M939A2 electrical issues- grounding

We have a 939a2 that has had electrical issues I think. When the builder outfitted the truck, it was pre-wired for a radio. Fuses, main ground lug etc etc, we mounted the bracket installed the Motorola m1225 and went to hook up the antenna and it arch out. Well hmm did some checking and looking and found nothing, we installed a 24v to 12v regulator, it still popped but not near as bad, had no issue on the radio then. 3 years later the regulator is removed because of voltage drop while transmitting. Other issues are starting to happen with the light/siren controller box as well that are running straight off the battery. Last ditch effort before replacing the radio, I used a MM and checked voltages from the chassis and antenna to battery for 13volts.

Question is has anyone ever had a bad ground in the alternator, or missing ground straps that would cause strange voltages with the truck on/off or am I just seeing stuff.

Will be getting more info come Monday, Tuesday as Ill have more time to check stuff then. Any advice would be great!

Thanks
Ian

We don’t know how they wired it. Sounds to me they are pulling 12v from the wrong battery set. Need to use the ground side batteries in order for something like this to work. This means the batteries which have their “-“ going to the vehicle ground.

you could have a faulty generator if this all started when you fixed the regulator. these factory 60 amp alternators aren't the best and they tend to fail very easily and do strange stuff when they do.
Thank you
TMG