coolant level sensor, how does it work?

[balidawg]

Okay, now that makes sense. Since my last post I veirified exactly what you said and you are correct. I cut open my old leaking tank and could see that the magnet on the float surrounds the sensor reed switch when the float is at the top position. If the float goes lower due to less coolant, the magnet moves away and the reed switch on the sensor opens causing the dash light to come on.

My dash light stays on because the aftermarket (non-BMW) coolant tank I bought off ebay probably doesn't have the magnet on the float. Kinda pissed about that, but that's what I should expect from cheap Chinese crap. So I can either short the leads on the sensor plug to keep the light off, or buy and re-install a proper tank.

Huge word of advice: test the new tank BEFORE installing it by installing the sensor, plugging it in to the harness connector, turn the tank upside down, and turn the ignition key on. If your coolant light is on, return the tank and get a proper one.

FYI: you can remove the sensor without removing the tank from underneath the car by reaching your hand over the transmission fluid cooler and twisting the sensor clockwise. It unscrews a quarter turn in the opposite direction you would expect (like a left hand thread).

[hcoles]

Sorry man, but you will loose sleep over this. The best plan for you is carry a new tank and enough coolant for spare, just in case.

about the level senor operation, asuming it's a heater/temp sensor combo type, it should be able to detect the coolant any time, not only when engine is cold. It does not depend on the coolant temperature; it depends on the coolant heat absorption. Here is my version to explain this:

1. Before turn on the heater, the CPU measure the pocket temperature, say T1 (e.i 200F with engine at normal operating temp)
2. Turn on the heater until the pocket is T2 (i.e T2 = T1 + Delta; where Deta could be 200F, which was decided by the designer)
3. Turn off the heater
4. Start the timer counter, and record the time elapse t_count until the pocket is cool down to around T1.
5. if t_count > t_constant, then turn on the light; else, do nothing.

t_constant is predeterminted value by experiment, say 1minute.

You see, T1 could be any temp, freezing to hot. If no coolant around the sensor element to absorb the heat (Delta=200F), then it will take more time for the pocket to cool down to T1.

It's neat concept, but I don't think the system is reliable in this application.

Sapote

Good explanation. Thx. Who would have guessed this is how it works? I have the low coolant warning and the coolant level is fine when checked. 99- 328i only 190k miles.

[balidawg]

Sorry man, but you will loose sleep over this. The best plan for you is carry a new tank and enough coolant for spare, just in case.

about the level senor operation, asuming it's a heater/temp sensor combo type, it should be able to detect the coolant any time, not only when engine is cold. It does not depend on the coolant temperature; it depends on the coolant heat absorption. Here is my version to explain this:

1. Before turn on the heater, the CPU measure the pocket temperature, say T1 (e.i 200F with engine at normal operating temp)
2. Turn on the heater until the pocket is T2 (i.e T2 = T1 + Delta; where Deta could be 200F, which was decided by the designer)
3. Turn off the heater
4. Start the timer counter, and record the time elapse t_count until the pocket is cool down to around T1.
5. if t_count > t_constant, then turn on the light; else, do nothing.

t_constant is predeterminted value by experiment, say 1minute.

You see, T1 could be any temp, freezing to hot. If no coolant around the sensor element to absorb the heat (Delta=200F), then it will take more time for the pocket to cool down to T1.

It's neat concept, but I don't think the system is reliable in this application.

Sapote

You could not be more wrong.

The coolant level sensor inside the expansion tank is very simple, it is a reed switch that closes when the magnet on the coolant level float (the red plastic stick) is near it and turns on the dash light indicating the coolant level is low. There is no heater or anything else. It is a simple height measuring device, on/off based on the proximity of the magnet to the float.

[DStein]

This information was already amended earlier in the thread. We have also validated that Sapote is a jerk (lol).

[Dave1027]

FYI: you can remove the sensor without removing the tank from underneath the car by reaching your hand over the transmission fluid cooler and twisting the sensor clockwise. It unscrews a quarter turn in the opposite direction you would expect (like a left hand thread).

Revisiting this thread because I'm getting ready to replace my coolant level sensor for the first time.

So you remove the sensor by turning it clockwise? Why in the hell would they make it like that?

[balidawg]

Yup. Because of its orientation it cannot turn counter clockwise as the tank's outlet is right there. It's only a quarter turn, so not too bad. When you see it, you quickly realize it can only turn one way. See the attached photo.

You can remove the sensor without draining the coolant, because it sits in a sealed compartment and works through magnetics.

The windshield washer fluid level sensor is the exact same part, so you can swap with that one as a test.

[curry]

Well they had a float in the e36 expansion tank and it didn't work well either.

[Dave1027]

Yup. Because of its orientation it cannot turn counter clockwise as the tank's outlet is right there. It's only a quarter turn, so not too bad. When you see it, you quickly realize it can only turn one way. See the attached photo.

You can remove the sensor without draining the coolant, because it sits in a sealed compartment and works through magnetics.

The windshield washer fluid level sensor is the exact same part, so you can swap with that one as a test.

Thanks. Makes perfect sense.

[TheGill]

Sorry but critter explanation is total bs. Pls exlplain how the light could be triggered by simply pushing the float down. Clearly a magnetic resistance sensor. Not similar to the THREE WIRE system you describe (which IS how both oil level and maf work. No more arrogant bs pls. Shut your mouth if you don't know what you talk about.