Well, here's one I haven't seen before.....

Truck is in my sig.

On a cold start, (and it HAS been COLD lately) alternator is not putting out. At all. Gen light doesn't come on immediately though..... I sit and let the engine settle down some, then, off to work I go. After driving for a little distance, the gen light comes on. Drive a bit further, and the volt gauge starts to slowly come up. Eventually, it will get to its 14.7 volt "normal" position. Gen light stays on. Stop at the sign, shut engine off, restart, volt gauge goes right up, gen light stays off.

The truck will charge normally afterwards, so long as I don't let it cool all the way down again. If I go to lunch, charging system works fine. If I skip lunch, and don't drive the truck till quittin' time, charging system once again freaks out. Quite consistent.

All related connections appear to be clean, and tight. PCM and alternator are original, so far as I know. (yeah, 21 years old, and 202K+ miles.)

My question becomes: Has anyone ever seen this before? I am REAL curious which part is actually failing.... The voltage regulator in the PCM, or the alternator itself.

 

On my 2001 2500 Cummins the gauge slowly creeps up to 14 volts on start up.Sometimes the headlights will change brightness.

This is the original alt. and seems to be normal after many years.

 

This is new behavior for mine.... Used to be, seconds after startup, volt gauge would be up where it is supposed to be. The slow creep just started recently, when the weather turned cold again......

 

To make a short story long...

Years ago, I had a Chevy Luv pickup that exhibited some odd behavior too. The alt light would come on, it there was a sweet spot in the RPM's that it go off and stay off. It turns out that I had a broken wire, right near the molex plug on the alt. When the engine was torquing, just the right amount, it would complete the connection.


If it's possible for you to do this, maybe remove the alt, let it sit outside over night, then get it tested in the morning at an auto parts store. Otherwise, I would check for loose fuses and relays in the engine bay fuse box. Maybe some contacts are separating in the extreme cold. Hell, maybe even a relay is bad.

 

See if ground cables just before the battery terminals get abnormally hot. Double check ground points. Do voltage drop tests on the rest of your battery cables.

 

Just for giggles, try giving the alternator a few good taps with a hammer right after you start it and see if it starts charging.
I had an alternator that would sometimes hang in the cold like you're describing. I found that the brushes were hanging slightly and a couple good taps would let them move to the armature. It lasted about three months like that until I got nervous and finally changed it.

 

I have a spare...... somewhere.... It's old, but, worked when I got it. Might just swap it out, and see what happens.