No 12v Power at PDP

[Dirtdiggler]

I had an earlier thread, but I've isolated, well, something. There is no 12v power at the PDP. This is a m1079a0 (1994). Has been sitting with dead batteries for 5 years. Has new batteries now.

I have a plan to jump the starter and see if that brings current then the alternator. I'll be testing tomorrow, but really interested to hear what others think might be the issue.

 

[GeneralDisorder]

Connections. Batteries and PPD and the connections and grounds for the cab.

 

[Ronmar]

12 and 24V run from batteries to the Polarity Protection Device 12 and 24v batt terminals between spare and air filter. The alternator is also connected to those same 12 and 24V Batt terminals on the polarity box. THE power cables to the cab PDP are connected to the 12 and 24v load terminals on the polarity box. Those connections are out in the weather and are a common point of failure. easiest to access it by lowering the spare tire and crawling in where the spare resides. one at a time strip and clean the 4 sets of connections(6 cables) and see what that gets you. While you are in there right below the polarity device is an instrument shunt(measures current) which has one large cable to battery ground and one large cable to starter motor case. this is your main ground so probably should clean those while you are in there...

The grounds are typically crappy on these trucks so strip all those to bare metal and re-assemble then paint over.
starter motor to drivers frame.
alt to pax frame.
cab to pax frame behind grill.
inside cab hump to pax dash to the right of and behind/under PDP.
pax dash to main ground test point inside/under PDP.

The A0 uses 12V to energize the 24v ignition relay(control power relay K2) so no or weak 12V to the cab is a showstopper...

 

[Dirtdiggler]

12 and 24V run from batteries to the Polarity Protection Device 12 and 24v batt terminals between spare and air filter.

So, this is not the case on this truck. The only 12v coming from the batteries goes to the comms box in the back. There is an 8 prong connecting between the cab and the back, but that went to something when it was in use, and is not connected.

I have pictures etc. But there is and never was since owning (8 years ago) 12v coming from the batteries forward. At least this is what I'm seeing, and yeah, it's weird and a big part of why I'm so confused.

 

[Ronmar]

Well then sadly, I cannot help you beyond how it is SUPPOSED to be wired and operate.

I would still look to the polarity box and see what is on the 12V batt and load terminals. was it modified to provide 12V from a 24-12 converter, that is not working or not turned on?. That is how I have modified my truck in prep for a 24V only alternator.

Does it have a dual volt alt? The dual volt alternator will not regulate properly unless it can see 12V from the middle of the 24V series wired battery as it is as much a battery balancer as it is a alternator. when you disconnect 12V from the alt the alt will de-rate/not run up to fll reg voltage.

and of course as i said originally, the A0 used 12V to turn on the 24V ignition relay. unless this has been changed, no 12V in the cab, the main ignition sw will do nothing...

 

[Dirtdiggler]

I cannot help you beyond how it is SUPPOSED to be wired and operate.

Oh no, this is insanely helpful! It does have a dual voltage alternator (which was replaced within an oil change of getting it). And as I said earlier, it also had a parasitic battery draw which wasn't an issue when it was being run routinely (and was unnoticed because of routine operation). AND I'm beginning to suspect that perhaps it's all related, and if not, at least getting it back to stock (or knowing why not to) is going to be very important!

So thank you so much and I will keep you posted - back at it tomorrow

 

[Dirtdiggler]

Again, thank you!