can apply enough glue to to keep you from ever touching it again, and the more glue you use today the worse it will be tomorrow when you are touching it again.

For the record, I've been driving for 50 years and have never ever carried those things in the trunk of any of the many cars I have owned. I call Bullshit, again. As a kid that grew up with a father that bought and sold used cars from and to the many car dealerships (he was a wholesaler) my family has had far more cars than the average family will ever own, and if you keep your car in good running order, the odds that it will ever be the focus of a flatbed truck is very low. I have been driving an E36 since 2000 (two different '94 325s) and I cannot remember ever opening the tool box in the trunk on the side of the road, except that time when I had a tire blow out. I did have a catastrophic failure of the cooling system one night early in my life as a BMW owner, and I can say without reservation that there is not enough silicone sealant in the universe to do a roadside repair to the cooling system when the top hose connection breaks off of the radiator.

You put silicone grease on an o-ring so that it seats properly, not silicone sealant.

[Corellian Corve]

I recently also had this problem. Came down to two things

#1 faulty sensor. The sensor in the expansion tank had lost it's "top" so the float was able to move off the top of the reed switch

#2 The plug was in backwards. Be careful, because you can plug the wiring in both ways. Look carefully and make sure it's seated and and the "clip" on the harness is mated with the tab on the sensor.

[Manolito]

I have been driving e36s for a very long time and I have never seen a failed coolant level sensor. What I have seen a lot are cooling systems with air in them that cause the coolant level in the reservoir to surge up and down with RPM, which triggers a low coolant level light. One of the reasons these coolant systems get air in them is crumbling plastic inside the reservoir allows air to leak in to the radiator. If your reservoir is much more than 5 years old and you suspect air in the system, replace the reservoir and split open the old reservoir with a dremmel tool. You will see the problem.

[flyfishvt]

I have been driving e36s for a very long time and I have never seen a failed coolant level sensor. What I have seen a lot are cooling systems with air in them that cause the coolant level in the reservoir to surge up and down with RPM, which triggers a low coolant level light. One of the reasons these coolant systems get air in them is crumbling plastic inside the reservoir allows air to leak in to the radiator. If your reservoir is much more than 5 years old and you suspect air in the system, replace the reservoir and split open the old reservoir with a dremmel tool. You will see the problem.

Those sensors do indeed fail. I've replaced a few. The most common failure is listed in one of the previous posts. The tabs that hold the float on break. Ive seen sensors with failed internal reed switches. It's not as uncommon as you seem to think.

If air is getting in then coolant is leaking out. Yes that will cause a low coolant situation. That's exactly what the sensor is there for

There is a BMW factory document floating around that explains the warnings that show up on the OBC. They are listed according to their level of importance. Those levels are apparently based on driver safety. Things like brake light failure are at the top of the list. Low coolant is a low level issue. While it's definitely important because of the potential for catastrophic engine damage it is not very important from a passenger safety view point. Keeping all that in mind this is how it's supposed to work....you'll get a low coolant warning at startup if the level is too low. You'll get a warning when you turn the car off if the level is low. If you start the car and no warning is issued then drive the car and lose enough fluid to trigger a warning you will not get a warning until you turn off the car. That is because it's considered a low level issue. Apparently BMW engineers did not want their drivers interrupted while driving for anything other than a safety issue. I think this speaks volumes to the differences between the way US drivers think compared to the way Europeans think.

Finally, there are 2 different coolant level sensors used in the e36. They redesigned the harness connector starting with the 96 model year. 92-95 e36s used a connector that is identical to the harness connector on the top of the brake fluid reservoir. It's long, flat and wide. The pins are fairly large and the wires going to it are fairly large. Starting in 96 they changed the connector. It's a completely different size and shape. Much smaller. The pins are very tiny and the wires leading to it are much smaller gauge. The older style connector required a sharp bend in the harness right at the connector. After many heat cycles and years of exposure to the elements the wires can break. It's not uncommon for them to break internally so the insulation remains intact. The newer style connector prevents this issue. BMW used the same style connector on the washer fluid res. That was changed at the same time for the same reason. The 96+ e36s use that same style connector on the radiator temp switch.

After owning 7 e36s I've seen just about every possible abuse for a low coolant warning on the OBC. Faulty sensors do happen but so does the following list.

Sludge build up can prevent the float from droppjng. If you lose coolant and top,it off sludge can also prevent the float from floating.

The pins of the older style connectors can get corroded and lose contact. This will trigger a warning

The wires can break internally or they can break off completely. This is more common on the older style connector. When they break internally it's particularly frustrating to find the real cause because we usually assume wires are intact unless we see a severed one. Diagnosing this is pretty easy with a jumper wire.

The newer style connector has 2 small tabs that hold it onto the sensor. If one of them breaks the connector will jiggle loose. This isn't as common on the older style.

The internal structure of the expansion tank can fail. I'm not sure how this generates a low coolant warning but it certainly has caused quite a few while I've been a member here.

[JDStrickland]

The Coolant Level Sensor is nothing more than a simple switch, a reed switch if you care. There is a magnetic float that holds the switch closed (continuity) at all times until the coolant level falls enough that the switch opens. It is not a sensor in the normal sense of the term, it does not measure anything it is binary, either on or off. Nothing in between.

If the switch fails it will show a false empty and this should appear as an error message as soon as you turn the ignition on. A false empty is one where you have a message but the the tank is not empty.This error can be cured -- as a means of testing -- by shorting the pins of the connector together.

