Tachometer Drive

[gcw206]

Could not any information on this Forum.
I have a 1969 280SE W111 Coupe.
I have installed Pertronix Ignition, a stock MBZ "Red Coil" and resistor, and Bosch W9DC spark plugs.
All worked fine until a few days ago.
Sometimes (but not always) at right around 2000RPM, the tachometer will "jump" ahead.
I went to lube the cable drive, but found that there is no cable drive on this model.
I assume the tach is driven electronically off of the coil?
Where should I start the trouble shooting?

[ja17]

Yes, yours is driven electronically off the coil. I am not sure if your conversion is causing the problem?

[gcw206]

No, she worked fine for over a year; this just started happening about a week ago.
Think I will check all of the wiring and the fuse situation, maybe something loose?

[wwheeler]

I know when I had my 280SE tach rebuilt by Palo Alto, they added something to help reduce erratic movements. So possibly the electronics in the tach are going South. I would contact them and see what they say if you cannot find anything. These don't last forever.

[gcw206]

Hello,
It was suggested that I might have a bad/loose ground somewhere in the tachometer wiring system.
I was advised to run a ground wire from the distributor housing to a solid ground, and see if that helped.
I did just that, and it seems to have solved the problem. 
No more erratic tachometer readings after about 6 hours of driving.
Thanks; this is a great site.
Galen

[wwheeler]

Interesting. That was a good tip. Grounds can be one of the most difficult electrical items to troubleshoot.

[gcw206]

Hello,
Update to the Update;
1.  The added ground did not do the job;
2.  Tach started jumping again;
3.  I checked all grounds, signal wire spacing, etc;
4.  I added a 10K-1W in line resistor to the tachometer signal wire (check out You Tube).
Works great (so far).
Galen

[wwheeler]

If I remember correctly, Palo Alto speedo added a resistor to the tach when they rebuilt my W111 tach. I cannot be 100% certain, but seems like they made some alteration.