This car came with a mechanical voltage regulator and the alternator uses only one field wire. The second field brush is grounded on the back of the unit. Pitted contacts on the relay inside the regulator could cause them to weld themselves together causing a full-charge condition. If you placed a test light on the small field terminal on the alternator, it will be full brightness. Normal is considerably less than full brightness. Don't measure this with a digital voltmeter because it can't smooth out the pulsing / rapidly changing voltage to display it properly.
A similar problem will occur if the regulator is not properly bolted to the body. It compares system voltage to ground to determine when to cut back on the charging rate. By not being properly grounded, it has nothing to compare system voltage to, and it can't operate the relay that reduces the charging rate.
Your ammeter gauge is fine. All alternator current goes through it on its way to the battery. The gauge is nothing more than a brass strip with two bolts attached and a magnetic pointer. It is possible for the high current to burn a ring around one of the terminals causing an open connection. This would cause a no charge condition at first, but then no current could get through to run the ignition switch and starter relay, so everything would be dead, just like having a dead battery. Since they did have a few problems on occasion with the mechanical regulators, some people rewired the engine compartment to use the 1970 and newer alternator with electronic regulator. This was an extremely reliable system.
Is it possible someone did that to your car? You would have a regulator module bolted to the firewall and the connector would have two wires; a blue one and a green one. If so, you need that alternator with two insulated brush terminals. For a long time, these rebuilt alternators were shipped with a separate steel washer that you were supposed to use on 1969 and older cars. This steel washer had to be installed in place of one of the fiber washers which allowed newer alternators to be used on older cars, but you can never use older alternators on newer cars. The washer grounded one brush.
The problem was that the grounded brush still had its plug-in terminal so it was easy to mistakenly install the alternator in a car with the electronic regulator. Two different problems could occur depending on which way the two field wires were plugged in.
Every dark blue wire under the hood has 12 volts on it when the ignition switch is on, (ignition coil, electric choke heater, and alternator field on newer cars, and voltage regulator on all cars. If that blue wire was connected to the field terminal with the steel washer, you'd blow a fuse and the engine would not run. If the green field wire was connected to the brush with the steel washer, it would bypass the regulator and cause it to charge wide open. To tell if this is what's happening, unplug the green wire while it's charging. It should stop charging. If it does, the electronic regulator is shorted. If it still keeps charging, there's a steel washer under that brush terminal that must be replaced with a fiber one. You'll find a bunch of these at the local junkyard, (or any cadaver unit).
Friday, March 20th, 2009 AT 12:58 AM