1992 300D Coolant Temperature Gauge - PeachParts Mercedes-Benz Forum

BenzEnthusiast · #1 02-24-2013, 03:05 PM

Since I bought the car, I haven't been able to solve the issue with the coolant temperature gauge. It just always sits at the bottom and doesn't move.

I have replaced the sender unit for the gauge and I have tried grounding out the plug that goes on the sender unit to get the gauge to peg at 120 and still the gauge won't budge. I did notice however, that when I turn on the ignition, the needle on the gauge slightly rises. Does anybody have a way of testing the gauge itself? By the way, this is on my 1992 300D 2.5 TURBO.

compress ignite · #2 02-24-2013, 04:55 PM

1.The wire that connects to the Sensor has a break.
2.The Instrument Cluster Ground(s) are faulty
2a There is a brown wire that "Grounds" the I.C.
2b There are "Tabs" between the Left,Center and Right portions of the I.C.
That Must be Clean Bright and Tight.

Oh and FINIS The Temp Gauge IS BAD.

If all the grounds are good ,as well as the sensor cable...
Grounding the Gauge is the definitive test.

See reply #2 of this post (jeremy5848):

Tach and Fuel gauge problems on 93 300D (124)

Then look at the other post he references:

W124 tachometer is intermittent - why??

BenzEnthusiast · #3 02-25-2013, 12:57 AM

I know that the wire that connects to the temperature sender unit is fine because the feature on the climate control that turns the fan on gradually as the coolant heats up so that the heater doesn't come on all the way until the heater core is warm works correctly.

Also, I suspect the grounds in the instrument cluster are also fine because all of the other gauges and the clock work fine, but I'll pull the cluster and check all of the grounds to be sure.

So, how do I go about grounding the temperature gauge itself to see if I can even get the needle to move?

compress ignite · #4 02-25-2013, 01:25 AM

"I have tried grounding out the plug that goes on the sender unit to get the gauge to peg at 120 and still the gauge won't budge."

(Which is why the gauge is suspect.
Also [Ding,Ding,Ding] the gauge isn't working with a new sender.)

Howeveah!

This causes some concern with the "wire":
"I did notice however, that when I turn on the ignition, the needle on the gauge slightly rises. "
[I like my DEAD Electrical Equipment: Dead, Dead, Dead.]
And if there's something hincky going on with the Wire THIS MAY be an exemplar.

Being the Owner of "Obsessive about not spending money I don't have to"
I'd take the sending wire off the Gauge and the Sender/Sensor
(just take the ends loose from the connections) And Ohm that baby out.
(Heck, Continuity would probably tell Ya)
The leads on a standard DMM should be long enough to reach both ends
of the wire.

BenzEnthusiast · #5 03-04-2013, 09:01 PM

So, I finally got some time to pull out the instrument cluster today and clean the grounds and root around the engine bay looking for any broken wires. I found a small amount of corrosion on the grounds inside the cluster and cleaned it off. Also, there are no obvious wiring issues.

When I tested pin 15 that plugs into the back of the cluster (the one that is suppose to supply the signal from the sender unit to the temperature gauge) I got 60 ohms of resistance on my multimeter, which makes sense because the wire from the sender is shared by the climate control and the gauge.

So, I put the cluster back in and hooked it up to see if grounding the plug to the sender would get me a result this time and it didn't do anything like before, yet the temperature gauge upon turning on the ignition still moves a bit.

Then, I decided to try the test on my 300SDL which has a working temperature gauge and it didn't move when I grounded the plug on that one either, so now I am just confused.

Any ideas of what I may be doing wrong or what might be wrong with the car?

BenzEnthusiast · #6 05-01-2013, 02:41 AM

So, I finally got the gauge working!

I feel sort of stupid, but the issue was the sender unit. I replaced the single post sender unit on the thermostat housing which I had been told by a couple of people was the one that controls the gauge, but apparently on OM60x engines the sender unit on the side of the cylinder head is the one that sends the signal to the gauge. Anyway, glad to have it fixed and thanks for the help trying to diagnose the problem Compress Ignite.

compress ignite · #7 05-01-2013, 08:27 PM

A Happy Turn of events when a Mysterious Electrical Anomaly is solved!

Benzman66 · #8 02-16-2018, 08:54 AM

I have a 1995 300TE that I converted to a 300TD and am having the same issues. No temperature gauge, AC compressor, electric fans, etc. There were several wires that are still unplugged that aren't color coded for the diesel set up and I no longer have the donor car as a reference so any information would be appreciated being that I want to keep this car for a long time, thank you.