I have a 99 blazer that when it cold alternator put out 14.3 volts...after it warms up its putting out 12.8 to 13.3 volts. All cables are new and oversized wires. Problem is my cell phone doesn't charge unless i'm running at higher speeds. Is the alternator on the way out??? I'm thinking voltage should be steady 14+

Mike

 

It definitely should be around 14V. I would remove the alternator and have it tested someplace that they can apply a load to it to get it hot.

 

Swartlkk - Please correct me if I am wrong.....The field terminal is actually coming from the VCM or PCM on 98 blazers correct? If this is true, could the Control Module actually command the alternator off under certian conditions? Could flaky alternator operation actually be caused by the signal from the computer?

Appliance Guys problem could very well be caused by the diode tree overheating. (Especially if he has a failed diode.)

Second question, I cannot find through the search option the ECM bypass for the alternator. Could you link me to it or give me a good search query to use?

I apologize if I just hijhacked this thread, but I do believe it is a possablity.

Have a good one!

 

Yes, the PCM controls the field terminal on the alternator. It is possible that a flaky computer could cause the alternator not to charge, but isn't likely to cause long duration drops in system voltage.

I would suspect a problem in the alternator itself as you have stated.

Here's the thread with the most information on rewiring the alternator:
Voltage Gauge going nutty

Found using a search term of "alternator+rewire" with an author of "swartlkk".

 

strange...voltage not anyewere 14+ when hot. The amp gage is 2 notches in the neg driving around town. I might try an alt...mine has 160,000 miles on it.

 

I know my Blazer is a 96 with about 176,300+ miles and the alternator is runnin 14+ volts. If the wiring wasn't specifically made for the the type of Blazer you have, it could be causing the problem. I don't know, I'm just guessing, I don't have any electrical or mechanical training (part of the reason why I'm on here). Try flooring the gas a few times while you are still accelerating on the highway, that might straighten it out.

 

Appliancguy and I have different problems. Hopefully his is an easy fix which will probably be taken care of by replacing the alternator.

My problem is when engine RPM increases over 4000 rpm, my alternator stops charging. Lights dim, voltage meter drops ect. That was why I asked about the ECM bypass. The alternator tests fine.

 

It's most likely the wiring or computer. If you have a friend that's electrically or mechanically inclined you can ask him or her. Check to see if one of your pulleys is stuck or missing, or if your belt is stretched. I know I had a similar problem on my lawn-mower, but it was the blades instead of an alternator. "strange comparison"

 

I appreciate your input. I am an electronics tech for the federal aviation administration.

I am certian I dont have any pulleys missing. The problem only occurs when engine RPM is above 4000.

I am leaning towards a computer issue. I believe it is commanding the alternator off at that engine speed for some strange reason. It isnt 3900 rpm, or 4100 rpm it is exactly 4000 rpm. Weather conditions dont affect it or the load on the alternator.

Generally I can nail electrical issues pretty quick. I have sound troubleshooting skills but i am really scratching my head on this one.

 

What are you doin to put at that high of an RPM anyway? My '96 Blazer barely exceeds 2500 RPMs on a good day. If you are just drivin down the highway, you need to get your transmission checked, cause even at 75 mph 4000 RPMs is a little too much for any of my vehicles (unless you consider a friend-of-mine's old Ford with a bus transmission). And hey, maybe you can help me on an electrical problem I have with my Blazer, I made a new thread for it, cause it would be rude to change topics in the middle of a good conversation.