temp. gauge acting strange
 

Hello everyone my temp. gauge is acting strangely on my 85 300 SD after many hrs. on tne road. The last road trip after about 6 hrs. of driving it starts fluttering to the high overheating range but am pretty sure engine is not running hot just erratic reading. Has any one else experianced this and where do you think the problem could lie? The sender, or gauge or wireing? Where is the sender located?

Engine ground strap.

Temp sensor/sender is on drive direction left, in the head. Test resistances across it at varying temperatures - should give you a quick sense of its functionality.

Also worth just plain cleaning the contacts on it - the sensor's post, and the plug's cylindrical connector. I also remember I broke my connector's plastic housing a few years back, just being un-careful. Dumb! But all very easy to fix.

I agree with VSTECH, ground Strap. The one between the engine and Body, remove it and clean the contact areas.
I had this same problem with our 85 300D.

Had replaced the engine with a rebuilt one painted up all pretty. Pretty doesn`t make a good ground. LOL
Scratched off some paint, and all was good.


Charlie

what happens is the ground strap is connected with a transmission bolt, and the bolt strips. the HIGH amperage of the starter causes a temp welding connection, but as you drive a bit, the oil and grease loosen the connection, vibrations get the cable wiggling and it loses contact. grab the cable, and I bet it wiggles freely...
without engine ground, the ground on the instrument cluster, is a different value than the ground on the temp sender itself, and you get a weird high reading occasionally. The giveaway is the RAPID jump to high... no way for that to happen on an engine...

In extreme cases, a loose ground strap can burn out traces in the cluster and cause other strange problems (by the cluster ground supplying current to the starter due to the missing ground strap ground). This actually happened to me not long after I bought my first Mercedes (the 83 300D in my sig). Got it going within a day with help from this forum. Fix that ground strap quick!

My temp sender wire was broken had to rewire mine.Yes I clean ground strap every few years,I also added a second one.

erratic temp. gauge
 

Thanks for all responses I will check the ground strap 1st. Where can I find this on the 85 300 sd? It sounds like this could be the problem.

You're looking for a short thick black wire that runs between the engine and the body. (Don't be afraid to crawl underneath and investigate
:) ) Mine was on the driver side IIRC but i got rid of it and ran a wire directly from the battery's negative terminal to one of the starter bolts.

temp. gauge acting strange
 

I finally got around to check out the ground cable and sure enough it was very lose on the end that was secured to the starter on the bell houseing on the passenger side of the motor. I took the ground strap off totally cleaned both ends and tightened back on now temp. Gauge is reading normally and not bounceing around problem solved. Thanks for all the replies now to get on with other issues with this car.