94 s10 phantom problem

hi, its a 94 s10 blazer 4.3 cpi. when it idles... the volts drop to 12 volts, and the lights dim...the wipers go slow. but when i drive, its fine, and it reads 14 volts. now, if i idle and just tap the gas to get the rpms up to 1000, its fine. ive been a 12 volt technician for 9 yrs and have done everything i know. i tried a new battery, i put a hi-output alternator in, replaced the grounds, all the fusible links coming off the alternator, and still the same. as a matter of fact.... it seems it is getting worse. i hooked up my friends car to it with jumpercables , and still the same. for the 5 years ive had the truck, ive never had a voltage problem and its always been at 14 volts (this is an aftermarket gauge). i ordered an iac sensor from work today, in hopes that it fixes it. does anyone have any ideas? it seems every problem i have with this truck , no one has ever heard of it.

Provided engine grounds, body grounds and battery positive cables and connections are good, there are only two things that could cause the symptoms you are describing. Either a weak rectifier bridge in the alternator, meaning one or more diodes are open or shorted, or the voltage regulator has a switching problem. The voltage regulator is built into the alternator which means any way you look at it, the alternator needs to be removed and repaired or replaced.

What kind of volt meter are you using , Mine in the dash is very unreliable . I have an aftermarket installed and a digital that plugs into the cig lighter . My dash one reads what looks like ten when I really have 13.5 at idle . Rob

Sounds like the alternator is dying. Replace before it leaves you stranded.

ive redone the grounds. im using a autometer volt meter, i know the factory meter isnt accurate, that was the reason i got one. i also used my multimeter and it drops to like 12.08. originally i thought it was the alternator, so i replaced it with a 155 amp one, but still have the same problem. even if both the volt meters are inacurate, you can tell its losing voltage because the lights dim, the wipers go slow and the fan slows down. the problem actually started at the end of summer but it was just a faint difference. now its real bad.

After all of your diagnostics, repairs, and troubleshooting, one fact still remains: The symptoms are still present due to the systems inability to maintain voltage properly. You have two choices: #1 Take the vehicle to a shop that has equipment capable of testing the alternator/regulator output on an oscilloscope. Short of disassembling the alternator and checking each individual diode, the only way to accurately diagnose a bad diode is to watch the sine wave pattern on a scope. Then replace the alternator. Or #2 you can just replace the alternator with a good one and save yourself some time and money. This is not a "Phantom problem" that you're experiencing, you have a bad alternator.

Well im not going to screem altenator again i guess the guys didnt read all of all your posts... You replaced the alt, i understand that , but there are two things that come to mind right away...possible you got a bad alt from the store #1 and #2 could be the computer?

Just becuase you replaced it doesn't mean the replacement wasn't defective.

when the problem started getting bad, i replaced the alternator with one from GM. when that didnt fix the problem, i got a hi output alternator from my work. and it still hasnt fixed the problem. 2 bad alternators in a row? i know that is quite possible but it just doesnt seem right. oh, i forgot to put this in my original post....when i start it in the morning, it will be fine, and will stay fine until it heats up. if people are going to be sarcastic , then dont bother replying. just trust me when i say the alternator is not the problem.

No sarcasm here. Just trying not to dismiss anything before we know for sure.

Can you be sure that your battery is good?

There are two wires that connect to the alternator. The 16ga brown wire comes from the ignition switch, runs through the filament of the “Battery” light, to the voltage regulator. This circuit is “hot” any time the key is in the "RUN" position. This circuit energizes the voltage regulator which in turn excites the field in the alternator. Once the alternator has stator output, ground is removed from the brown wire in the voltage regulator and the “Battery” light goes off. The 10ga red wire goes through a fusible link directly to the battery positive terminal and is “hot” at all times. The alternator case is grounded to the engine via the mounting bolts. Ground cables between the engine and body supply the ground. Assuming each of these three circuits is functioning properly, (without excessive resistance) and the battery is good, insufficient alternator output or the inability to keep voltage constant, would indicate that the alternator/regulator needs to be serviced or replaced.

ok, just to be sure, i tried yet another alternator at work. but it didnt help any. when it didnt help, i hooked up a new battery to it, and it still didnt work. the grounds, and the fusible links are ok. what im tryin to figure out is how come when i first start the truck, all is fine. its when it warms up that the problem occurs.

I know this is an old thread, but I am new to this forum and I am having the very same problem with my 88 s10 2.8 ltr 4x4. I was wondering if you ever resolved this problem