bad altinator?

You can replace the regulator pretty cheap no need to buy a new or reman alt.

 

I hope you are using a Multimeter to check this and not the gauge in the dash?
For an alternator to charge the batterys It must produce at least 13.8 volt, at the batteries. Give the wire on the alt a tug also they have been known to come unsolder-ed and loose.
To test the alternator start it up, set the RPM to 1200, turn on all the lights and fans, and check the voltage if it is not above 13.8, test all the connections for voltage drop and if all is good and the voltage is low, the alternator is junk.
You may want to disconnect all the batteries and charge them independently with a regulated charger, then test them for voltage. A bad battery will draw a lot of power and cook the good batteries.

 

That's great that you have a spare to install. I would install the spare and note the difference in performance. Since you have two alternators, you can run one and refresh the other with Delco parts as needed. They aren't difficult to go through and will save you a boatload of cash. Normally, the failed component on an alternator is an easy fix once you get to know them. You can check them out with an ohmmeter and replace just the failed part which are readily available for cheap. There's really no need to be plunking down hundreds at the parts counter for a new or exchange unit when a single $40 part fails.

 

It's possible that the original alt. took a voltage hit that it couldn't handle at some point, from jump starting/welding etc. A new voltage regulator might make it good again.

 

The variable voltage thing can only be an alternator spinning variably. That can only be the belt slipping. Especially when you gas on it, it drops. Other problems will be a steady voltage whether high or low. The voltage ain't gonna jump around.

You check the tentioner by checking belt deflection on the longest run. I'm not sure of the specs on yours, but most is the belt shouldn't wiggle more than a 1/2" either direction on the longest run. You won't see signs of slipping unless left unrepaired for awhile. Another check you can do is put a ratchet on the tensioner release and it should take a good 100lbs pressure to move it. If it moves easily, the spring is weak.

Like a few said, it's easy to rebuild an alternator. They sell kits and you can save a ton of money.

 

In my 2006 387 pete I had the same symptoms it turnout that one of my batterie was dead I replaced the battery and everything returned the normal.I had replaced them exactly year ago .

 

Gunner 76 did the spare alt fix your problem I am having the same prob

 

It amazes me some of the questions asked by a so called owner operator. Especially on a generic truck like that that is super easy to work on. All of the time wasted on the computer couldve been spent popping off the alternator and running to to a Alternator shop andd had it fixed for probably 80 bucks. ..... I