Somewhere in and around 91.5, they went to the internal voltage regulator in the PCM. If yours isnt in te firewall, then its in the pcm. I would install an external and be done with it.

 

I think 92. My intercooled 91(.5) still has the box on the firewall.

 

Build date and model year are not necessarily the same. Manufacturers start building "next year's" model in August or September.

Look at the VIN to be sure, 10th character tells the year. A '91 would have an "M", a '92 has an "N".

 

How old is your drive belt?

It could be slipping, your alternator will charge at an idle but as soon as your RPM's increase the load on the belt will increase as the alternator puts out more current, especially if it hasn't been charging and the battery is low.

An easy to check is to start your truck and bring up the revs until the gauge starts to show discharge and hold it there for about 15 seconds then shut down your engine, now very carefully touch your belt at the alternator pulley and see if it is hot, if it is hot then it is slipping be careful it can be very hot and burn you.

It could be your belt is glazed or your tensioner could be weak and not keeping enough tension on the belt.

My last Chevy 6.2 diesel had this problem, I had a modified 120-amp Delco Si driven from the stock belt, you could watch the lights dim and the volts go down as the RPM's went up and the alternator pulley would get so hot that it turned blue.

To see how much power it takes to turn your alternator, full field it and try and turn the pulley by hand, it will be hard very hard to turn.

Depending on the amperage and the pulley ratio to can take somewhere around 10 horsepower to spin it at 120-amps.

Did your new alternator come with a new pulley or was it swapped out from the old one?

The easiest way to cure your charging problems is the way I did it, install a 160-amp Leece Neville in its place.

https://www.dieseltruckresource.com/...s-t247286.html

Jim

 

I just wanted to know that we figured out what the problem was or at least the dealer did after $400 in diagnostic charges.

It turns out that the crank shaft sensor that I put on at the beginning to solve the problem, but only seemed to help a little was the problem. They said I had purchased a faulty crank shaft sensor and it was giving crazy readings. They put a new one on and everything is back to normal. Thanks for your help.