The switch could fail with the coolant tank empty, but you get no message. This would require the float to stick in the raised position so that the switch contacts never opened even if the coolant level fell. It's also possible, although not plausible, that the switch contacts could weld themselves together and no matter what the float did, the contacts would remain closed. This is not plausible because there simply isn't enough current in the circuit to cause the kind of arcing that would weld the contacts shut.

The switch is very robust. Broken wires are a far more likely problem area.

On my car, I was getting the Low Coolant message upon Engine Off. Almost every time I parked the car. I would get the message but the coolant level was nowhere near low. This is a false message, but the sensor was actually working correctly. The trouble was a break INSIDE of the expansion tank, not a problem with the actual level of the coolant or the sensor. If the car really has a low coolant condition while driving it, you will see this in the temperature gauge, and then again on subsequent Engine On cycles. It is not possible to have a low coolant condition that is real, and then have a normal coolant level when you open the radiator cap after the coolant returns ot room temperature. If you have a normal temperature when driving, then a low coolant message when you park, and a normal level when you physically check, then you should suspect a break internally in the expansion tank.

There is a small hose (small diameter) that travels across the top of the radiator and goes to the expansion tank. This hose goes into a pipe inside of the tank, and the pipe goes to the bottom of the tank. I cannot say what happens to allow the coolant in the expansion tank to fall far enough that the switch opens, but one possible cause is a clogged radiator. My guess is that since the radiator has the cooling cores running horizontally, then the lower cores can become clogged and the flow is affected. When the t-stat opens, the radiator drains into the water pump (ultimately into the engine) and some of the fluid in the tank is drawn in faster than the fluid is replaced. This would allow the expansion tank to empty momentarily even though the system has sufficient contents. In my car, the pipe that the overflow hose uses was broken open, raising where the bottom of the pipe is. This affected the continuous level of the coolant. In the interest of full reporting, I had a hole in the radiator that caused me to replace it. I replaced the expansion tank at the same time, and discovered that the pipe was broken. I don't know if the radiator was clogged or not.

[flaviuschiopu]

Hi everyone,
It seems this is a long overdue explanation for how the coolant level sensor works on BMWs...
Everyone seems to agree the sensor is a reed relay that gets triggered by the magnet on the floater,the magnet on the floater causing the reed relay to close, right?!Only partially correct- But then, if the sensor is out of the dry-well or if the connector is disconnected, the "check coolant message" becomes permanent, also if the connector pins are shorted ( with a staple or something else), the message goes away!!! This seems to be "backward " !! So the explanation is simple:
The sensor should be "normal closed"- no check coolant message ! They achieve this, by having a SECOND magnet on the dry well !!!! This causes the reed relay( coolant level sensor) to close AS SOON inserted on the dry well! ( tested by me on an OEM tank). Now when the coolant level drops and the floater gets close to the reed relay, the magnetic field from the magnet on the floater will counter-effect the one on the dry well( sensor enclosure) to open up the relay= check coolant message !
This is how it works ! and this is why many after market expansion tank will throw this "check coolant level" message !- they don't have the second magnet on the dry well to keep the relay closed in normal operation !
OEM expansion tanks can fail also ( probably by this second magnet getting either loose or weak with the time- not being able to keep the relay closed in normal operation= coolant level OK, or by floater magnet falling on the bottom of the tank and counter effect the magnet keeping the sensor closed)
I hope this helps everyone understand once for all how this wicked system works!

[gdavid]

The windshield washer level sensor issue is probably just a bad connection. I moved the windshield washer tank when I replaced my coolant sensor 3 weeks ago and apparently I had a bad connection when I reinstalled it because I started getting a low washer fluid light. After cleaning the connectors and firmly reconnecting it went away.

It sounds like the coolant sensor was assembled upside down some how and this would have been a much more sensible manor to configure it in the first place. Why put a fitting at the bottom of a tank when it could have been much easier to service at the top?

[FamilyForce6]

Hi everyone,
It seems this is a long overdue explanation for how the coolant level sensor works on BMWs...
Everyone seems to agree the sensor is a reed relay that gets triggered by the magnet on the floater,the magnet on the floater causing the reed relay to close, right?!Only partially correct- But then, if the sensor is out of the dry-well or if the connector is disconnected, the "check coolant message" becomes permanent, also if the connector pins are shorted ( with a staple or something else), the message goes away!!! This seems to be "backward " !! So the explanation is simple:
The sensor should be "normal closed"- no check coolant message ! They achieve this, by having a SECOND magnet on the dry well !!!! This causes the reed relay( coolant level sensor) to close AS SOON inserted on the dry well! ( tested by me on an OEM tank). Now when the coolant level drops and the floater gets close to the reed relay, the magnetic field from the magnet on the floater will counter-effect the one on the dry well( sensor enclosure) to open up the relay= check coolant message !
This is how it works ! and this is why many after market expansion tank will throw this "check coolant level" message !- they don't have the second magnet on the dry well to keep the relay closed in normal operation !
OEM expansion tanks can fail also ( probably by this second magnet getting either loose or weak with the time- not being able to keep the relay closed in normal operation= coolant level OK, or by floater magnet falling on the bottom of the tank and counter effect the magnet keeping the sensor closed)
I hope this helps everyone understand once for all how this wicked system works!

This is interesting. I did a cooling system overall including new expansion tank and sensor. I get a low coolant warning on EVERY key off but at not at start-up or while running. There's plenty of coolant and the system is bled of air. Guess I'll have to replace the new expansion tank to get this fixed.