This is my first post here on the forums so please bear with me. I just picked up a 1999 Blazer 4x4. Drove it about 30 miles to get it home from the person I bought it from. It was raining so we have the headlights and wipers running. ran fine all the way home. Once home took it for a drive around the neighborhood and parked it in the driveway. Sitting there for a couple of minutes looking over the inside of the truck the battery light comes on and looking at the volt gauge looks like it was only at around what would be 12 volts at most. Revved the truck up to 2,000rpm and no change at gauge.Turned the truck off and of course it would not turn over . It tried and then click click. So I'm thinking the alternator is toast. I jump the truck off my car and it turns over fine and starts. Removed jumpers and she stalled out in a few minutes again I assume alternator. I pick up a new one and replaced the old one. Jump the truck and she turns over and fires up. Volt gauge at this point shows a tick over 14volts..perfect. I let the truck run for a few minutes with my foot on it at 1,500 rpm or so. Again the whole time 14 volts on the gauge. I turn the truck off and restarted just fine but the volt gauge only shows about 12 volts...when it was just 14volt....hummm. Turned it off and restarted a couple of times letting it run for a few minutes above idle each time. And the volt gauge was slowly dropping the whole time till it stall out. I am thinking a bad alternator at this point.
So today I get out the multimeter and jump the truck and it starts fine. I check the battery while running and it shows 14.2volts...perfect! Gauge on dash shows just over 14 volts but that is close enough. I turn on the lights, heater, radio and still get 13.8 volts...perfect. I take it for a short spin and the whole time the gauge shows 14 v. I check again with multimeter and get 13.8 ...good still. I turn the truck off and read 12.8 volts at the battery ...perfect. I restart the truck and the gauge on dash now only shows 12volts at best. I check at the battery and only have 11.98 volts!!!! So it would seem the alternator is not making power. I turn everything off and recheck connections, look at grounds etc and all look good. Restart it and it ran for a few minutes before stalling out. Took the alternator up to AutoZone and it checked out good at 14.7 volts on their test stand.
So it seems it was fine when I first jumped it cold both days but once it ran for a brief time something is happening and it's not wanting to charge. I have looked and looked around the internet and can't seem to find anyone with this problem so I am hoping the experienced group here can point me in the right direction.

 

did you ever take the battery in to get it checked

 

Yes I had the battery checked and it came back ok. After reading the forums here I learned the PCM controls the alternator on the 99' Blazer. Going by what was stated in that thread I took an ohm reading on the PCM #3 connector at pin 5 and the small plug on the alternator end and it too checked out fine. At this point I am thinking the PCM itself is the problem. Other than this charging issue the truck is fine.

 

You might have a grounding problem or some sort wiring problem. Chasing wiring is never fun but work from the alternator and head back.

 

Save your money on the PCM, very doubtful that's the problem.

First you need to fully charge the battery before we can get any kind of accurate results from the tests.

There are two wires on the alternator. First is a larger gauge red wire on the back: It must show battery voltage at all times. Put a test light on the alternator stud, not the wire or terminal, and wiggle the wire around. Follow the red wire back as far as you can, make sure there's no blinkey action with the light.

The second wire is a smaller gauge red wire on the side of the alternator. It is in a 4 place connector, and it should be the only wire in the connector. Unplug the connector and start the engine. Check voltage on the small red wire in the connector and post your results.

 

Yes the large BAT wire is always showing the same voltage the battery. I have measured it at the stud, the nut and the wire connector itself so I feel safe to say I am good there.
I will check the voltage at the small side connector in the morning. I believe that is the "L" connection. I believe it should be 5 volts from what I was reading earlier tonight.

In the mean time I am trying to get my head around how to convert the truck (99' Blazer) from PWM to Battery Reference. I am just trying to figure out the new wiring configuration. I am assuming the red wire that goes into the new connector cavity "B" would be the 1 old wire from the original wire from the stock connector. But I can't figure out where cavity "C" wire will come from.

 

What you're trying to do is utilize the remote sense terminal on the voltage regulator. The alternator is equipped to operate like that, but from the factory they chose to use PWM from the PCM to control the regulator. Which is actually a very accurate way to control the regulator, but only in theory. What it doesn't take into consideration is the actual voltage that is being applied at each electrical component. In the GM TSB it instructs to connect the remote sense wire to the alternator output terminal. That takes care of the flickering lights, but it doesn't solve the underlying problem of insufficient system voltage. We really don't care what the voltage is at the output terminal, we need 14.2 volts where the accessories get their power! This is where the remote sense circuit comes into play. It needs to be connected as close as possible to the junction where most of the accessories get their power. On 2nd gen Blazers, that happens to be a large red wire connected to the underhood fuse panel, that's where it needs to sense the voltage. The regulator will keep voltage at 14.2 volts where ever the sense wire is connected. You should notice a voltage drop of ~1 volt from the alternator output terminal to the fuse panel connection when you're done. Doing "The Big 3" isn't a bad idea, but this mod will fix the root cause.

The original PWM wire at the regulator goes to the same cavity on the new connector. The remote sense wire is added, and will override the PWM. Pretty sure you're right, with cavity C.

Obviously the original problem of no charging needs to be taken care of. Quite simple to troubleshoot. Battery voltage to the output terminal, (which you do have) and PWM to the small red wire while running. If all of the battery cable connections are good, and you show PWM on the red wire, you got a bad alternator, (wouldn't be the first time that happened)

Keep us posted.

 

Ok so I took a reading at the single small wire connector with it disconnected from the alternation and it was reading 11.60v. Plugged it back in and with the truck running had 14.3 volts at the battery and at the BAT stud on the alternator. Turned the truck on and off a couple of times within the first couple of minutes of starting working on it. An it measured the same. I let the truck run for a good 10 minutes room and it was still reading 14.3v. Now with the truck good and warmed up I turned it off and restarted it and only have 12.3 volts at the battery and BAT terminal on the alternator, turned heater and lights all on and had the same reading. Clearly not charging. The BATTERY light is on at the dash. Disconnected the small 4 plug connector and it still was reading 11.6v.

 

The 11.6 and the 12.3 readings are battery voltage. The PCM is commanding the alternator to charge. There's something erratic going on in the alternator or regulator. This might be why the last owner took it off his vehicle, and the rebuilder didn't catch the problem. Bad alternator, try another one. The alternator testers in the auto parts stores are impressive looking, but they're worthless.

 

Is there a certain voltage that the small red wire from the PCM should be measuring?