So I'm kinda stuck on this one. My girlfriends Blazer is not charging. The alternator tests good but the gauge says 12v or less the longer it runs. I've verified this with a multimeter. Also, my tachometer isn't working and the speedometer is pegged at max. I just replaced the fuel pump (with the battery disconnected, of course), but I'm not sure that would cause this problem. I've been doing a lot of work on this recently, new plugs, wires, battery, cap & rotor, but this is the first time I've had the electrical problem. I'd appreciate any help you guys can give me.

 

How did you test the alt, and did you check the voltage at the battery with the DMM?

 

I pulled the alternator and had it tested at the auto parts store, Autozone to be specific, and they said it tested good.

 

The machines in the auto parts stores look impressive, but the results can rarely be trusted. Starting in 1998, the alternator and voltage regulator are controlled by the PCM using a duty cycle. Not sure if the machine can't duplicate it during the tests, or the machine operator is not properly trained. They are in the business of selling parts, easy to do if "the machine" says it's bad. Best to avoid the situation.


The 10ga red wire on the rear of the alternator must show battery voltage at all times. The 16ga red wire in the electrical connector on the side of the alternator must show battery voltage while the engine is running. Ideally it should be 14.2 volts. If you apply battery voltage to the small red wire, the alternator must charge. If it does, the alternator is fine, and the problem lies elsewhere.

 

I could not have said it better. Thanks cap!

 

Figured it out. The fusible link underneath the battery platform had burned completely through. Fairly easy fix. The tach and speedometer were fixed by replacing the fuse that was labeled for the radio, kind of a head scratcher, but whatever, it solved the problem. Thanks for the help guys.

 

So, what if the voltage on the red (16g) wire is 9.75 VDC?