Problem is with my daily driver. 2004 Ford Explorer. 4.0 engine...specific problem is the battery light. With the engine at idle, the battery voltage reads 13.6 -14.00 (the BAT light is OFF). When the RPMs are increased the BAT light comes ON and the voltage at the battery drops a full volt (taking the reading from the battery posts).

This is after I replaced the alternator and belt...any ideas on what to check next?

James, I would normally suspect the alternator also but .... on a lot of newer vehicles, almost everything, including the charging circuit, is controlled by the powertrain computer. I cant say for sure on yours but it might save you money in the long run to pay a good shop to at least look at it and see if they can diagnose the problem. You have already probably wasted at least an hours worth of typical labor costs by throwing an alternator at it.

Thanks for the reply Bill. I had held off on the alternator swap, but the guy tonight said that the "diode pattern was bad" on the original alternator when he tested it so I went ahead and bought one...only to get met by that battery light as soon as I revved the engine...and to think that the wife offered to take it up to the shop this morning...:clonk:

James, Are these symptoms the same as the original alternator? We see them out of the box not functioning correctly.Best way is to have it tested with good equipment,like Bill said,it will take the guess work out. Steve

Steve...other than getting a little higher voltage with the new alternator, the symptoms are the same (higher revs cause the voltage to drop and the battery light to come on). I'll post back after I get it to the shop.

"Diode pattern is bad"??? What does that mean? Or is that the latest Autozone "speak"?

Okay...I bit the bullet and had the car checked out today at my local repair shop...the result was that the brand new alternator from AZ was bad...and guess what, it was another "bad diode". Anyway, I drove it up to the AZ this evening and swapped it out for another unit in their parking lot and everything seems to be working great. Another hassle to deal with, but at least this alternator is an easy one to replace...:)

glad you found the problem. There were 2 alts for that truck depending on production date. You may have gotten the wrong one first. charbilly- diode pattern - results of a test on the wave pattern of the alt. Done with oscilliscope or ripple voltage done with an analog meter. Some good alts will give a pattern similar to a bad alt. It depends on the design of the alt to know if it is correct or not and I doubt if the part store guys know the difference, so bad diode pattern becomes the saying of the day. By intimidating the customer into not wanting anyone to know what he doesn't know causes end of conversation. Next time ask what bad diode pattern is and you will find he doesn't know either. My old boss used to say "If you can't convince them confuse them". I have found that by giving the facts it causes confusion cuz electricity has some strange quirks.