Temp Guage Jumps

[BRIAN123]

My temp gauge needle flickers. Dead cold it reads 40, most of driving time it is around 80, and then it jumps between 80, 90 & 100. Sometimes it is pinned at 120. It never overheats or has any fluid leak. Coolant level is steady. My driving style or use of the heater has no affect one way or the other.
Where should I start?
What danger lies ahead if I do not fix it now?
Pennsylvania climate still in the 30’s for another week. It is kind of hard to work on in the garage.
My baby is a 1990 Sl 500 with 106,000 miles

[balsa]

Try it first with changing the temperature sender unit it should be on water pump. This was the solution on my 1986 560sl and the part is cheap and easy to fix.

I hope it helps.

Balsa

[JPMOSE]

Brian,

One of my Mercedes did the same thing (my 300D). When it reads high, I would tap on the dash, near the gauge, and the needle would drop to normal. I called someone in Phoenix, who advertises on Ebay for clock/gauge repair, and was provided a good fix. I don't know if the W129 gauge is similar to the W123...but....on the back of the instrument cluster (fuel/temp/oil/economy) there are two small 7mm nuts that I removed and cleaned (although they didn't appear corroded or dirty) and reassembled. The gauge has worked fine ever since. The gentlemen told me if this didn't do the trick to run a small wire from the nut to a common ground.

Next time yours reads high...see if tapping the dashboard makes the needle drop...then you know it's your gauge.

[BRIAN123]

That tap did the trick just fine. What a relief!! Now I can go in and try to fix it when my fingers defrost. At lease now I do not have to worry every time I drive it. Tapping brings it down to normal.
BTW , it creeps up with vibration, when I hit a small pot hole or road seam it would go up. This explains why my mechanic said “I had it running for an hour and the needle never moved”

After cleaning the nuts, shouldn’t I run that ground while I am back there? It will sure beat removing and installing the cluster twice. I will just have to live with the “was it the nuts or the ground” question for the rest of my life (I hope).

Big Thanks, my wife like the car more the less I spent on it.

[JPMOSE]

Well yes...I have to agree that it would make a lot of sense to install the wire anyway. On my 300D, the cluster pops out easily. On my 560SL, I have to remove the steering wheel -- which means disconnecting the air bag. Not really so hard either, but it is extra steps. I would assume the W129 chassis is more involved too.

Before installing the wire, I would do some more research and make sure this is applicable to you set-up.

[Mike Miller]

You can remove your cluster without removing the steering wheel on the R129. Move the power adjustment all the way down and out. You will be able to get your hand behind the cluster to pull the plugs and then slide it out toward the center of the car.