24 volts on 12 volt ignition switched post in PDP

[dwlindsey]

In my M1088 A1 I've had the Power Distribution Panel up to get 12 volt ignition switched and 12 volt unswitched out to two new fuse panels for accessories.

I've also had the instrument panel on the dash off to paint it. All the gauges and controls came off for painting.

I just put it all back together and I now have 24 volts, where 12 volts should be. The 12 volt ignition switched post has 24 volts ignition switched on it and the dash gauges have 24 volts on them.

I just pulled the PDP up and looked at it the bottom of it where I added wires, the only thing I did to the 12 volt ignition switched post was to add a wire to take 12 volts off to a fuse panel That wire goes first to a breaker, which is now off, so it's absolutely in original condition, electrically.

When I took the instrument panel off, I labeled everything and plugged it all back in according to the labels. I did replace the lever light switch with the push button one.

I really can't sort out why I now have 24 volts where 12 volts should be. I don't see what I might have done to cause that.

I would REALLY appreciate any hints as to where to look. I'm baffled

 

[GeneralDisorder]

Start tracing back in the schematic and on the truck from the point you have 24v and shouldn't and test/check till you uncover what you did.

You are in uncharted waters for us - we don't know what you did or didn't do or where you veered off course into the rhubarb.

This is HOW you learn electrical BTW - screwing up and lighting things on fire. As pops always said - not gonna figure out how to do it right till I figure out how to do it wrong.

Pick a good ground for your meter, and start point-testing each connection in the circuit based on the schematic till you see it go from 24v at the last test to 12v at the current test point and then start tracing backward and forward till you find where it's connected wrong.

 

[dwlindsey]

I didn't do anything between the batteries and the PDP, so I really can't imagine what caused this

Is there anything 24 volt behind the instrument panel that could be back feeding 24 volts to the 12 volt post in the PDP?

 

[GeneralDisorder]

I don't think so.... I mean there's diodes on the circuit board style PDP that could potentially back feed if they failed shorted. Those panels can and do have problems with failed diodes and cracked solder joints. Horrible design using a PCB for that thing in this application. Absolutely the worst of the three generations of PDP's used on the FMTV. I've had to repair them for causing no-crank issues from failed/cracked components.

 

[Ronmar]

Ok, the LMTV uses 24v gauges with 12v lighting… Look in the schematic, and identify the gauge power fuse. It is sourced from 24v switched ignition power…