Electrical system blues
Mike Allen
Popular Mechanics
Dear Mike: The alternator on my Chevy pickup isn’t keeping the battery charged consistently.
My mechanic has replaced the voltage regulator twice, and now he wants to replace the alternator for a second time. He charges me $50 every time I come in to get it checked, before he even really does anything.
What’s going on? And is there any way for me to test these things myself?
A: I’m going to assume that you’ve checked all the wiring in the charging system, including the ground cable to the engine block, frame and the battery’s negative terminal. Poor connections can make even a brand-new alternator/voltage-regulator combo perform inadequately.
Check for voltage drops along both the positive and the negative legs of the circuit. You’ll need a simple, $15 multimeter for this.
Your GM vehicle uses a Delco alternator. On most Delco alternators you can simply insert a screwdriver into a D-shaped hole in the back of the case and ground out the voltage regulator. This is called full-fielding the alternator: It bypasses the voltage regulator and forces the alternator to put out 100 percent of its capacity. Take your voltmeter – or use the one on your instrument panel, if you have one – and monitor the alternator output while you do this.
In a minute or two, you should see the voltage rise steadily from 12.6 volts with the engine off to 15 to 16 volts with the engine idling. Pull the screwdriver out, and the voltage should return to 13.5 to 14.5 volts, depending on the state of battery charge and temperature. Don’t let it get above 16 volts or so – pull the screwdriver out before you fry something.
If the voltage doesn’t climb steadily, it’s a bad alternator. If the voltage does climb, but returns to less than 13 volts, it is probably a bad regulator.
Dear Mike: I own a 1993 Ford 1720 diesel tractor.
I have always replaced the battery with a unit from the dealer. The dealership would have me believe that its batteries are the only ones built to withstand the vibrations and demands of a diesel.
Is there any downside or any possibility of damage to the electrical system – a normal 12-volt – if I use a battery of equal or slightly higher cold-cranking amperage purchased from a Wal-Mart?
A: Short answer: no, not really.
The dealer is partly correct: Diesels and trucks tend to vibrate more and could potentially damage a battery by cracking the lead plates inside it. So I would look for a truck-rated battery, which will have a more robust interior construction.
Better yet, look for an AGM (absorbed glass mat) type of battery. This construction has the electrolyte soaked into an inorganic blotter between the plates. The plates are stuffed tightly into their individual cells in the battery case, and thus can’t move or break. Because the hydrogen that’s electrolyzed out of the electrolyte is captured and recombined with oxygen to make water, there’s never any need to add distilled water.
An AGM battery is somewhat more expensive than a battery from Wal-Mart, of course, but it should outlast even the truck battery the dealer wants to sell you. And it should be around the same price, because the truck dealer has a substantially higher markup on parts. Shop around.
Dear Mike: I have a 2002 Volkswagen Jetta. It has a 1.8-liter turbo engine and a manual transmission. When I accelerate, the rpm gauge will shoot up before the engine catches up. Why is this happening?
A: Your clutch is worn out. You’ll need to remove the transmission and replace the clutch disc, and probably also the pressure plate and maybe the flywheel. I’d also replace the rear main seal while I was in there.
Mike Allen is an editor for Popular Mechanics magazine.Questions or comments of general interest should be sent via e-mail to driveit@nytimes.com. We regret that questions cannot be answered individually. Distributed by The New York Times Syndicate.
