Tachometer Issue on W116:
 

I am having a strange issue with the Tach in my 116.120. It sometimes flops all the way to 6000 and then back to zero. At times, it will flop to 6000 and remain there.

I have checked all the grounds.

At times, I can hear something clicking in the passengers footwell in time to the erratic operation of the tachometer.

Could this be the transmission overload protection relay???

Anyone experience this???...Bert

DO you have a known good tack amp tp try?

My W123 was doing that...I bought a pair of Tach amps from a fellow member to replace my dead one and one was doing that very thing..he has since replaced it for me....

Thanks for the tip, Bone...

Where would I find them on a 116??? What do they look like??? I have never experienced this problem before.

Also, my temperature gauge reads really low, even with a new sending unit. It reads about 140°F with new Beher 80°C Thermostat. I know this is wrong since the auxilliary fan will come on if I allow it to idle for long periods.

I have a cluster out of a 116 with a V8. I was thinking about changing the gauge.

Also...my fuel gauge reads good at full, but when it drops to 1/2 tank, it starts dropping to 1/4 and then to Empty, before popping back up to 1/2 tank again.

This sounds like a sending unit issue to me. What is your opinion??? Thanks for your time...Bert

grounded
 

although you have checked your grounds it sure sounds as if that is the most likely cuprit. basically all your guages are screwed up except the oil press and it is mechanical.

Dr. Bert:

I totally feel your pain. Same exact symptoms I am having right now too, including the erratic fuel gauge and the clicking relay from passenger side (but that shouldn't have anything to do with the operation of the tach...right??).

I checked for voltage, swapped out the amp for a new one, even tried a new instrument cluster, still nothing. Interestingly enough, when I made another ground, it temporarily solved the problem but it soon came back, the spastic tach.

Being that this post I'm replying to is 8 years old, I'm assuming you solved the problem. Would you be so kind as to share your valuable knowledge/insight with me? Also, underneath the dash on the driver's side, there are three relays. I know the top one is the compressor relay and one of the other two are for the aux fan. Do you know what the other one controls? You have no idea how long I have been struggling with this issue...it is literally the last thing that needs to be fixed on the car and it is 100% complete.


Much appreciated

Mine was doing that once on a previous W116, and I think the problem was the ground up by the radiator was gone because someone cut it. I reattached it, and suddenly the glow plugs and tachometer worked. Maybe that little 7mm bolt connection is corroded.

Good idea, but mine was a faulty amp....FWIW

I tried cleaning the leads as well as the exposed metal - no dice. Next I'm going to try all new relays. Do you happen to know where the sending unit is located under the dash? I'm thinking that could also be the culprit.