I'm experiencing a strange situation with my temp gauge on my 85 300D....When I first start it up, the gauge shows cold (below the 40*C mark) and I've watched it quickly climb to about 100*C (as expected) over the next few minutes. It hovers around 100*C for a while then it pegs to the top of the gauge (like as if it's grounded?)........At this point, I can shut the car off and put my hand on the radiator, the engine is just starting to feel warm, it's nowhere near 100*C.....At first, I thought it must be a bad gauge, but I see the same symptoms after switching it out with another one (both could possibly be bad I guess) but it seems like something is causing this "grounded" condition as soon as it starts to warm up....Could it be a bad idle temp sensor (on the thermo housing)....??....any ideas?

I'm sure one of you experts here knows what's going on and how I can resolve this issue.......THANX for your help guys!

Sounds like the lack of a good ground in the instrument cluster. I had the "jumpy gauge" problem and wound up attaching a wire from an instrument cluster fastener to a good ground spot. Cheap and easy.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

You could also have a defective sending unit on the head. I had this one happen to me.

That is not what would be expected for a normal engine, the temp should rise to somewhere between 80 C and 85 C then stay there unless you are driving up a steep hill or carrying a heavy load.
The sensor is the first thing I would suspect. A bad ground will be flaky all the time, you do not have symptoms of a bad ground.
The temp you want to check is not the radiator but the thermostat housing. If it is indeed hitting 100C, which could occur of the thermostat was sticking, then the housing will be very hot. And the head will likewise get very hot very quick.
I believe the thermo sensor is on the head, not the thermo housing. At least thats where it is on my 123.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

If it was a bad ground, wouldn't it effect things too (like fuel gauge)...??

When you say "on the head" do you mean on the side of the block between glow plugs #2 & #3??

Are both of these sending units interchangeable (same part #)? If so I could swap 'em and see if it resolved the problem and proved the other one was a bad unit....

Get an accurate reading on the engine temp for a quicker and safer diagnosis. For about $25 you can pickup an infrared temp sensor at Harbor Tools. It can be usefull at times like these.

The latest info.....I bought a handheld temp gun (cool!) and measured the operating temps....I thought maybe the sending unit was bad so I replaced it....Same results, temp gauge starts to slowly creep up as car warms up and the jumps to the top....Maybe a wiring issue? So I took another gauge and wired it to ground and new temp sensor....YEP, you guessed it, PEGGED to the top too.....So what could be causing this?? Any ideas???

Check batt-chassis and eng-chassis grounding. Also, what is the voltage regulator, on the alternator, putting out as a charging voltage?

The reason you see it first in the temp. guage, with grounding problems, is because of the simple inductor(guage) and resistor(sensor) circuit.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I would try running a new ground from the left side, back of the instrument cluster.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Hmmmm, sounds like we're on to something here.....I tired to eliminate the existing wiring by running a wire from the battery ground to the gauge and the second wire from the temp sensor to the gauge......It sounds like you're saying maybe the battery isn't grounded properly??

When I ran across similar problems, the engine-chassis ground strap was found to be loose at the engine. In fact, this eventually caused starting problems. With 300K miles, original worn engine mounts, missing engine shocks and a dire need for a valve adjustment, my guess is all of the vibration had caused the bracket it was attached to to work loose.

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Sam

84 300SD 350K+ miles ( Blue Belle )

99% sure it is in the cluster. M-B in their wisdom made the ground connections in their cluster go through mechanical connectors like screws and bolts when the cluster was assembled rather than wiring the sections together. An easy fix is to simply run a wire from the metal on the section of the cluster with the gauge to a known good ground in the dash (look for a grouping of brown wires all bundled up and connected to a big bolt on the chassis behind the cluster and that's a ground point) and see if that fixes it. Usually that will do it...you can't hurt anything by trying this. You're simply replacing a ground which has gone bad.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz