1990 Other GMC Models alternator charging problems

Tiny
PTOBRIEN
  • MEMBER
  • 1990 GMC
  • V8
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
I noticed my 1990 6.2 diesel van had been slower turning over. Then two nights ago I tried to start it and the battery was dead. I had the battery tested. It was good. I charged the battery and the truck started. But it is only showing 11 volts and drops steadily while driving.

Just to be sure I replaced the alternator and replaced the side mount battery bolts. The one fuseable link I did find has continuity.

I am completely stumped.
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Wednesday, December 12th, 2007 AT 7:36 PM

4 Replies

Tiny
MHPAUTOS
  • EXPERT
Hi there, when testing the battery did you do a load test as this will show the battery condition under duress, a hydrometer reading is only the charge and not a full condition indicator. Check the alternator charge rate at the alternator and the condition of the terminals on the alternator harness.

Mark (mhpautos)
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Thursday, December 13th, 2007 AT 4:15 AM
Tiny
PTOBRIEN
  • MEMBER
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.
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Thursday, December 13th, 2007 AT 10:15 AM
Tiny
MHPAUTOS
  • EXPERT
Hi there,

Is the charge light coming on when the ignition is turned on. If not, check the globe next. Then check the alternator out put as some alternators require this globe(resistance) to start charging.

Mark (mhpautos)
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Thursday, December 13th, 2007 AT 3:49 PM
Tiny
PTOBRIEN
  • MEMBER
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.
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Saturday, December 15th, 2007 AT 4:15 PM

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