MotorHome AC power question.

[DickWells]

Got a retired couple, here at Hidden Valley TP in Deming, NM with a problem. Have to run the main engine in order to get AC power. It's a big 2 year old Tour Master with Mercedes power (Pusher Diesel), and an Onan diesel gen-set. I found 120 V on both legs, coming to the main breaker, and 120 V on each of the single pole breakers on the same buss. The breaker works. Shut it off, and the buss is dead. The inverter won't work on the Onan. Nothing works on any of the 15 or 20 amp circuits. Everything works as soon as you start the Mercedes! No wiring diagram to be found. Can't find any tripped GFI's. Something tripped back by the engine, that is supposed to control cross-feed between the gen-set and the AC IN? I didn't get into the engine compartment. No confidence in finding anything, because I'm just over my head with this one. Beats me, how something is keeping power from those circuits, when, with the breaker panel pulled out, I find 120 V on each wire to them, and they're marked in black ink. Makes no sense to me, but I've never been involved with a complete system like this, before.
Anybody got any ideas?
Thanks, Dick Wells

[JFerg65]

This one is also out of my league, but it sounds like you are on the right track. It is acting like an automatic isolator switch on a house with back up generator power. Sounds like something is keeping the system isolated to the engine / rig power only.

That is if I read your post correctly. There has got to be an automatic transfer switch or manual over ride somewhere.

Good luck on this one. I'm sure someone will come riding into the rescue here....they always do!

[DmaxMaverick]

A couple possibilities here.....

If it has a standby inverter (battery/alternator powered AC), it will have 2 isolators. One for the batteries, and the other for power switching (prevents different AC sources from operating at the same time). One of the isolators is likely not switching. Usually, just turning off the standby inverter will resolve the issue.

If the shore power ground is plugged into anything that's either earth ground, or chassis ground, the generator isolator won't switch. GFCI issues are very common on the coaches, and can backfeed the ground to the isolator.

The coach should have a power distribution isolator/switcher, which prevents any of the 3 possible AC sources from working at the same time. This is often the problem with AC issues. It may have a manual switch for selecting any of the sources. Some of the later electronic switchers don't have a manual selector, but should have trouble indicators.

Good luck. RV shops just love these problems. You should have a time with it. You might try posting this (or I could move it) in the towing forum. That's where the RV techs mostly hang out.

[DickWells]

Thanks, Fellas: I'll copy this, and carry it over to the rig, and see what I can come up with. Yesterday, I saw something like 200 mini fuses in the DC system. A nightmare, that has shades of an airliner dash. Still don't understand how the AC could be interupted (beyond) the individual #12 and #14 fused lines to the various AC circuits, but I never claimed to be an electrician. Just a dubber/sometimes DIY handyman/wannabe.
I'll keep in touch.
Dick:)

[DickWells]

DMAX: Please do move the post. I set out to post it on that Forum to start with, but changed my mind.
Thanks, Dick.

[rustyk]

DM is on the right track. The usual heirarchy for 120VAC is:

1st: Genset
2nd: Shore power
3rd: Inverter

(Sometimes 1st and 2nd are reversed)

The isolators or transfer relays stage this order. It sounds as though the inverter is the only thing supplying 120VAC. This points to the isolator/relay for the generator and/or the shore power.

There should be NO 120VAC breakers in the engine compartment (except possibly one for the block heater - but that's a variation from code, and unlikely).

As DM mentioned, the first test is to shut off the inverter, but I don't think this will change the symptoms. One of the functions of an inverter in a contemporary coach is to switch over to being a converter (120VAC to 12VDC) to operate the 12V circuits and to charge the house battery (most don't charge the engine battery). It's possible the switching circuit is not operating properly; more likely is that the inverter isn't getting 120VAC. (I assume this is what you mean by "the inverter won't work on the Onan"; does it work on shore power?)

Check house battery voltage (engine off); inverters will shut down if battery voltage gets below a preset level. This would explain why the inverter powers up only when the engine is running. If battery voltage is low, it again points to one the 120VAC isolator relays, as the inverter isn't charging the battery from 120VAC.

The 12V (battery) isolator is not a likely suspect here.

[DickWells]

Thanks for all the replies. These folks left town, this morning, armed with printouts of your posts to (maybe) help whatever tech they get with, down the road. They're good folks, and I'm sure they will get back to me with the final solution. We can communicate via E-mail.
I couldn't come up with anything concrete, since I didn't want to keep getting behind their panels, etc. That last post about low house batteries sure did hit a nerve, though. I'd bet the problem is in that area. However, I'll never be able to understand why I had 120 V on both legs (through) the Main breaker, to the dist. buss at the breaker panel in the kitchen????
This was true, with no gen-set running, and with the inverter turned off! How could there be 120 V available through the #14 and #12 wires off that buss, and not have voltage to the various outlets? Makes no sense, since it was obvious (to me, anyway) that those six or eight circuits were hot, by my digital VOM, (BEHIND) the panel! How could those circuits be isolated beyond the dist. panel, when to my way of thinking, any isolation should take place before the dist. panel, from the 3 possible sources of AC?
Anyway, thanks again for your replies.
Dick Wells:p

[rustyk]

You're right - source selection/isolation for the 120VAC should be before the circuits you tested. This is definitely an odd set of circumstances. I take it shore power was connected?

The only similar one I've encountered was with a fellow Barth owner at a gathering, and the problem was that they'd had a new 50A shore power cable installed.

The "tech" had tightened only one connection inside the plug (the receipt stated it was a "factory installed" plug on the cable). After testing all the internal circuits (all good), I tested it and it tested fine. But I had to push the plug down onto the cable to get readings, doing it solo (needed three hands). Only when another owner showed up, and held the plug, did the pressure relax and the gaps open up. It showed 120 VAC, but nothing would power up because the barely-touching wires in the plug wouldn't carry the current.