Electrical Problem, 90 Chevy 3/4 ton

Discussion in 'GM Electrical Tech' started by michaepr, Oct 26, 2011.

  1. michaepr

    michaepr New Member

    I wonder if any one can assist me in diagnosing this problem;
    I have a 90 Chevy long bed 3/4 ton truck .. Silverado.. Camper special factory equip with dual batteries and isolator and camper hook up
    5.7 Engine with AC.

    What is happening is I can be driving down the road, and for no apparent reason the Battery symbol light will come on and the gauge showing battery charge will drop to the red area... then if I stop and turn everything off.. lights, ac. heater etc.. and start the truck it will clear. Then it will run about 100 miles with no problems and suddenly come on again.

    I thought it might be the battery so I changed it and the cables etc as well for good measure. With a new 1000 CCA battery. The problem did not show up for about 475 miles.. then it came back. then it will go off.. and come back sometimes. I cannot get it to duplicate with any particular combination of accessories.. heater .. ac.. lights.... etc.. it seems to come back when it feels like it. Then it will clear.

    Its driving me nuts.. any help will be appreciated ..

  2. csltrains96

    csltrains96 Rockstar 100 Posts

    You've already replaced the battery, so my next suggestion would be to have your alternator tested. Not so much for any shorts, but for output capacity. If the alternator is not keeping up with your electric load, the battery will take the brunt of the load, and eventually power down. As soon as you pull over and turn all the electric devices off, you give the alternator a chance to catch up and recharge the battery, even if it is at a lower output capacity.

    Also, as many others on this forum will point out, check for any bad grounds that may be interfering with proper alternator function and battery charging.

    Good Luck!
  3. michaepr

    michaepr New Member

    Thanks for the suggestion.. I have ordered a new alternator.. napa did not have one of the amp rating that this truck needs.. so .. when It comes in we will see if it works..
  4. michaepr

    michaepr New Member

    Well finally got the alternator and installed it. So far so good. checked the voltages at 2000 rpm 14.2v and battery voltage prior to start was 12.8 v so it seems if all is well.
    However, I noticed with the alternator is packaged a warning it says 1988 to 1990 Chevrolet and GMC trucks, may experience an intermittent illumination of the charge indicator on the dash. This usually happens only when the wipers are in use. This is caused by negative voltage being emitted by the wiper motor.
    Solution GM has released a diode (91H4003) or diode connector kit (12085207) which needs to be installed in the brown wire at the alternator connector to prevent the charge indicator light from coming on.

    This is kind of similar to the problem I have been experiencing.. I did not notice if it was related to wipers though..

    Can ANYONE tell me if they have heard of this problem or fix offered by GM?? Dont want to have to buy another alternator.. they are a bit pricy.. !!
    Last edited: Nov 15, 2011

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