i swapped a GC alternator into my 88 xj with an external VR and everything was fine. one day it was charging 14v as usual but as i drove, the voltage went up to 18v. so i tried 2 new regulators and it does the same exact thing. but if i ground the VR, the voltage drops to 12v and stays there. all connections are fine, as well as all the grounds. tested it with 2 new batteries, 3 new VR, new wires, terminals etc. i am at a loss right nw and no one seems to be able de assist me. i would greatly appreciate any help besides being told to clean grounds and check connections since that has been done with no results

 

Check your rectifier bridge. You could have a shorted diode. That's the unit inside the alternator. It'll have to be checked with a multimeter for one way continuity. There are 3 pairs of diodes in the bridge.

 

weird thing is that i tested two alternators at work and they supposedly passed

 

An alternator may be different, but it's usually a good idea to test any diode out of circuit so there's no chance of a false positive.

 

There are two ways you can check the output current of an alternator. If you have a GOOD multimeter, you can check for the presence of AC in the DC output, or put the output on an oscilloscope and view the wave form.

The only other way I know is to take the rectifier out, unsolder all the connections, and test each one individually for one-way continuity.

If you still have a system that uses a separate voltage regulator, you may be able to still get separate parts for the alternator. I myself have rebuilt many GM alternators in the 60s and 70s. These were very simple systems because all the electrics were solid state and internal to the alternator. The maximum output amperage was determined by the size of the armature and field windings. I've used diode bridge rectifiers for 40 amp forklifts in 75 amp cars. The 90 amp uses larger brushes.

The rebuilding of parts today is usually left to companies that do that specifically. A lot of companies don't bother anymore because you can get a new Chinese part for less than it costs to rebuild one. This is part of our "disposable society". Don't EVEN get me started on that!