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So I got the rig back from the shop that was repairing my "lifetime warranty" roof - for the third time - and it didn't start up so well. The guy started the generator because, he said, my batteries were low, then started the coach, and I drove it to the storage lot about 10-15 miles away. No problem.

I went back after a couple days, and click, nothing. Dead. Doornail worthy dead. Started the generator, got some juice flowing, and some things came back but not the engine. I had low volt readings on the panel, the transmission shift light wouldn't come on, the odometer LCD flickered. No matter what, I couldn't get the engine to start; I even brought Mrs. Goofy to it so I could manually push in the starter solenoid while she turned the key. Still nothing.

Replaced the engine batteries with new ones. Nothing. Then, I notice there was a cable hanging down behind the battery compartment. Not one of the huge, heavy cables that comes in the compartment but a thinner cable, maybe 1/4" thick, like the kind you might buy at an auto parts store. Anyway, completely loose, just hanging in midair. Nice.

Luckily I had it marked with a black cable tie (black for ground, of course, most of the cables have red terminals on their own, but I have "blue" cable ties for the engine positives and "yellow" cable ties on the coach positives to keep all those straight.) Anyway, I hooked this dangling cable onto the engine ground, nothing. Oh wait, the mobile service guy comes and plays with the battery terminals for 45 minutes, alternately tightening and loosening them, then loosening them and tightening them, accomplishing nothing. Oh wait! The engine turns over!

"Turn it off" he says, let's make sure it's right. I dutifully shut the engine off, it won't start again. The panel voltage ranges from 0 to 5.7 to occasionally 12.4 or 13.6. The mobile service guy continues to dick around with the lugs on the batteries, I finally get frustrate and send him away. I don't need to have someone twist a pliers on the batteries at $100 an hour. I can do that myself for less.

Finally, I decide maybe "ground" isn't "ground" and I switch the dangling ground from the battery for the engine to the battery for the coach. Everything springs to life, the engine starts, everything works.

So my question: isn't "ground" "ground"? Is there more than one ground? And if so, why did it work once and never again, and why should this matter?

(I've taken the rig to Cummins for regular service and asked them to check the electrical, so maybe they'll have an answer. And all of those became necessary because where I'm parked in a storage lot it would be nigh onto impossible for a tow truck to get me out thanks to the uber-sharp turns I have to make. Solo I can make it with a couple inches to spare. With a tow truck on the front, no way. Somebody suggested the truck could "tow" me around using just a chain and once we got to an open area hook up correctly, but without the engine I wouldn't have brakes, would it?)

Anyway, are there TWO grounds on an RV? And, uh, how?
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I have had this trouble in a car. You must keep the chassis and the drive train grounds connected.

The ground from the engine, the one that returns the battery for starting and the ground from the computer must be at the same point. There is generally a wire that joins the two. It will run from the battery to a solid ground point on the chassis.

It is important to have this ground as many sensors, think oil pressure, only have one wire. The sensor is simply a pressure activated resistor. As the resistance changes, the voltage to ground changes. The computer senses this and reacts accordingly.

If the ground from the chassis to the engine is missing, then the engine is a floating ground and the computer will measure the voltage across the sensor plus what ever voltage drop there is between the floating ground of the engine and the chassis.

If the computer is sensing a bad reading caused by the floating ground, low oil pressure, low oil, etc, the computer will not let the engine start.

Cheers
Qazulight
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I'm thinking, why is this post even here?
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I'm thinking, why is this post even here?

Because it's about my RV? Because the board is called "RVing Fools?" Because that's what we do here on this board, talk about problems/travels with our RV's?"

You're new here, apparently.
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Bonding, we called it in Telecom, making sure all is a common ground within a ground plane. That ground plane was isolated from building ground, except at a single point, that prevented ground currents from other equipment, or via lighting from passing through delicate equipment causing disastrous results..
In a vehicle with dual battery systems, the engine may be insulated by engine mounts, is I think there should be a hard wired link from the engine block to the frame, which the 'house' system may use, or maybe only within it's framework, so they do need to be bonded together. A clamp-on Ammeter would show if there were leakage currents, but as long as everything is negative ground, I wouldn't expect any..

Ages ago, my brother added a CB radio in his salmon boat, a wooden boat... Then noticed he wasn't catching fish in close to the boat, only further out.. Hmm, turned out that radio was reversed polarity, caused a voltage on the boat and it's trolling lines, got rid of it, fishing results returned.. It had also caused his copper tubing feeding the onboard stove to corrode, electrolysis effects a lot of things in a saltwater environment..

Anyway, Grounding, bonding isa huge consideration in many areas..

They sell heavy braided cables for doing that bonding...
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