Hi,
I see several posts on alternators but none deal with my issue. My problem is the alternator (not stock and unknown brand) in my 70 Cutlass causes my volt meter (mounted under dash) to spike all the way over (above 16 volts) when driving normally. At idle, it seems to function OK and reads about 14.2 volts. Someone told me it could be a sign of faulty diodes. Can anyone help diagnose the issue? I'm inclinded to replace it with a rebuilt Delco but if it is not the alternator, I don't want to waste the money. The other question, in the event I need to replace it, is the 4 wires. Two go to the Battery terminal on the back of the alternator and then the two smaller "jump" wires connecting into the side to "excite" the alternator into charge mode. Can I buy a 3 wire and just hook both of the 10 Gauge wires to the battery connnection as it appears now? Is there such a thing as a 4 wire alternator? Thanks so much for your help.

 

First, there are internal and external regulator alternators. Since you say this is not original, I'm guessing that it's an internal regulator style. The most common are the 10SI and 12SI Delco alternators. These are three wire designs, were there is a single threaded post for the heavy gauge +12V wire, and a two wire plug, usually with a brown and a red wire. The fact that you may have more than one wire going to the threaded post does not make it a "four wire" alternator. The number of "wires" actually refers to the number of connection points, of which there are 3.

The fluctuation of your voltage is due to the voltage regulator, not the diodes.

 

Diodes usually either open up or short out (and stay that way), so I am thinking regulator also. Does yours have the regulator on the firewall? Is there a bypass harness on it?

 

The prior owner converted to electronic ignition. Other than a 6AL box, a coil and awindshield wiper motor, I don't see anything. I can't even find the horn relay and the aftermarket horns he put on there don't work so I have more investigating to do. Based on your comments, it sounds like I can fix the problem by replacing the alternator with one that has everything contained internally. I believe the one on there has everything internally. It's an aftermarket chrome unit. No label or numbers. Thank you. By the way, nice looking ride. I would love to put a picture of mine in my signature block but I don't know how to take a 680 kb sized photo and reduce it to a 19 kb photo. Do I need special software? Thanks again.