I have 68 firebird 350 that I put a HEI in 2 years ago using a relay on the fire wall. Worked great. But this week the car wouldnt shut off. Did some testing . I used both the pink and yellow to trigger the relay to enable the 12v to the HEI. I tested the pink and yellow wires, car not running, I separated the wires from each other and both work as they should. Pink on and yellow start. I started the car, then separated the yellow wire from the pink and the relay. Car still was running, I turned the key off and was still running. I measure the voltage at the pink wire / relay trigger joint it had 6.7 volts while the key was off. It only has the extra voltage when the car is running. Its got me scratching my head. I put new ignition switch thinking it was a cure before testing the wires. I also swapped out the relay...
It could possibly be feeding back voltage into the system from the alternator. next time you turn the ignition off and its still running disconnect the 2 wire plug that plugs into the alternator and see if the car shuts off.
I pulled the two wire plug and the engine stopped. It also started and stopped as it should with it unplugged. Voltage regulator is common between the wires and ignition so Ill swap it out tomorrow. It may have been in the car at least 27 years.
