Voltage problems with a 1995 Buick LeSabre

My 1995 Buick LeSabre will not hold a constant voltage. I put a volt meter on the alternator and find that it fluxuates between 12 and 12 volts. I have put another used alternator in it but still have the problem. I have cleaned the terminals and grounds and even the fuses. As I drive along my lights will dim and the check engine light comes on as the voltage drops, but then later the problem disappears. My mechanic and I can’t seem to find the problem. Any ideas.

The alternator output voltage should be between 13.25 and 14.8 volts while the engine is running around 2,000 RPM and have a good load on the system. I assume the voltage regulator is built into the alternator for your car. If it isn’t then that needs to be checked out.

There may be a problem with the field lead on the back of the alternator and ties to the warning light in the dash. You should see close to the battery voltage on that lead.