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Thread: coolant level sensor, how does it work?

  1. #26
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    Quote Originally Posted by Sapote View Post
    Well, this happened to us couple weeks ago driving home from holiday-travel. It's 100% true that the coolant level light was not ON after the tank ruptured, and luckily I saw the temp needle at the H (hot) zone before the over-heat red light ON. If my wife drove the car on that day, I pretty sure she would not know it until the overheat red light ON.

    I thought about why the hell coolant light did not turn ON too, and here is the reason: The weak point on the tank is the upper left rear corner (view from the driver), and I has seen many posted photos of ruptured tanks at the same corner. The rupture section did not extend down to the tank bottom, and so there was about 2" of liqid left on the tank, but all engine coolant was dumped out on the ground.

    So, if the ruptured section was on the bottom side of the tank, then it would save more engines from overheated.

    Sapote
    There is an overheat light ?

  2. #27
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    Quote Originally Posted by Alekm View Post
    There is an overheat light ?
    When the temp gauge goes into the red, the red area lights up. I know this from experience.

  3. #28
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    Quote Originally Posted by Sapote View Post


    C'mon, the temp gauge needle is small, moves slowly, and located away at the right corner, while the coolant light is big rectangular at the middle of the instrument display, and when it turned ON, those orange color photons would hit your eyes really hard that you would cry.

    Sapote

    I agree, it should be hard to miss, but I see it happen way too frequently, that someones' driving around with the coolant light on and they don't even know.
    Quote Originally Posted by B4SH View Post
    If anyone knows, it's this guy who knows literally nothing about everything.
    Quote Originally Posted by fcvapor05 View Post
    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


  4. #29
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    Sorry to bring up an old thread but I hate seeing information that is wrong or incorrect as others may be confused by it and it defeats the object of the forums to help people with correct information.

    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.

    Hope this helps, Dan

  5. #30
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    Thanks for the update.. I also found this out when I replaced my expansion tank in the summer of '08. What the hell is that thing rattling around inside the tank? Oh, it's a little something wrapped around the tube that the coolant sensor gets stuck into. So it would appear that the coolant sensor works very similarly to how the old E30 and E36 coolant sensors work, except that you can remove them without losing any coolant.

    I didn't think to go back and try to find this thread to update, but cool that you did!
    Quote Originally Posted by B4SH View Post
    If anyone knows, it's this guy who knows literally nothing about everything.
    Quote Originally Posted by fcvapor05 View Post
    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


  6. #31
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    Yeah, at first i was like "hey I thought that thing was a magnet."

    Then I thought, "man that sapote guy is a jerk".

    Then I accepted that you learn something new everyday, even on an old thread.

    If you look at the sensor though, it sure looks intricate for just sensing magnetism. I can see how it could apear to be a heated sensor.

    My theory on why the light doesn't always come on: Engine Sales!

  7. #32
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    The coolant 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.

  8. #33
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    Quote Originally Posted by Sapote View Post
    Look Critter,

    What sort of coolant sensor are you talking about? Is it for sensing the chemistry of the liquid? There are only two kind of sensors that related with coolant: level sensor and temperature sensor. So please be more clear about what you want to say. No body could read your mind if you keep saying coolant sensor.

    Please be helpful and not wasting other people times.

    Sapote
    You gotta be shitting me, right?

    Finished reading the rest of the thread. Good discussion and info.

    Sapote, the guys really showed you some grace on this one. Critter was attemtping to explain it as best he could.....

    Be glad you got help. Also as an aside, when you start a thread, asking for help/information, and it is provided, an acceptable response is "Thank you" and is much appreciated.

    So I will say it for you. Thanks go out to Nathan, Critter, GP, and the others who contributed and took valuable time away from their day to try and help Mr Sapote.
    Last edited by danewilson77; 12-02-2009 at 09:19 AM.

    by Dane Wilson, on Flickr

  9. #34
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    Quote Originally Posted by danewilson77 View Post
    So I will say it for you. Thanks go out to Nathan, Critter, GP, and the others who contributed and took valuable time away from their day to try and help Mr Sapote.
    Even though our two-year-old discussion of the operation of the coolant level sensor was .. well, quite wrong?

