Is there a way to know if the pcm/voltage reg is working? My volts at startup are 12.42 and will raise slowly but not reach 13v within 2min-I shut down the truck because the alt gets super hot and I don't want to fry it

I know if a reg goes bad the volts can over charge. But can a reg go bad and under charge?

Parts that are new-Alt, drivers batt.

What I have done--Isolated pass batt incase it was bad(holds a good charge). Cleaned both batt terminals, starter terminals and contactors. Ran a jumper wire from the neg batt post to the alt case to eliminate any ground issues. Removed heater grid power from batt. new drivers batt All of my gauges work, ac works, lights ect.

I am sure the alt is working as if I give it throttle, the volts will climb a bit faster. The truck itself runs great, fires right up. When this issue first started I had smelled burnt electrical--pcm regulator maybe?

 

Always check grounds first when dealing with electrical issues.

 

Doubt you smelled the PCM, if it got that hot other things would have fried.

From your description of the voltage, I assume you are not relying on the factory gauge, but an actual VOM?

Could be the PCM asking it to charge fully though, below is a pretty good write up on diagnosing Alternators written by one of our members here.

"I had to troubleshoot my charging system recently and came up with a fairly simple way to isolate the problem. Here is a step-by-step way.

First you will need a DVM to measure voltage. Also the batteries need enough charge to start the engine, so use a battery charger to get them charged enough.

The charging system is made up of the PCM mounted on the firewall, alternator, and don’t forget the 120 amp fuse in the PDC on the driver’s side fender well.

1) Start the engine and measure the battery voltage at one of the batteries. It should be more than 13 volts but probably less with a charging problem and that is why you are here.

Measure the voltage at the alternator output terminal (B+) leaving the DVM negative connected to the battery negative post. It the measurement is greater than 13 volts then the fuse in the PDC should be checked or the nut holding the heavy gage wire on the alternator is loose.

If you get this far then the next step is to measure the voltage coming from the PCM. The two small nuts on the back of the alternator are the ones to measure with the lower one positive and the upper one negative. These are the field terminals on the picture. If the voltage reading is greater than 8 volts the PCM is calling for full charge and your alternator is bad and not putting out. If the voltage is less than 3 volts your PCM is toast or the wiring between the PCM and alternator is broken. The normal voltage reading at the field terminals will vary from 3 to 6 volts when working correctly.

It’s always a good idea to clean battery terminals and check for bad grounds before spending money on replacement parts.

My alternator turned out to be bad and I had 9 volts at the field terminals. It was the diodes inside that were bad."

 

My 96 has, 3 times in the last 2 years, done a similar thing. Starts right up and the volts don't get up to the 14 or so as usual. I continued to drive and was bothered by it so I stopped, shut it down, and then checked the alternator which would sizzle spit. When I started it up again the volts were normal and when I got home the alternator was normal temp. The next time it did the low volt thing I just shut it off; counted to 10 potatoes then fired it up.........I don't know what's up but I bring some wire to wire up the start/run solenoid when it takes its final dive. Let us know what you find out, cd