Faulty battery or faulty charging system?

Dear Car Talk

Dear Car Talk | Nov 01, 2001

Dear Tom and Ray:

We have a '95 GMC Yukon. It has a slow electrical discharge that eventually causes the battery to go dead. Then my wife finds herself stranded. The battery has been replaced, so that's not the problem. The fuse box has been sequentially checked in order to isolate the leak. No leak has shown up. We have kids in college and can't handle a new car at this time. Can you suggest another approach to identifying the cause of this electrical leak? -- Rich

TOM: This is all your fault, Rich. I think you're living a lie. You don't have a current drain at all.

RAY: I agree. I think this is a classic case of the customer who thinks he knows too much. We have some customers who come in and tell us what's wrong with their cars. They don't tell us what the car is doing; they skip that step and just tell us what to replace. And a lot of times they're wrong.

TOM: You went to your mechanic, I presume, and asked him to figure out what's draining current from your battery. Only, you led him down the garden path, because we don't think anything is draining current from your battery.

RAY: If you had simply told him your battery was dying intermittently, a good mechanic would have considered three possibilities: a bad battery, a current drain or a faulty charging system. You've eliminated the battery by replacing it. You've eliminated the current drain (assuming it was checked correctly) because you checked and found no drain. And that leaves what I consider to be the most likely source of the problem: a faulty charging system.

TOM: Normally, the battery starts the car, and then the charging system takes over. The alternator (which is the key piece of the charging system) then provides all the electricity the car needs. It provides electricity to generate a spark for combustion, electricity to run all of your accessories (like your power windows, air conditioner and lights) and electricity to recharge the battery for your next start.

RAY: But if the charging system isn't working correctly, everything will take its electricity from the battery. The battery never gets fully recharged, and eventually it dies and leaves your wife stranded.

TOM: So here's what you do: Go back to your mechanic and apologize for lying to him and leading him astray. Tell him you'll always give him the symptoms in the future, and never try to solve the problem for him. Based on your description of an intermittent dead battery, he should test your charging system. And my guess is he'll find the problem there. Good luck, Rich.

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