    At least it's resolved now. The magnetic-switch explanation makes somewhat more sense than the heated-wire explanation. I still don't think it's a reliable indicator of catastrophic cooling loss (i.e. ruptured expansion tank).

  10. #35
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    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.

  11. #36
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    Quote Originally Posted by valvtronicdude View Post
    The coolant 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.
    Yeah, same principle works with the screen washer tank. I couldn't believe the cluelessness exhibited in earlier posts.

    My coolant light came on suddenly yesterday while driving under ideal conditions. Today, I checked the level and it's at max. So my guess is either the float in the tank got stuck at the time of the light, or my level sensor is kaput. I need more observation time to see which is which.

  12. #37
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    Bidman08 is spot on

    Quote Originally Posted by bidman08 View Post
    Sorry to bring up an old thread but I hate seeing information that is wrong or incorrect as others may be confused by it and it defeats the object of the forums to help people with correct information.

    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.

    Hope this helps, Dan
    Dan, you are the man!

    1) I just removed my ET and cut it open to extract the dipstick/float which has a simple magnet on it. Then I looked at the coolant level sensor under magnification to see that it is a simple magnetic switch. I put an ohm meter on the sensor leads and it read open circuit, then I held the magnet from the float next to the sensor and it read closed (0 ohms). This is a super simple mechanism that just closes the sensor switch when the float's magnet is near it, at low coolant level when the float is at the bottom.

    2) My testing indicates this level is sampled when you start the car. If I jumpered the sensor's plug on the car and started it, the dash light would be off. Pull the jumper out and it stayed off. Shut down engine and restart and the dash light was on. Apply the jumper and it stayed on.

    EDIT: And I just now realized that my jumper tests in #2 contradict how the sensor works as measured in #1. Crap! And after reading other posts in other BMW forums I see the same confusing observations that I made. What the heck is going on with what should be a bone-headedly simple thing?

    Someone REALLY needs to delete Critter7r's posts from this thread, they are so wrong, and this forum thrives on correct advice.
    Last edited by balidawg; 07-28-2013 at 01:34 AM. Reason: clarify contradiction

  13. #38
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    Quote Originally Posted by balidawg View Post
    My testing indicates this level is sampled when you start the car.
    Level detection works at all times. Here's the real reason for intermittent lights:

    http://forums.bimmerforums.com/forum...-light-fix-E46

  14. #39
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    Quote Originally Posted by GoingNuts View Post
    Level detection works at all times. Here's the real reason for intermittent lights:

    http://forums.bimmerforums.com/forum...-light-fix-E46
    Okay, but can you kindly explain how this works? If you're implying that incorrect coolant bleeding leaves air in the float cavity and causes the float and its magnet to sit at the bottom of the ET, then yeah ithat makes sense.


    But here is the thing, my float is at the top. I know this because I can see the little white sucker in there and it is pushing the red stick to the top. So the float is definitely not near the sensor yet the dash light is on. Please explain how this could be.


    Also, I can push the red stick all the way down with a skinny screwdriver until the float is at the bottom and the dash light is still on. I let go of the red stick and the float goes quickly to its topmost position and the dash light is still on.


    Something is really misunderstood here. Anyone who truly understands how the sensor and dash light work, please explain.

  15. #40
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    I realize I have lost cred on this subject, but your sensor may be bad. Or unplugged.
    Quote Originally Posted by B4SH View Post
    If anyone knows, it's this guy who knows literally nothing about everything.
    Quote Originally Posted by fcvapor05 View Post
    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


  16. #41
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    Ah, you're not the first one to give incorrect advice. Done it myself a few times...no worries.

    I tested the sensor carefully and it functions as expected. It is normally open until a magnet comes near it, then it is closed. This is how the Bentley manual says it should operate and also makes sense, because the magnet on the float passing over it is what closes the circuit to turn on the dash light.

    I checked the leads in the plug that go to the sensor and they measure 12V.

  17. #42
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
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    Have you tried swapping the sensor with the one in the bottom of the washer reservoir? They're the same part number.

  18. #43
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    I have not. But I did remove the sensor and perform tests on it and it's working fine. I will pull out the washer level sensor and check it as well. Thanks.

