C70 tachometer quit

[coondog706]

Please advise testing of tachometer. My c70 was running and the tach went to bouncing at the needle then died. The battery light also came on. I pulled the alternator and the shop put a voltage regulator on it. I didn't check voltage before i carried it to be repaired. I reinstalled the alternator,checked the belt and had the light come on and no tach. I did check the voltage after that and had 14.3 at the battery and at the alternator.
The tach reads about 4000 with the key on but drops to zero when cranked. Any help would be greatly appreciated. Thanks,coondog706

[MCC]

Do the other functions of the instrument cluster work? If so you may have to replace the tach. There isn't really a test for the tach other than checking the W Terminal ( yellow wire) for an ac signal to the dash panel.

[coondog706]

Thanks for the info MCC. I did't mention that the light is still on even though the voltage reads ok. I suspect a diode in the alternator ,because everything else is working except battery light on and tach is dead. Thanks, coondog706