For those just joining us, original problem, voltage gauge on the dash of a 1993 K2500 (4×4 3/4ton) Suburban showed a solidly low voltage, around 10V when running while both battery posts on the battery isolator showed a good 12V with the engine off and 14V with the engine running, as well as the alternator post.

The isolator and second electrical system are additions that I added for touring, mostly for running the electric cooler over long periods without running down the engine battery.

For those involved in the general debugging conversations:

I moved both battery systems and the alternator to the same post on the isolator to rule it out, no change.

NAPA didn’t sell alternator diodes alone and a voltage rectifier rebuild/repair kit was nearly the cost of a new alternator, so I saved myself the trouble and went with the latter. This unfortunately didn’t solve the problem. I should pull it and return it, but I don’t think the time is worth it. I debating keeping the old one as a spare and eating the core charge.

I took some jumper cables from the ground post on the engine block to the body inside the engine compartment. The voltage on the dash returned to near normal and I saw the newly installed ammeter show a reading, obviously charging the batteries. Bad ground.

I went for the low hanging fruit first and pulled the battery ground attached to the negative engine battery terminal that goes right to the passenger side fender. I took a steel brush to the fender, and the contacts, tightened it up good, and retightened the battery posts for good measure.

Problem solved.

I should add another ground strap, I thought I had one, but I’m not going to put more effort into this project unless the symptoms return.