I recently replaced a ruptured coolant expansion tank, and a new tank cap. Bleed the system, and everything is normal, except the coolant level light turned on after about one minute engine running. How does this noncontact level sensor work, how dow it detect the coolant level?
2000 E46 323i automatic wagon
Sapote
BMW sure loves their heating elements. The coolant sensor (and the air inside the little pocket it resides in) gets heated up to a temperature above the temp of the coolant and then measures how long it takes for it to cool back down to a predetermined temperature.
I'm total confused. I was asking about the level sensor, and you were saying things about the temperature sensor? Please pay more attention to other people question.
Thanks,
Sapote
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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
I'll try to explain it better.... The coolant sensor is a temperature sensor with a heating element attached. The heating element heats itslef up to a temperature above the normal range of engine coolant (for reference, we'll call it 400 degrees, because I don't know the exact temp). Then it shuts off the power to the heating element and waits to see how long it takes the heating element to cool down to where it's within the normal range of coolant. As long as the heating element cools down within the prescribed amount of time, the car knows the coolant level is ok (i.e. above the point of where the sensor is mounted and cooling the pod the sensor is in).
The reason this works, is because when the coolant is below the level of the sensor, and there isn't any coolant around the pocket that the sensor resides in, the temperature inside that little pocket won't cool down within the prescribed amount of time and your coolant light will come on.
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Ok, so after many tries to understand what you tried to say, I finally realize that you were trying to explain how the coolant level sensor works, not the coolant temperature sensor works.
All right, you were saying the level sensor at the void pocket at the bottom of the tank was actually a system of a heating element and a temperature sensor with timer counter. Well, let's see how many wires require for interfacing to this level sensor. Since there is no metal chassis ground in contact with the sensor, so:
a) 2 wires require for the heating element
b) at least one wire for the processor to read the tempurature sensor
So, in order to design this sort of sensor, at least 3 wires are required, but the actual sensor only has two wires.
In addition, relying on counting the elapse time for the surounding air to cool down to B degree after the air was heated up to A degree to detect the coolant level is questionable. What happen when the coolant is 0 F deg cold in winter, or when it's 200 F deg when hot? What I want to point out is there are too many variable which will change the elapse time, and not only the coolant level inside the tank. And thirdly, this sensor will be so slow to response, since the air in side this fairly big void will change its temp slowly.
The theory sounds reasonable, but the fact does not add up to support it: two wires, external temp influence...
Could you forward or provide reference to the source of your information?
Sapote
2 wires is enough. The resistance of the sensor varies with temperature. (very similar to a hot-wire MAF)
But, I agree that it's a lousy design. Seems like if the expansion tank breaks and dumps all your coolant, the coolant level sensor won't even notice, since the air pocket is already heated. This seems to be confirmed by peolpe who have had the tank fail during driving.
learn something new every day.
ASE and BMW Master Certified Technician
2 wires is enough; good point and I agree. But for this design, I would think a proximity sensor (capacitance) is a better solution, and the method has been widely used in other industries for detecting liquid level.
Relying on the coolant to absorpt the heat in the heated air pocket to detect the existance of coolant is way complicate and unreliable. If anyone had been working around this level sensor, then you also noticed that the air pocket involved was not air tight, so external air temp could influent the cooling elapse time of the heated air inside.
Any way, if this is really how the sensor works, then I don't know why after the new tank and cap were installed, and the level light was ON, since the sensor does not depend on any components inside the tank, beside the coolant itself, which I know is full to the max level.
Sapote
Last edited by Sapote; 01-16-2008 at 06:08 PM. Reason: more info
See below for the explanation of the mechanics of the beast. BMW loves their heating elements, and they love measuring resistance (that's how the car knows when a bulb is out). Therefore, 2 wires, power and ground for the heating and resistance measuring for the temperature.
The oil level sensor works on the exact same principle, except that it's actually submerged in the oil.
"What happen when the coolant is 0 F deg cold in winter, or when it's 200 F deg when hot?"
Well, think about it, it's not measuring the actual temperature, just how long it takes for the sensor to cool down from whatever temp it gets heated to (say, 500 degrees) back down to another prescribed temp. I'm sure BMW know at which temperature coolant freezes and boils at, so I'd assume that their alogrithms for this range would take those two temperature points into account and if the temp sensor stays at a higher temp longer than it would if the coolant was at it's hottest possible temp, that means your coolant is low, because there's no coolant outside the sensor cooling the pocket it's in.
"The theory sounds reasonable, but the fact does not add up to support it: two wires, external temp influence..."
The facts don't add up? Do you mean the facts that you don't know? Or did you mean the facts that you don't understand?
"Could you forward or provide reference to the source of your information?"
13 years in parts and service at 2 BMW dealers. 7 years in parts and 3 years as the warranty administrator at 1 dealer, and over 2 years as service manager of another.
To be honest, I don't know from where I know a lot of the information I know. I just know that I know it. Thirteen years of exposure to BMW bulletins, recalls, general service information and handling just about every imaginable part gets you that kind of knowledge. Could you provide me with reference or the source of how you learned to breathe? No, you just know.
And what I want to know is why that piece of ^%$^%$ did not turn on when my expansion tank exploded and my wife drove the car on red??????
Where'd he come from??
But to answer your question... if she didn't notice the temp guage was in the red, what makes you think she didn't notice the coolant light was on? I'm sure she says the coolant light wasn't on, and I'm sure she even believes that the coolant light wasn't on. But how do you know she just didn't see it? Happens all the time.
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
[
But to answer your question... if she didn't notice the temp guage was in the red, what makes you think she didn't notice the coolant light was on? I'm sure she says the coolant light wasn't on, and I'm sure she even believes that the coolant light wasn't on. But how do you know she just didn't see it? Happens all the time.[/quote]
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
Critters info would also explain why the low coolant light comes on when there is exessive air in the system....
I'm sorry but how does it know if the air was trapped in the cylinder head, or in the water hoses, or the radiator, or even inside the tank. If there is trapped air inside the tank, air would be up high at the top, and this very cute sensor would never be able to detect it, unless the tank is full of air, or we should say the tank is empty.
Sapote
So. Is it reasonable to believe that:
1. The low-coolant light will come on only if the car is started cold while the coolant level is below the bottom of the expansion tank. The rest of the cooling system warms up, but the airspace holding the level sensor does not.
2. The low-coolant light will not come on [in time to save your engine] if the coolant tank ruptures during driving and most of your coolant suddenly pours out.
2a. Because there is probably still coolant in the bottom of the expansion tank;
2b. And because the airspace containing the sensor is still very hot.
Is it true that the highest pressure occurs immediately after the car is turned off? This sounds reasonable: The airflow over the radiator is stopped, and the water pump is stopped, but the engine block is still hotter than the coolant.
I'm kicking myself now for not replacing my expansion tank five months ago, when I replaced the rest of the cooling system. This thread has me unnecessarily worried.
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
Last edited by Sapote; 01-17-2008 at 02:48 PM. Reason: more info
If what you guys are saying is true, the temperature and time measurement then that is not a very good design. WHy?
Well, the tank is raptured and as I pour coolant in the tank, the coolant just seeps down on the floor. When I turn the car on, the coolant light should light up right away but it does NOT. It should not wait to see if the coolant temp will cool down in a certain time period. If the float is so low that you can not even see it then the damn dash board should light up and a voice should come through the speakers saying PULL THE $$%$##$ OVER RIGHT NOW!!!.
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