coolant level warrning light on - sensor wiring problem: '97 S430

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02-09-2008 | 11:06 AM

vojin

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Joined: Feb 2008

Posts: 1

coolant level warrning light on - sensor wiring problem: '97 S430

My coolant level warrning light is on. Coolant level is fine. Anyone knows how 97 S420 does connects coolant level and washer fluid level sensors? Any information is appreciated. Here is what I have learned so far:

I changed a few coolant level sensors with no results. We broke one apart to see what is inside. (read relay and a resistor in parallel) Finally I disconnected the sensor and shorted the coolant sensor connectors - which made a light disappear and I was driving like that for a while. (this is opposite of the normal mode (?!) - I believe the read relay is open when the level is good and it closes the read relay switch when the level is low).
Last week I started the car - battery was very low because it was not driven - and the light came back again. I thought that something did reset itself and now it should be good with a sensor. No way.
After searching the web - I found this: https://mercedesforum.com/m_32516/mpage_1/key_/tm.htm
If you look at the last posting - there are wiring diagrams of coolant and washer level sensors shown.
There is a voltage divider supplied by 8V and sensors get it through 357 ohm resistor making voltage at the divider to be:
3.5V (everything OK),
1.9V only coolant OK,
2.6V only washer OK,
0V both levels low.
The output goes into an Analog to Digital (A/D) converter to generate an 8-bit number supplied to a dash control computer (microprocessor) that sets the light according to the number corresponding to the above voltages.
(What unnecessary complication !!!!!!! - great German engineering)
(However, this posting was for 1996 C200 - it is likely, but not certain the same is used on 1997 S420)

I changed the coolant sensor.
We took off a lot of unnecessary items before figuring out that only left headlight needs to be off. The right screw is a hell to take it off.
It did not fix the problem ! I have the same result:
- If the washer fluid is low - washer light is ON and coolant light is OFF. - If I fill up the washer reservoir, the washer light goes OFF, coolant goes ON.
I measured the voltage at the coolant sensor and it is 0V at both terminals.
The sensor does have 110ohm resistance when OK and 0 ohm resistance when I put the magnet near - meaning low level. Thus, it is OK.
I do not know if the diagram for S420 (97) is the same as 96' C200 - they may have changed it. I think I better have a diagram or advice how it does exactly work for '97 S420.

If you have any idea, or have run across any posting or any useful information - I would appreciate it very much !
If I ever figure it out (without the last resort of bringing it to the dealer and paying $1000 for it) - I will post it. So far I am driving with washer light on.