240D is my temp sensor electrical or mechanical?
 

I was going to order a new one of these to try to track down what is making my gauge jump around, but was asked if it is electrical or mechanical.

How can I tell?

My 240D is a 1979

Its electrical, on the left side of the cylinder head near the glowplugs.

MB hasn't used a mechanical temperature gauge in decades, I'd suggest finding a more competent parts supplier.

Thanks for the answer.

My car is three decades old, by the way... I would say that is definitely "decades" :P

And well, I won't say who the parts supplier was, but their customer service, responsiveness, and past proof of knowledge will keep me going back to them... and the forum on the site they provide :D

lol, i meant decades as in like the 50's or 60's.

My '71 250 still has a mechanical temperature gauge. I think they used them until the end of 114/115 production.

Make sure you have good ground to the engine before blaming the gauge. A loose or oily engine ground will exhibit this symptom sometimes.

IN 1973 they stopped using the mechanical temp gauges. The W114/W115's post 1973 had electric temp gauges.

Good to know. Thanks for the info.

X2

My temp gauge would read normal, then would suddenly rise or peg to the top. turn off the engine, then turn the key to activate the gauge, it would read normal. start engine, the temp would be steady, or would get jumpy, or peg to the top.

flip one of the switches on the dash would make the needle jump up, or drop back down to a normal reading. scary feeling to look at the gauge, and it is in the melt down zone.

so figured it was a ground in the instrument cluster.

wasn`t until the battery seemed dead one morning,, turn the key, nothing. but this is another story. chasing down this problem is what fixed the gauge.

Problem was the ground strap from the engine to the frame on the drivers side.
the adapter plate the strap is mounted to, has this pretty silver paint, which doesn`t make a good ground. so a little sand paper getting down to clean metal did the trick.

the gauge is steady as a rock.

Charlie

Thanks for all the info guys... I will check that ground before ordering the new sensor.

You know, I was just testing this out... and I disconnected the wire to the sensor and got a 0 reading....

So if there was an error with reading high, then that would mean that the electricity is leaking through to ground, wouldn't it?

How could that happen with a bad ground connection? Wouldn't it actually read low with a bad ground?

Yes.
The sensor is old. Being that the sensor itself is like $3 and takes 5 minutes to change, everyone should change theirs as maintenance since they're all over 20 years old now and have been through the ambient-180*f-ambient thermal stress a dozen or two thousand times.

If the gauge randomly jumps up and down ~10*c quickly, the sensor is bad.

By the way, there are two different sensors or something like that in the side of the block - both look similar and have the same type of wire connection coming out of them.

The back one is the temp sensor for the gauge. What is the front one? (towards the front of the car)

The front one is the preglow temperature sensor.

Hey one more related question.

When I reach up behind the instrument cluster and wiggle the electrical connector (way up on top - big connector with a bunch of wires going to it) around, the gauges jump - particularly the fuel and temp gauges, which have both been "jumpy" for me - but almost never jumping at the same time.

Wiggling it, I can see that they will jump independently.

Could it be that this connection just needs to be cleaned?

How do you get that connector off? Just pull?