    The thing that is confusing to me is the wiring/dash light side of it. The dash light is on when the sensor plug is not connected and off when I short the sensor plug leads. This seems opposite to what it should be based on how the sensor works.

    If anyone here has been down this road (I have searched all the forums, googled, etc., for hours) and understands why my dash light circuit seems to behave opposite of how it's supposed to, by all means jump in and straighten me out.

  19. #44
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    Just swap them and see if the sensor is good or bad.

  20. #45
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    Okay, I just thought of something. Maybe the way this mechanism works is that the float magnet has to be at just the right vertical position to keep the switch closed and the dash light off? Any other float position, too high or too low, and the dash light comes on? If this is is true, then it implies that the coolant level must be kept precisely correct, otherwise it's dash light city. Hmmm...this theory is starting to make sense given how critical the ET's coolant level is.

    Anyone care to comment on this theory? This mystery is killing me...

  21. #46
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    Quote Originally Posted by balidawg View Post
    Please explain how this could be.

    I quote myself in the link I quoted:

    Quote Originally Posted by GoingNuts View Post
    PROBLEM ANALYSIS
    The air pocket in the cooling circuit is compressed when the pump is running causing more tank coolant to enter the circuit making the tank level to drop to below MIN level. When the car and pump are off, the air pocket in the cooling circuit expands returning the tank level back to MAX. This produces a low coolant light while car is running, but the coolant level as indicated by the float is perfectly good while the car is off.

  22. #47
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    Quote Originally Posted by GoingNuts View Post
    I quote myself in the link I quoted:
    I understand you here. However, all my testing has been with the ET cap off, therefore no compression could take place. Sorry I didn't mention that before.

    My coolant light is always on, regardless of engine running or just ignition on, cap on or cap off.

  23. #48
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    Actually, it's the air pocket in the tank that gets compressed when the coolant heats up because the coolant expands, which causes the coolant jug to be more full when HOT, and less full when COLD. That's also why many people that are only very slightly low on coolant come in and complain that their coolant light is on in the morning, but after they drive a few miles, it goes off.
    Quote Originally Posted by B4SH View Post
    If anyone knows, it's this guy who knows literally nothing about everything.
    Quote Originally Posted by fcvapor05 View Post
    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


  24. #49
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    Here is how it works.

    Quote Originally Posted by balidawg View Post
    I have not. But I did remove the sensor and perform tests on it and it's working fine. I will pull out the washer level sensor and check it as well. Thanks.

    The thing that is confusing to me is the wiring/dash light side of it. The dash light is on when the sensor plug is not connected and off when I short the sensor plug leads. This seems opposite to what it should be based on how the sensor works.

    If anyone here has been down this road (I have searched all the forums, googled, etc., for hours) and understands why my dash light circuit seems to behave opposite of how it's supposed to, by all means jump in and straighten me out.
    The switch is normally open which means that the magnet is suppose to close the switch when coolent is at the right level. It is designed this way so that in the event of a failure in the external cable, switch or magnet you will get a check coolent message to troubleshoot. By jumpering the wire harness sockets you are similating a closed switch with sat coolent level. This verifies your harness and computer are working properly.

    I'm currently trying to determine if my switch is bad or if the tank is the problem. I have a check coolent level indication that will not clear. Whats making this difficult for me is that I need to unmount and lift my tank to get to remove the switch. The mounthing bracket under the tank prevents me from getting access to it with the tank installed.

  25. #50
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    The way a reed switch works is that it has an actuation point and release point. The magnet needs to be pass the upper portion of the switch for it to close the contacts and it will keep it closed untill it hits the release point. Therefore there is a range on the switch where it will close and stay closed and open and stay open. With the switch engaged the coolent level may need to drop ~ 1 1/2" (best guess) to cause the switch to open. To clear the alarm you would need to fill the tank to the correct level in order to get the switch to close again. One reason people get intermittent alarms is due to them trying to top off the tank when the coolent/engin have not completely cooled therefore not filling the tank to the correct level. Air pockets in the tank which can be created when inproper bleeding is perform can also force the magnetic below the the swich actuation point giving you a false impression that the liquid level is low when infact its correct. I'm guessing thats why they place a bleed valve directly above the level switch.

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