What is my warning light trying to tell me? For more than the past year, the dashboard's Coolant Warning Light has been coming on. And when it's on, the light is usually less than full-on compared to the other start-up warning lights. But the Temp gauge is always reading in bounds (@43 minutes analog). And the coolant level is full up. I've wrangled the expansion tank's sensor out and cleaned the contacts. But it hasn't really changed the dash light from simmering on after it warms up.

Yesterday I drove 75 daylight miles and the Coolant Warning Light was sometimes just glowing and some full-on. On the way back in the dark it was very in my face and on my mind. This morning I checked and the coolant level was spot on. Coolant drops less than 1/4" per 1000 miles. Replaced coolant system 100K / 8 yrs ago when it blew a head gasket. Coolant's clean, and there's no white smoke. Any ideas what's going on? Could it be warning about the transmission fluid level? It slow leaks, and has it's own parking "spot". Not sure about thread jacking my own post. But are they related?

Reservoir tank level sensor is kaput!

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

You can do 2 tests.

1) unplug the plug to the coolant expansion tank level sensor and drive for a day or two. If it ever comes on then the problem is your wiring in the car somewhere.

2) short the level sensor plug and the light should be hard on. If it does not then again something in the wiring.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

So I'll check the wiring, though it's never been a problem (except in the B pillars).
New sensor's prices: $10 for Meyle, $35 for MB, and Hella in the middle. Any reason not to go Meyle?

A little background on the bulb self test. But first a little theory on the alternator because it has a key role in the bulb test on a 124.
The alternator has 2 terminals, D+ and B+. B+ is the main output to the battery and D+ is the feedback wire. When the alternator is not turning, and you turn the key to the II position, the D+ wire has no voltage on it. Without going too deep think of the D+ having a path to ground when the alternator is not producing output, and having alternator voltage on it when the alternator is running. The instrument cluster has a bulb that is connected between the dc bus (what MB calls Terminal 15, which becomes live when the key is in position II) and the D+ of the alternator (which MB calls Terminal 61). Note these terminal numbers have nothing to do with actual pin numbers on many device connectors. However you will see references to them all throughout the wiring diagrams. I believe this is actually a German standard as I know BMWs use this convention too.

This voltage difference causes the battery indicator to glow when you have the key in position II and the engine is not running. Once the engine starts turning, and the alternator makes voltage, then the voltage at the D+ rises to the system voltage. This puts dc bus voltage on both sides of the bulb, making the voltage difference across the bulb 0 V, which means it does not light up.

They use this voltage difference to also self test the following indicator bulbs:
Low oil level
Low Coolant level
Low Washer fluid level
Low Fuel level
Low Brake fluid level/Park Brake applied
Brake Pad Wear

Each of these lights is connected to terminal 15 (the switched power leg) inside the instrument cluster. From each light bulb, the wires go to the various sensors. All of the sensors switch the wire leading from the bulb to ground. Typically the ground is located at the sensor location.

To achieve the self test feature, a tap is placed between the bulb and the sensor connection inside the instrument cluster. A diode is wired in each tap line (to provide isolation, otherwise all the bulbs would light up if one of the sensors mentioned above operated). And to provide reduced brightness on self test, a resistor is placed in series. All this is connected to the D+ connection in the instrument cluster.

So when you turn the key to II with the engine stopped, the power flow through the low coolant light is battery positive -> key -> fuse 5 -> instrument cluster pin X3:6 (X3 is the big round connector) -> the low coolant bulb -> a diode -> a resistor -> X3:9 -> the voltage regulator D+ terminal -> ground through the alternator mounting -> engine block -> engine to body ground bond cable -> battery negative. This results in a dim glow of the low coolant light.

Assuming you have a low coolant condition, the power flow through the low coolant light is battery -> key -> fuse 5 -> instrument cluster pin X3:6 -> the low coolant bulb -> X3:12 -> S41 (the coolant switch) -> W2 (ground behind the right front headlight).

So the takeaway from all this is:
dim glow = bulb being illuminated from the self test circuit and the D+ terminal on the alternator. Or a weak contact in the low coolant sensor that is falsely triggering even though you have proper coolant level.
Bright glow = bulb being turned on by the sensor, or a short circuit to ground somewhere between the instrument cluster and the sensor.

Here is what I would do.
First unplug the sensor and see if the self test works correctly. Turn on the key and see if the bulb comes on at half brightness, confirming the bulb check side of the circuit is working. Start the engine, the light should go dark.

Now find the end of the connector you unplugged from the sensor. Short the two connector terminals together with a piece of wire while watching the low coolant light. It should come on full brightness while shorted and should be dark when the short is removed.

If this is the case then you have a bad sensor. I would go with the Hella as they made the original sensor for MB. Meyle is cheap junk and genuine MB is just a Hella sensor in a MB package.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Dumb luck struck. I paused a parts order, thinking I might need something else and get free freight. And I delayed another day after reading the judgement on the sensors. The luck was stumbling onto my old expansion tank from way back when I rebuilt the coolant system.

And it still had the original sensor with the MB Star. Yesterday I pulled out the air filter box and unbolted the expansion tank, allowing just enough room to wrangle out the old sensor, spill some coolant, and pop in the cleaned old spare.

The failed sensor was a Hella, one of the 2 electric post connectors was green tarnished into the core. So it lasted 100k-ish

When I switched the key to 1st position the warning light lit dimly, just like the other warning lights. And today I drove it all warmed up, and no more Radiator warning light. That's going to make driving more relaxing. Thanks guys.