Dear Jim,

The battery keeps dying in my 2000 Plymouth Grand Voyager (3.0L, V6) whenever I drive with the headlights on.

My boyfriend says it must be the alternator, since the battery dies while the car is running. That's what they said at the repair shop, too, but now I've had two brand-new alternators installed and the problem's still there.

The mechanic did mention a bad voltage regulator, but he says replacing the alternator takes care of that. So what's the problem here, Jim? Got any ideas?

-- Lisa Moreno

I do, sister, but I can tell you this mechanic is on the right track at least.

Your dead battery does sound like the result of a bad voltage regulator (VR), which is the part responsible for setting the output voltage of the alternator.

In other words, the VR gauges your car's electrical needs and makes sure the alternator is cranking out enough power to get everything done.

Alternators and voltage regulators can both cause charging problems when they go bad. These are two separate units, but on most vehicles the voltage regulator is included as part of the alternator assembly. When you change out the alternator, you're usually killing two birds with one stone by replacing the voltage regulator along with it.

That's why your mechanic is confused -- it sounds as if you have a VR problem, but it's not being fixed with the new alternator.

Why? Because your 2000 Grand Voyager isn't like most vehicles.

As with other later-model Chryslers, yours doesn't incorporate the voltage regulator into the alternator. Instead, you'll find it in your car's engine control module (ECM) -- the car's computer.

It's not a bad location for the voltage regulator, as long as it's healthy. But when something goes wrong with the unit, suddenly the ECM isn't such a great place for it to be, because your vehicle design means replacing the whole ECM just to fix the broken VR inside.

Unfortunately, replacement ECMs run upwards of $500 -- more than you'd pay for just a new alternator.

So there you go, sister. Yes, the problem is with the voltage regulator, but it sounds as if you'll have to tell this guy (or maybe someone else) where to find it.

Tip of the Week:

Cruise control stopped working? Could be several things -- damaged wiring or a broken vacuum hose, for example -- but before you do anything else, consult your vehicle manual and check for a simple blown fuse. It could save you some bucks at the repair shop.

Buffalo Jim, owner of Allstate Auto & Marine Repair in Las Vegas, writes a weekly column. Send your automotive questions to him at buffalojim@lasvegasniftynickel.com. If Jim picks your question to answer in Nuts & Bolts, you'll also be the proud recipient of a deck of official Buffalo Jim playing cards, presented in a classy, tin box bearing his hairy image.