Hello,
I’m new to this site and a new owner of a 1998 blazer LT 4 dr 4WD. Anyways... I have a question regarding the alternator. Two days after I picked up the vehicle from the dealer the alternator went out. Understandable considering the vehicle has 97,000 miles. Okay so I replace with a New Napa alternator and a brand new premium Deka battery. Parts that ive known and trusted for years. (I didn’t bring it back to the dealer because I have issues with other people working on my car) Lol. After a hundred miles of driving, I notice one night that my headlights were dimming in and out, the battery light was on and the volt gauge hanging around 12. What’s weird is the alternator would randomly work and then cut out, what’s really weird is the alternator would actually pull the battery volts down when the engine would tach up during acceleration, then things seem to be fine during cruising speeds- around 14 volts. Logically I would think if something was wrong- lets say with the regulator, then charging would go UP with increased RPM not down as in this case. I’ve checked and rechecked all the cables, connections, etc ect. I would consider myself a good shade tree mechanic but this one got me stumped! What would cause volts to go down with increased RPMs? Would something in the ignition system cause this or a short of some sort? The engine runs perfectly fine. Please help! I go to school and work full time and don’t have much time to troubleshoot these kinds of things. Any help would be appreciated!

 

only thing I would suggest is check the battery.

 

Just a guess, but it sounds like a bad rectifier diode in the alternater.
Maybe take it back and get it checked......warranty

 

Sometimes these can be a cause of a bad belt ( I have seen odder things happen over little things )

Best also to use a volt gage and check the sorce ( directly at the battery )

Its very odd for a New (not rebuilt) alt to go out that fast, but not out of the question.

Idle surge can also be caused by a faultly fan clutch (unless you have an electric fan). Those are easy to test, just give the fan a spin while the truck is not running.
If stiff and does not spin freely (w/ no abnormal sqeeks) fine, if loose and spins freely bad.

 

I had alternators go bad in my Pont GA and the problem was the connector at the altenator from the 12v source or battery. I had to replace it and since it came off when replacing first alt. Then I drove around and reved the motor to 2.5K and the alt batt light went off and sure enough it took out the alt/rectifier. So i got a better connector and soldered it on and no problems since.