LMTV no power to cab

[Rtra9631]

I have 27 volts going to the top terminal of the polarity protection device and have 9 volts coming out and going to cab, tested the x terminal on dash for 24 volts and only have 9 volts. Tested the 12 volts side and I have 14 volts going to and coming out of PPD. Tested the 12 volt terminal and have the 14 volts. Would this indicate the PPD is bad or a bad connection internally. Vehicle is hooked up to another vehicle via the slave cables.

 

[GeneralDisorder]

Just move the leads on the PPD output studs over to the matching input studs. That will take it out of the circuit so you can test. Then check connections and check the forward voltage drop of the diodes for each side. Should be about a 0.5v drop across each.

 

[Rtra9631]

Just move the leads on the PPD output studs over to the matching input studs. That will take it out of the circuit so you can test. Then check connections and check the forward voltage drop of the diodes for each side. Should be about a 0.5v drop across each.

Will try that, if it is the PPD can they be bypassed or are they necessary?

 

[Ronmar]

As long as you never make a mistake connecting batteries or jumpstarting, the PPD can remain bypassed. This could also be as simple as a bad connection at the PPD. Since it is out in the weather, it is not an uncommon occurrence...

 

[GeneralDisorder]

Will try that, if it is the PPD can they be bypassed or are they necessary?

They are necessary for preventing battery connection mistakes by poorly trained operators (not technicians - obviously we are expected to know MUCH better) - but if you are dealing with 88M's..... Battery change out is an operator level task just like changing a tire or a headlight bulb and the PPD is there so when they connect the batteries all kinds of wrong it doesn't smoke the diodes in the $3k alternator and all the electronics on the truck.

If you never mis-wire the batteries then it serves absolutely zero purpose other than to be another point of failure.