Hi,

I've checked through all the intermittent coolant light threads bu t no luck so far with my car's symptoms.
2006 Base Crossfire, 6-speed. Coolant light off & on: temperature fine, AC fine (fan blows high when AC engaged); replaced coolant bottle sensor - my next step is the windshield washer reservoir sensor, since it seems as if the FCM and Fan unit are working as they should, and if the 50 amp fuse was blown that wouldn't happen. Any ideas? and many thanks

 

I had that once too, turned out I was confusing the low windshield fluid light with the coolant light.
DUH !

 

That light is, as you apparently know, NOT a "Coolant Light" but a "Generic Light indicating an issue with engine cooling" light.

The items that I know can light it:

• Low Coolant, High temperature, or a bad sensor falsely reporting either of those.

• Problems with the ECM not 'handshaking' with the Fan Control Module (FCM) - this can include either module being bad.

• FCM sensing current flow to the Fan Motor that is out of specs (too high or too low?)

• Blown Fuse to the FCM.

Other things that might light it:

• Who the hell knows? Our cars are, to some extent, kinda over-designed in some aspects.

 

I am having the same problem with an intermittent low coolant indicator light on the rescue SRT. Engine temp is normal, coolant level is good. The light seems to go out if I roll both temp controls to the heat side past detent. I have seen it go out and come back on without rolling the temp controls. The coolant level sensor is original-05. I have in the past also experienced the heat intermittently coming on with the A/C running full cool at the controls which would indicate an issue with the heat core solenoids , or its associated circuitry. I suspected the IC pump, but the SRT runs full power. I suppose I will start by replacing the coolant sensor.

 

So, we replaced the sensor and guess what? We HOPEd the situation would CHANGE, but the problem remains. Engine temp is fine, seems to come on either with a sagging temp needle or slightly above halfway. I have noticed a lot of fluctuation in the temp gague since I have been watching it like a hawk. I have heard that the thermostat is weak when this occurs. It's probably original. The intermittent low coolant indicator light seems to be the last light to go out after starting the car. The fan motor speed control has been in various positions and I have seen the light come and go. Mostly it will go out it seems , when it is on, if I turn the fan control to the off position , but it is no guarantee. I realize that it wouldn't be instantaneous as there is data to process with the signal before the light is energized. This coolant recovery tank is only a year or so old....Any ideas? Haven't had the heat come on during AC in quite a while by the way.

 

The cause of the intermittent low coolant light I was experiencing has been solved. Apparently the last rib of the serpentine belt came off of the belt and lodged in the cooling fan for the radiator. This was a Gates belt and only a year old as we put it on when we did the SC swapout. (faulty belt Rob?) With the radiator fan being energized and not turning, I suppose it was backfeeding, triggering the intermittent light and the reason for the AC compressor to become intermittent was a result in a rise in temperature since the fan was not turning. All is well now.
To: Skip19, I hope you got your issue squared away as I would not have been able to fix my own had you not started this thread and obtained the knowledgeable input from Pizzaguys reply....Peace

 

Are you sure the belt was correctly centred on all of the ribbed pulleys?
In the past belts have had this kind of damage when the end rib ran on the end of one or more of the pulleys which cut it off.

 

Coolant light Gremlin - Still on & off

So my coolant light continues to go on/off. Replaced radiator overflow tank sensor; cleaned washer fluid tank sensor, 50 Amp fuse fine. Now thinking FCM or ECM or handshake. There is a pattern: light comes on if temp gauge goes slightly above mid line; off when it drops when gauge falls to slightly below. Seems electrical & sensor-based. Fan & AC working fine. I think I may just live with it until something more definitive arises...

 

Any advice

Coolant light comes on when a/c is turned on it will go off after few minutes when the a/c has been turn off. The car temp just goes tiny bit over half way , when the coolant light is off its sits just below halfway. It does come on randomly but off again at times.

Checked engine coolant and level is fine
is it likely to be the cooling fan or fan control module ?

Thanks for any advice

 

Any advice

Coolant light comes on when a/c is turned on it will go off after few minutes when the a/c has been turn off. The car temp just goes tiny bit over half way , when the coolant light is off its sits just below halfway. It does come on randomly but off again at times.

Checked engine coolant and level is fine
is it likely to be the cooling fan or fan control module ?

 

Yea, it sounds like an indication of failure of the engine cooling fan or Fan Module. Any time the A/C is on, the fan should be turning, at least at low speed. If it is not, then either the motor or module is bad.

Start the car up when it's dead cold. Fan should be still.
Turn A/C on and interior fan on low or medium speed.
If the outside air temperature is above about 36F, the engine fan should turn about 120 rpm.

 

Okay i will check this ... any way to test the fan module or is it case doing a swap and a see if that works .

i will test the fan with a 12v battery

 

I agree w/pizzaguy - especially if coolant light on/off is directly connected to the A/C on/off. My coolant light gremlin not synchronized with anything (at least nothing that I can identify) - but it hasn't caused me any problems either...

 

This is one of my pet peeves: you can either buy a good Merc scan tool for $100-$150. Or you can randomly throw parts at it and will quickly get to the same amount. For $20 Torque Pro can tell you a lot but is limited to standard OBD-II elements and not the special codes that Mercedes uses.


Electrical and sensor issues, particularly intermittents, can only be found with either instrumentation & logging or luck. I prefer not to rely on luck

 

In general, I agree. One of my pet peeves is when someone has a battery that is discharging and they are told "change this", or "it's probably that". The ONLY way to address a battery that is being drained is with a damn meter. PERIOD.

IN this case, well....

You may end up swapping, but the way to do this is to understand the operation of the Fan Module (FCM), and it is a very simple concept.

How it all works:

Whey you turn the key to "ON", the ECM and FCM both power up.
1) The ECM sends pulses to the FCM, if the pulse is short, the FCM just sends a pulse back. If the ECM does not see the pulse come back, it assumes the FCM is in trouble and lights the Coolant light.

2) If the ECM wants the fan to run, it sends pulses that are longer than in 1) above, the longer the pulse, the faster the FCM will turn the fan. (If the engine is cold, but you turn the A/C on, the ECM sends pulses long enough to make the fan run slowly.) The FCM still replies to every pulse with a pulse.

3) If the FCM is bad, it won't send the pulses (not the case in your situation as you said the light goes out with the A/C off).

4) If the FCM tells the fan to run, but it senses the motor is not flowing as much current as expected, or TOO MUCH, then it fails to send the pulses to the ECM and the ECM lights the light.


I strongly suspect that your problem is a bad fan motor. Of course, if the power feeding the FCM is not reliable, that would cause the FCM to possibly not send the pulses back under load as well. I can say this, in eight years on this forum, I have seen many fan motors fail. In that time, I have seen FCMs fail at maybe 20% of the rate the fan motor fails.

 

What Mercedes scan tool might you recommend?

 

Even simpler: if the motor is running and the AC is on, the fan should be turning.

 

Connect 12v from the battery direct to the fan. ( observe polarity )
If you find yourself looking around for a 747 taking off, your fan is OK.

 

What about the sensor itself or a bad contact in a connector to it?