I'm needing assistance with a charging system problem. Vehicle is a 1979 Magnum, factory 100 amp alternator, external voltage regulator (1970 & later electronic type/ triangle connector). The Magnum uses the shunt wire to bypass the amp gauge so charging current does not pass thru the bulkhead or the ammeter. There have been no charging system problems for years and everything was fine when I parked the car last fall for the winter season.
Over the winter I replaced the ignition switch (bad mechanically, not electrically) and the starter relay (just because it was original). This past Saturday, I replaced the battery because it was 12 years old and did not turn over the engine. Sunday, I fired up the car for the first time this season. My voltmeter (Auto Meter) spiked to 16+ volts on start up but after a few seconds came down to the 14-15V range. I consider 14-15V at idle (1000 rpm) normal. After a few minutes running, the voltmeter spiked to 16+V again and remained. I connected my digital multi meter to the alternator output post and read almost 18V. I checked the voltage regulator ground and it is clean-I also use a braided grounding strap from one of the VR mounting bolts to the engine. I had a spare VR (not new but know good) and swapped that on but it did not change things. There is a ground wire from the alternator (alt. ground post) to the left cylinder head where the negative battery cable connects-that is good. I let things sit for a day then started some checks. Battery voltage, key off-13.1V. Blue wire at VR connector, key on-12.9V. Blue field wire at alternator, key on-12.9V. I checked continuity of the blue and green field wires, each has about 0.5 ohms resistance, and no continuity between the two field wires. I have not yet pulled the bulkhead apart but my readings appear to be normal.
I next bench tested the alternator according to the FSM-"Rotor Field Coil Current Draw Test". The result was 4.2-4.3 amps. According to the FSM, "Field coil draw should be 4.75-6.0 amps at 12V. A low rotor coil draw is an indication of high resistance in field coil circuit. (Brushes, slip rings or rotor coil)." This is where I'm stumped. Is this the cause of the high voltage output of the alternator? To double check the alternator, I took it to a local parts store for testing and they said the rectifier is bad.
I don't want to throw parts at this, but rather learn. I have a handle on electrical basics but that's it. Is the problem solely the alternator or am I missing something? Sorry for the long post but wanted to provide as much info as possible.
Thank you.
Steve