12 Volt system problem

[EndeavourNiche]

I just got my first motorhome an old 1980 Roll A Long on an E350 Ford Chassis. It's old but very nice for it's age. Everything works on shore power and generator but in order to get the lights to wok on 12V battery power I have to first start the engine wait a few seconds and the lights will come on. Then I shut off the engine and the lights stay on for about 2 minutes and shut off. Batteries are fully charged. However when I test the voltage at the control center from the batteries there's nothing until I start the engine and wait a few seconds. I'm leaning towards a bad relay. I checked the contactor/switch in the control center and it closes as it should when connected to shore power or when the generator is running then it opens as it should to allow 12 volt power but there is none. Any ideas?

[Bordercollie]

In our old 79 Delta Class C, our first Class C, the previous owner and his wife relied on a "genius" son and others to keep the old beater functioning. There were numerous examples of cheap and dirty DIY fixes. Your mention of a positive cable routed accross a grounding block and shorting by rubbing through the insulation, blowing fuses, etc, sounds more like an DIY thing. Anyway, glad you dove in and were able to diagnose it before a fire started. I was lucky enough to find an auto/truck electrical technician with RV experience to untangle some of the "fixes" during our early RV'ing days. 30+years later, I'm still learning.

[rvten]

House battery disconnect switch on?
Ok I missed your post that you found the problem.

[Harveyj]

The worst case of poor routing I have ever seen was in a TT parked next to us a few years ago. The manufacturer had installed a generator in an outside bay and the fuel tank in the adjacent bay. The "rubber" fuel line was routed through a hole cut into the metal divider panel (without a grommet). The vibration from the generator caused the fuel line to rupture and gas flowed into the inside of the TT. The owners had to have it towed back to the dealer as they could no longer stay inside. It was only 1-1/2 years old.

[EndeavourNiche]

Ok I solved the problem. When I first got it and looked in the genny compartment I noticed 4 boxes with genny solenoids in them that looked new and 3 film canisters full of fuses. That should have set off my alarm but for some reason I just put it off to the previous owner was over prepared. Today I tested the solenoids in the boxes. 3 are bad but look brand new and 1 is a good new one. So then I knew there's a problem that's burning up solenoids and blowing fuses. I started at the front solenoid by the house battery and traced the cable back cutting all the ties holding it to the frame and dropping it to the ground. Under the control box the coach builder attached an aluminum block to the frame to connect all the grounds to. battery cable was run over top of the block and tied tightly to it. When I cut it loose I found that the cable had chafed against the block and was exposed to it. Apparently not a constant short but enough to cause problems. So I fixed the bad part of the cable, rerouted it and everything works as it should. Had I not found the short the potential for disaster was high. So yeah Bordercollie it was a problem due to age and incorrectly routing the cable in the first place. Thanks everyone.

It's not hard to imagine the havoc, damage, and cost to modern electronics that a stupid mistake like this by a coach builder could cause.

[EndeavourNiche]

I've been over thinking this problem. Obviously it can only be one of two things. Dead house battery or a faulty switch or relay. Since the battery is good I just have to find the switch or relay. So far I can't find it anywhere. That's where I could use the experience of someone who's owned or worked on this particular motorhome. Chances of that are slim to none though.

[EndeavourNiche]

Thanks for the help and suggestions everyone. @Bordercollie, My skill level with RVs is higher than the guys that work at my local RV dealer/service center. Everytime I go there I'm offered a job. Example, The last time I went there I took a circuit board from my furnace in my TT to them to use a circuit board tester to see if the board is bad. They tell me "We don't have a tester, if we think the board is bad we just plug in a new one". This kind of practice is why so many people test everything at home and it works fine then get where they're going and the furnace or whatever fails again. Then they take it back to who worked on it and they say the board is bad again must have been defective and install another new one. Which of course will work fine until you get away from home and start using it.

This is my first motorhome but I've lived in and worked on TTs and TCs for years. I've been living in them full time since 1992.

[Bordercollie]

Check tightness of allen head set screws at converter/charger output terminal block. Check for 13.6 volt DC output at the same terminals. Check for any blown fuses or open circuit breakers between converter/charger and house batteries. That control center may have some sort of house battery disconnect/connect button connected to a latching relay near the house batteries. You should hear it snap when you push the house battery disconnect/connect button. These gizmos go bad and stick open. You may need to get help from a local RV service.Old RV's often have tricky electrical problems caused by corroded or loose connections or aging parts.

[tenbear]

Have you monitored the house battery voltage during the entire cycle from before starting the engine to after the lights going out? Might provide a clue???

[ronfisherman]

When you installed new batteries. Could you have missed a ground or positive wire?

[EndeavourNiche]

Both batteries are new and fully charged. The light on the control center indicating that the battery is being charged does not light up as it should. I disconnected all battery cables and checked them with shore power connected. The cable shows 4.9 volts. So I have 12.89 volts at the battery and 0 volts at the control center battery cable connection. Then if I start the engine a few seconds later all the lights come on and stay on for about 2 minutes after I shut down the engine. Not a slow fade out. Immediately, like a switch was flipped. I can't trace the battery cable from the lug in the control panel. It goes up into the ceiling. There is no identical cable anywhere on either battery or solenoid. There's a cable to the generator and that's it. The cable to the generator comes of the solenoid on the starter battery. The other solenoid just has a positive cable to the battery, a neg to the battery, and I think only one small wire going across to the starter solenoid. There's also a small wire coming from somewhere that's spliced into the wire between the alternator and the regulator. I've traced it back to the chase used by the control panel wires and it goes up past the control panel into the ceiling.

[CodyClassB]

bad house battery ?