Testing a Tach
 

I have been working through ignition issues and have and found a local wrench that will let me put a functional CD unit he has on the car to determine if that is the issue.

I read in the manual that a faulty Tach can cause the unit not to spark. It reccomended unpluging the Tach to see if the unit would spark.

If the replacment CD unit cures the issue, I don't want to plug a bad Tach in and blow another unit.

The Tach worked perfectly before the ignition failure but I would still like to test it anyway----Anyone know of bench test that will tell me the condition of the Tach.

Thanks

If you verified correct operation of the tach to a known input pulse rate, would that be good enough?

A small reed relay (low mass, capable of high pulse rate) could be wired to operate from 117 VAC from a small wall transformer at 120 pps, you could do the calculation what rpm that would correspond to for a six-cylinder engine. Whether you want to bother with that or not is up to you.

A simple check with a multimeter or Ohmmeter while the tach is hooked up to a bench 13.8 Volt supply to see if the tach. input lead has any voltage present with respect to ground, or if a low impedance or resistance to either ground or the Battery + supply can be measured ... would tell you if the tach. could be a 'killer' for the CDI-unit! I would not think any measureable Voltage present should be acceptable. Any resistance measured at the input lead of 10,000 Ohms or more would seem OK to me. Good luck!

------------------
Warren Hall
1973 911S Targa

Thanks Warren,
All the information you have passed on, and previous threads on ignition issue, plug wires, distributors and testing have been very helpful. (not to mention they have kept my frustration level in check.)

Rather than putting the wrenchs CDI in, he directed me to www.carquip.com in Boulder CO. Its a few miles from where I work, so I ran over on my lunch hour. They had a bench tester made and showed me how they test the unit, they put mine on and it was bad, they found a used one that checked out fine.

After reading all the threads on aftermarket CDI's, I decide to stay with OEM parts, no reson to throw another variable in the mix.

Looks like I'll have it running over the weekend.

If anyone contacts Carquip and would like someone to eyeball the parts before purchase I'd be glad to do what I can.

I think I have all the items to test the tach here at work. I'll do that Monday.

I'll keep my finger crossed.

Thanks, Todd

I'm not sure piggy-backing questions is a good idea, but I have one: My tach wiggles sometimes. Not often. For just a few seconds. Wiggles maybe 400rpm total. Then settles down again.

Why?

------------------
'83 SC