Tach does not work under 4K rpms ??

[manoa matt]

Fixing up a friend's 79 spider for sale. I've gone through it stem to stern and eveything mechanical and electrical works. Well except the tach below 4K.

Car sat for a year. I came in, rebuilt, replaced, reconditioned. Picked out the best parts from my lot. Opened the gauges and cleaned them, existing tach may have been replaced with an earlier version (same yellow and redline as my 78, 1800) Car has a dry coil (primary resistance is within specification, secondary/high resistance is only around 6-7K. Specs for the OE Mirelli coil is 10-11K). Regardless it runs great and will rev up to 6K under high load going up a hill and no misses. Ignition pickup and control module are new, with correct gap at pickup to lobes.

Car starts and runs beautifully, timed perfect at 10deg BTDC. Upon startup the tack needle will make one flop up to 1000RPMs then drop. The needle will only register at or above 4000rpms.

I have an electronic plex distributor in my 78 with the stock tack that yellow lines at 6200, redlines at 7000. I have never had a problem like this before with my tach or anwyone elses car. I have a few spare tachs I can try that might be the later versions designed for the electronic ignition.

Any thoughts?

Matt

[TX82FIAT]

I can't recall but i think a stock distributor works off a vaccum advance. Many fiats were and continue to be upgraded with an electronic Marelli ignition. If you have the Plex mechanical advance they have been know to create bouncing tach similar to what you describe. They had a fix for the tach problems that involved putting a capacitor in-line on the tach wire.

I'm not an exeprt on this but do recall reading about many folks installing the Plex units for relaibility and having tach issues. Don't know if this is any help.

[manoa matt]

The 79 and later tach fixed it for my friends 79 that was having the issues. My 78 has the original tach and a plex but works fine, go figure.

[narfire]

Thanks for the post. I just got my 72 up and running and installed a coil and dist from a 79, electric anyway, and my tach works,original 72 tach, for a moment then dies off and thats it until the rpms are up around 2000.
Going to swap out with a tach from a later car and see what happens.
Then to figure why the fuel guage and oil pressure guage (do have oil flowing though) are not registering.
Chris