Alternator/Battery question

[slfjas]

Background:
03 7.3 PSD
Q4x4
EB 350
about 38,000 miles on the van

Last summer, I had both starting batteries replaced. One night, while camping, I discovered they weren't charging, so we coasted back down towards town, and, after a long tow and a delay in Durango, discovered that they were somewhat poorly hooked up. However, the alternator was also dead. Short story: bolts all tightened, and alternator replaced (standard alternator, not heavy duty, billed as a rebuilt).

All's been well 'til last week, on our most recent trip. The battery light on the dash comes on at any voltage output below about 12.5 - 12.7 volts. Once I rev the engine enough to bring up the volts, the light goes off. There are plenty of times in normal driving that the voltage output falls below 12.5, and the light will come on. As soon as the engine is turning faster, the light goes off.

Is this a problem? Is the dash "idiot light" set too high? Should the alternator be putting out more volts? Or is this a battery problem? The alternator should be under warranty, as are the starting batteries.

By the way, it never seems as though the batteries run down- so far the van has always started OK.

Any help appreciated!

John
'03 7.3 PSD
Salt Lake City

[daveb]

While the dash light is going on and off, does the light stay on if you turn on the headlights?

The alternator should put out a minimum of 12.8 and should normally run between 13.3 and 14.2
You might be able put a battery charger that has a gauge on the batteries and see if it ever tappers off, just make sure the separator is open and disconnected from the chassis system. If it charger doesn't tapper off you could have a short somewhere, most likely one of the batteries. Actually it sounds like an alternator problem but I'd check each battery first before looking at the alternator just to make sure. Many electrical shops can test the alternator W/O pulling it. I would avoid dealerships unless you're under warrantee. One of our spare trucks does the same thing until it's warmed up. It starts so the shop won't do anything about it. Make sure you tell us what you find...good luck.

[John and Dana]

John,

You can pick up a cheap digital volt meter that plugs into the cigarette lighter at most auto parts stores for less than $10. It will give you a better idea of what is happening with the voltage.

When your batteries are fully charged it should read around 12.8 volts.

Turn on your head lights and see what happens to the batteries under load. They may drop a few 10ths of a volt but it should stay pretty constant.

When the engine is running your voltage should be between 13 & 14 volts like Dave said.

Good Luck!
John