new battery won't hold a charge, what could be draining the power off? on 1990 GMC Vandura 1500

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We keep replacing the battery but none of them have held a charge. The auto shop says there's nothing wrong but we have to charge the battery if we want to use it. Sometimes we can be somewhere and the van has ran good up to that point but if we turn it off, it won't start again. I am not sure if it is 5.7L V8, or 5.0L V8.
(1) Answer
You have a draw that is draining battery and not allowing to crank engine. Sounds like a major draw to cause such rapid drop in voltage. Sounds like diode problems in alternator. Have another shop check it. Had a draw like this happen one time with friends vehicle and found coin in the cig lighter causing this.