When You Are Hooked UP

Wiring hook-ups to your pickup camper and maybe even a camp trailer can keep things hot under-hood, even with under-hood batteries disconnected.

I went to swap out a glow plug relay today. Disconnected the batt negative cables first and then went to wrench off the nut holding the hot wire side off the glow plug relay. Surprised me when I contacted the engine block with the wrench and got sparks. Took me a minute or so to realize that somehow the hot wire on the glow plug relay was still being fed by my camper's battery. That's in spite of the fact that everything in the cab was dead.

I don't know how it was wired because the dealer wired the camper hook-up for me. The hook-up feeds the camper's battery and keeps it charged when I'm driving, but however the dealer hooked it up also keeps that glow plug relay hot. Once I disconnected the camper battery all went OK with the repair.

Yep. That's how that works. The only way around it, if wired correctly, is to isolate the RV harness from Batt+ with an isolator relay. I normally recommend this, unless you are particularly fond of dead starting batteries when someone leaves on one little lamp.

(normally) The GP Batt+ lead (large wire) is always hot (through a fusible link from the batteries). Only the relay coil lead is powered through the ignition switch.

Yep. I wired mine through a 40a relay that is triggered when the key is on the "run" position. That is from experience when I had a trailer with a bad battery and it drained the truck battery overnight.