Anyone know where the diagnostic plug is?

 

I think yours is diagnostic capable by turning the key for the ignition on and off 3 times, then on again to reveal engine light cadence

 

It is. I pulled the codes that way. Thanks.

 

what code or problem are you chasing?

 

Code 46. Low voltage. Replaced the alternator, ignition switch and relays.
Looking at the schematics now.

 

Bouncing voltage.

 

man, the pcm regulated charge systems can be testy, ground cables can be suspect, reading posts can be worse, try using a volt meter near the battery/alternator/fuse block checking anything close to 13 volts first with no or good load on accessories, looking for inconsistency

 

I believe it is under the hood, near the relays on driver's side fender area.

 

Could be the voltage regulator, which on the 91.5 & up is integral with the ECM (computer)

But grounds are always suspect and usually need to be upgraded on these and lots of other older trucks / cars. They were barely enough to work when new LOL

 

I tried an external Mopar regulator (without using a relay) and it bounced just as bad if not worse. PCM power, grounding issue, don't know.

 

Did you disconnect it from the computer?

Perhaps follow this?
https://www.dieseltruckresource.com/...n&onlynewfaq=1

 

Yep, at least on the gassers it is, rubber cover to protect the terminals.