My friends 280CE developed an odd battery drain situation today. Backstory-It just when in for some work and the shop installed all new belts (Even though they weren't all needed) The car drove great for the next week or so but one morning the battery was dead. I knew that it was old and needed to be replaced anyways so we figured pop in a new one and all will be fine. Well the 280 was brought to Pep Boys for a quick install but for some odd reason after the new battery was put in the battery light would stay on and drain the battery. I went to look at the car a few minutes ago and noticed that the alternator belt was loose which would cause it to not charge. However the 'tech' that was there said that the alternator was pulling current in reverse (or something along those lines) and that it was causing the drain. I've never heard of anything like that happening before but who knows with electrics. The alternator is a re-manufactured unit and is only about a year old so I don't see why it would suddenly decide to act up because a new battery was put in. I'm guessing that the Pep Boys guys don't know what they're on about and did something to cause the drain to try and milk more money out of an unsuspecting customer. My friend will be creating an account tonight to give more details about what's going on. Any assistance will be most appreciated.

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1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

A bad diode or two in the alternator can cause the battery drain, I sure hope the "mechanic" didn't hook the battery up backwards for a second or two because the end result is a fried alternator such as you likely have.

The light in the cluster is what energizes the field, if a doesn't go out that means it's not generating any current, the diodes prevent current from bleeding out of the battery and to the alternator when the engine is stopped. They are a one way valve of sorts, but they don't like reverse polarity at all.

The key here is that the light won't go off since the battery was replaced, the only thing that would cause that would be reverse polarity.

The alternator has a large wire on the back that is always hot, even when the car is off. That is the conduit through which your battery is being drained via the bad diodes.

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"I was a dirty bird, Carol's not grungey - she's *****in" John Milner....American Graffiti

I knew something sounded REALLY fishy when I first heard about all of this. Why do I get the distinct feeling that they fried the alternator on purpose so they could get more money out of my friend? Wouldn't be the first time i've heard of people who don't care trying to take advantage of people whom they think don't know any better

This was so helpful, thanks for pushing me to join PeachParts.com Fulcrum525.