Recently my batteries died in my F250 psd. I replaced the batteries, but noticed the guage only showed 11-12 volts. I brought it to an auto parts store that tests alternators in the vehicle, it shows 119 amps but only 11.5 to 12.5 volts. I am wondering if it is the alternator, or could it be the external voltage regulator?

 

This is a common problem...Alternators that don't put out the correct voltage...Start out at ~12.5 volts, then jump to around 13.8 to 14.5 volts after a few minutes of running...The alternators in the PSD have a two-stage output.

Your alternator is bad, and there is no external voltage regulator, it's built into the alternator.

I've had the alternator replaced twice in my '02 under warranty already.

 

Thanks. If there is no external voltage regulator, then what's that thing screwed to the back of the alternator?

 

Paarothead

My truck when it sits for a length of time summer or winter has that jump that you mention upon start-up. When it has run, shut down and started again it doesn't do it, reads normal. Is this the "two-stage output" or do I have a problem. It's been doing this for as long as I can remember and everything runs strong.

 

Your alternator seems to be fine.

The way it works is...
When the engine is *cold* and you first start it, the PSD alternator puts out a "low" charge, maybe 12.5 volts for a minute or three. Then it kicks up to the normal charge rate of 13.8 to 14.5 volts, where it stays. If you start the engine when it's warm, the second, higher charge rate starts immediately.

This is designed in, I think it's to protect the glow plugs and other electronics from burning out from a voltage surge since they're still operating when you cold-start the engine. If the engine is warm, then the "two-step" charging doesn't happening, the alternator automatically goes paddle to the metal right away.

The problem is that the late model alternators seem to be less reliable than desired. Like I previously wrote, I've had two fail on me in my truck's first 30,000-odd miles. The first time was at maybe 6,000 miles, it wouildn't go more than 12.5 volts and the batteries eventually ran down, then no starting. No red idiot lighht, either. The second time was similar, low output, except the alternator ran real hot and I could see some of the windings inside looked discolored from heat.

I don't know if the black module on the back of the alternator is the voltage regulator or not..If so it could possibly be replaced by itself, but someone better be sure that's what the problem is, not something internal to the alternator.

 

It seemed fine to me, has more than enough power for the trailer lights ,headlights, radio and amplifier all cranking simultaniously.
Was probably a stupid question but thanks anyway.