So, i just finished rebuilding my sisters eclipse engine and i was all ready to fire the car up, but I had no good battery. So I pulled up my z and gave the eclipse a boost.
Durring that boost, my friend notices that the tach in the z is going crazy, like dropping to 0 rpms and then kicking back up, but all the while the engine remains at a constant idle. Also, the car would keep blowing the 10 amp fuel gauge fuse. (im guessing because the tach was doing such crazy stuff.)
After that, my tachometer would read double and never give me an accurate measurement of what the engine was doing. It even seemed to be that the car was running on the battery alone. I took the car to autozone, and they couldn't determine if it was the alternator or the voltage regulator. I took the alternator off and tested it by itself and it was only putting out 3 amps!
I put in a new alternator and everything works again! It even fixed the problem i had before where the "charge" light would never turn off, and the voltmeter reads around 14 volts where before it would read really close to 16 volts.
EXCEPT... there was a wierd problem, because i was at a stop light (after i installed the new alternator) and i heard some relays clicking under the dash and the engine shut off... but it started right up with no problem. I'm going to drive the car around tomorrow and see if it happens again.
either way, i just wanted to report what happened, because when i searched for "double" and "tach" and all other tach related problems, i didn't come up with anything, so hopefully this will help someone in the future. BTW this is on a 1977 280z.
-Oliver