Tach Signal - Confounding
 

I will be indebted to anyone who can solve this mystery:

Tach for Euro 83 SC motor stopped working each time after driving for a few minutes. IU then tested the tach on a bench at PA Speedometer - it came out faulty. I got a new tach, which did not work either -- except that the needle did not register any RPM. Took the car to back to Palo Alto Speedometer, who tried 6 different tach boards without success.

Maybe the problem is on the car end, we wonder. We try a different CD unit, no success. Check the wiring, normal. We put the signal on the oscilliscope and compared it to another 83SC. My signal is a series of peaks, off the instrument. Normal SC siganal looks more like a series of rectangular steps. Signal is the same at the CD unit and at the tach.

I have taken the car to Dieter at D&M Motors (brother of guy who runs PA speedometer), and he has never seen anything like this in 25 years of wrenching. Before I decide to invest a huge amount of money and time trying to fix this friggin' tack, does anybody have any ideas?

Jamie,

It sounds like your CDI-unit caused the initial problem, and is continuing to fry any tach that you feed that high-Voltage signal to ...

I believe there has to be some conductive corrosion or contaminant causing a 'sneak current' path from the high-Voltage section of the CDI-unit over to the TD signal output terminal! I don't believe that the TD signal should be above 10 Volts or so! Do you happen to recall just what the highest Voltage signal peak that you saw was ... ??? It does not surprise me that hooking up a new CDI-unit to tachs that had already been fried ... didn't magically fix your problem and cause a good tach reading to reappear!

Hm,

it sounds to me like you are frying the tach board with high voltage spikes from the CD unit. That is why you see the tach ceasing to work after a few minutes. It is most likely a capacitor that burns down due to overvoltage.

I am not a wrench myself but would guess that there is something wrong with the grounding of your ignition system that feeds HV spikes back all the way to the tach.

I would also check the ignition coil and the HV wires as well and try to swap them.

The oscilloscope should be the tool of choice to track down this issue.

thanks for your replies! I will go through it tomorrow.