It'll start and run fine, some times for days, then I see the gauge bouncing from where it should be down to just over 12 and then drop to right at 12, then lower and the battery idiot light comes on. I pulled the alternator and it tested fine at the parts store. I cleaned the corroded top 3-prong plug and it was good for a couple days. Then the same symptoms and I replaced the 3-prong plug. Worked fine for several miles now the same symptoms again. If I keep it well over 2000rpm the idiot light shuts off and the gauge sits just below 12.
I'd suspect bad connections cables. On these 25+ year old cars, the wiring is probably corroded under the insulation. May want to go over your grounds and battery cables and connections and replace anything that looks iffy.
Charging is pretty simple on these cars. If the alt and battery are fine, it's prob the wiring
You probably have already read this and have shied away because it looked too difficult or complicated. The question is: how bad do you want to fix the problem? How much money are you willing to throw at the problem to fix it and not succeed?
Voltage drop testing of connections and grounds.
Use a Digital Volt Meter (DVM) to measure the voltage drop across a connection or wire. Adding length to the test leads may be required, and does not affect the accuracy of the test. Use 16-18 gauge wire for the test leads if you have to lengthen them.
Voltage drop testing must be done while the usual load is on the circuit. If it is a starter, it has to be tested while cranking the starter. If it is lights, A/C or fan, they must be turned on high while testing. Fail to do this and you will not get accurate results
Voltage drop increases with the increase of current in a circuit and it also increases with heat. Put a maximum current load on a bad wire or connection and it gets hot and drops more voltage across the wire or connection. As it heats up, resistance increases which makes more heat. Round and round you go in a vicious circle until something catches fire or fails.
1.) Most grounds use the negative battery post as their starting point. Keep this in mind when checking grounds.
2.) The voltage will be small if the ground is good: less voltage drop = better connection.
3.) Be sure that the power to the circuit is on, and the circuit is being used in its normal manner. For instance, if it is a light circuit, the lights on that circuit should be powered on.
4.) To measure grounds, place one DVM lead on the battery negative post and the other on the wire or connector that goes to ground.
5.) 5.) Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections (sensors are low voltage devices and small drops can have a large effect on the devices dependent on sensor accuracy)
A voltage drop lower that spec is always acceptable.
6.) See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .
Replaced the battery first, didn't do it. Replaced the very aged pigtail, didn't do it. Went and got a new alternator (which of course came with another new pigtail...), running and charging fine - on the open road and around town getting good and hot under the hood - for a couple days now.
Thanks for all the info!