Tachometer problems

[MackGuy]

Looking for suggestions to get my Tach working correctly. I have a 1973 (.5) 911T

It always "works", but it reads low. I have no idea how low, but at idle it says 0rpm. In 5th gear around 65 itmph says ~2100rpm. It just seems like some sort of offset, but not sure how that makes any sense.

Any suggestions of where to start? I suppose it could be that the tach needs to be rebuilt, but I would expect it to have problems other than just reading low if that was the case.

Thanks!

[E Sully]

I would make sure all the wiring connections and grounds are clean and solid. Check to make sure the points are clean and dwell is properly set. The basic signal to the tach is the black/purple wire from the points. The only other way I can see to check the Tach before you send it out would be to swap it into another 911.

[304065]

Search here for "monostable multivibrator." No kidding, this will tell you how the tach works.

What is probably happening is that the capacitors that charge while the tach needle is being moved are worn out. As such, you aren't getting the same duration of charge so the needle doesn't move as much, the spring tension is the same so the instrument reads low.

You would need to test the tach by driving it with a 11V square wave with a 63% duty cycle into about 420mA of current. If you have the means to do that, that's all the explanation that's required. If not, best to send it to North Hollywood Speedometer and have them overhaul it, they do an amazing job.

[MackGuy]

Thanks, I may give north Hollywood a try.

On a related note what does UPM on the tach indicate?

[304065]

Originally Posted by MackGuy

Thanks, I may give north Hollywood a try.

On a related note what does UPM on the tach indicate?

"Umdrehungen/Minute"

"revolutions per minute"