1970 Tach problem
 

So it's pretty much all over the place - no acurate readings at all i think. Is it an electrical or mechanical tach? What is the remedy?


Thanks,

Allan

Check your points if you have them still.

Allan,
Have you checked/monitored your system Voltage to see if it is outside nominal factory specs of 13.2 - 14.5 Volts? Tachs are sensitive to transients and high Voltage on the system bus!

The tach is electronic ... ancient discrete solid state design. Relatively easy to fix if familiar with that age electronic circuits and are familiar with traditional troubleshooting methods using oscilloscope, meters, and signal geneators ... after you have the case and board disassembled, of course!

A more modern remedy, if you are willing to get a PC board made or use a prototype board ... would be to fabricate a replacement board based on an LM2917 integrated circuit. It is discussed in the following thread:

http://forums.pelicanparts.com/porsche-911-technical-forum/226183-1-49-lm2917-ic-does-tachometer-mfi-speed-switch-rev-limiter-shift-light.html?highlight=lm2917

Thanks Warren. I'll check the the volts this weekend and take it from there. Interestingly, I was having problems melting rotor buttons - I soldered a piece of brass into one of the buttons to remedy this. I never checked the volts. Maybe the two are related.

allan

Allan,

The type of checking I was referring to is an extended test during a normal extended drive of an hour or more ... with a meter setup as below as a minimum. More serious tests require a meter that can log readings to a laptop PC.

If Voltage is nominal during a test drive, points are the next most likely problem.

What CDI are you using, MSD? Burnt rotor resistor or cap button contact is another problem, altogether ... haven't heard of misbehaving tach during such adventures!

http://forums.pelicanparts.com/uploa...1123126225.jpg

Bump-n-fix-pic ...

I'd try to use an nice big analog needle meter if you have one. Easier to see it swing...

Check your grounds, I've found the 90% of electrical problems have to do with bad ground connections. Get a good analog meter digital meters are worthless.

Thanks guys.

digital meters are worthless

- well they are great fro precise readouts -- but you want to see changes here - it's nice to have 1 of each...

Give me an old Simpson any time

yes, give me one too ...