Proper Voltage at Coil
 

Stock 71 GTO with points

Reinstalled engine after removing for a dipstick tube replacement and car wouldnt start

Determined that the stock Pontiac in dash Tach, which had worked fine prior to engine removal, was grounding the coil. Once disconnected the car ran ok, set dwell (30 degrees), timing (12 degrees), and curb idle at 750 rpm

When I went to start it the following day the car wouldnt start
I was able to get it to start today and run for about a minute but it ran rough and would eventually stall out

Testing the power to the positive side of the coil I have 8 volts with the key on and when the car was running it was ranging between 8 and 14 volts to the positive side of the coil before stalling out

How much voltage should I get with the key on? 12v?
Should it be a constant 14v to the positive side of the coil with the engine running?

Does this sound like a coil issue or an alternator issue?

The tach has remained disconnected since our initial start up issue, not sure why it was suddenly grounding the coil having worked fine before pulling the engine.

Thanks for any thoughts/opinions
DV

Assuming points ignition, with the key on, the engine NOT running and the points open, the + terminal of the coil should read battery voltage to ground. With the points closed the + terminal of the coil should read around 8 or 9 volts to ground.

With the key on and the points open, you should get battery voltage (12 volts). If the points are closed, you will get a lower voltage - around nine volts. Running, it should average around ten volts. I use the word “around” because the exact numbers are dependent on the specific electrical resistance of your particular coil.

Thanks guys for the info.