1990 Other GMC Models alternator charging problems

Thanks for the reply. Yes I did have the battery load tested.

This morning I cleaned off the negative engine ground. I was hoping that would solve the problem as it was soaked with oil. It didn't.

The truck was reading almost 12V before starting. It dropped to 11V after I got it running. I turned the lights on and didn't see the voltmeter surge up to indicate the alternator was kicking in. After running with all the headlights and fan on it dropped to about 10V

The new alternator tested on its wire about 12" from the alternator terminal. The negative test lead was grounded on the engine. That reading is still 12V.
Both the alternator post and the ring terminal on the alternator wire are brand new.

In other words, it looks like the alternator is not getting its signal to start charging.

It is charging! Thanks for your input. I don't think it was the Check engine light. Here is my best guess as to how this happened.

A belt was squealing for weeks and weeks. I was to preoccupied with other things to deal with it. The belt went at some point.

Then the transmission wouldn't shift out of second until I was over 43 MPH. I did not put two and two together.

I took it to a big truck mechanic who wanted $2600 for a new transmisssion. (Even after I told them that the belt was off).

Thank god for good friends. I took the truck out to a buddy and he said the belt's off man, your not getting enough juice to the electronic transmission.

One new belt was $12 and the truck started shifting correctly again.

Then the battery got weaker and weaker. Still I didn't connect the problem to the broken alternator belt.

My guess is that the battery got severely depleted. I know it did because I had it tested at a battery store.

The guy there told me an alternator doesn't really charge a battery but maintains the voltage of the battery and provides electricity to the vehicle.

I think that after the alternator lost it's power because of the belt. The battery went dead and as a result the alternator wasn't getting its intiial kick from the battery it needs to start making its own electricity.