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Old 10-09-2020, 07:34 PM   #1
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THOR #15089
Air Conditioiner Runs on Shore Power but Short Circuits Generator

I have a 2018 Siesta 24ST, which was at Wakarusa June 15 for a warranty replacement of the roof air conditioning unit. Since then the a/c has worked normally on shore power; however, today when testing/exercising my generator, I ran the generator, then started the a/c as a test load. As soon as the a/c tried to kick on, the generator stalled and stopped, and the generator data screen displayed message "short circuit!". No breakers win the main breaker box tripped, and no breakers had to be reset to restart the generator. Repeated attempts resulted in the same result each time, but on shore power again, the a/c started and ran normally. The generator will run any other appliance on this or any other circuit; only the a/c causes the stall/stop.

I believe this was the first time since Wakarusa that we attempted to run the a/c on generator, so I'm not sure this is related to the installation or something else. I'm baffled, because it seems that, with the automatic transfer switch, exactly the same AC current would be reaching the breaker box, regardless of whether from shore power or generator. I'm looking for any ideas on what to check next. Thanks.

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Old 10-09-2020, 07:43 PM   #2
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I think that the shop in Wakarusa needs to be called...
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Old 10-09-2020, 07:48 PM   #3
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I'm thinking they swapped the hot and neutral on the new AC, but that's just a guess.
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Old 10-10-2020, 02:44 AM   #4
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I'll check that out, thanks!
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Old 10-11-2020, 09:35 PM   #5
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Need Advice from Electrical Expert!

Per 16ACE27's recommendation, I checked the power center to see if the new a/c's neutral and hot wires were reversed: They're not; but I tested some circuits with an ohmmeter, and came up with something I can't explain. If I tested between the neutral bus and each circuit breaker's black wire contact screw, every circuit EXCEPT the a/c circuit showed a reading of between .05 and .5 kohm. But the a/c circuit showed a reading of .5 mohm. Since my meager understanding is that since a mohm is 1000 times a kohm, it seems like the a/c circuit is 1000 times or more as "short circuited" as any other AC circuit. (a/c = air conditioner; AC = alternating current). Is this normal? Is this the reason that, when the a/c starts up on generator, the generator display states "short circuit!" and the generator shuts down? And if this is a short circuit, why doesn't a circuit breaker trip when the a/c runs on shore power?

I am really out of my depth here; any advice would be appreciated.
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Old 10-11-2020, 10:34 PM   #6
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Quote:
Originally Posted by steve2777 View Post
Per 16ACE27's recommendation, I checked the power center to see if the new a/c's neutral and hot wires were reversed: They're not; but I tested some circuits with an ohmmeter, and came up with something I can't explain. If I tested between the neutral bus and each circuit breaker's black wire contact screw, every circuit EXCEPT the a/c circuit showed a reading of between .05 and .5 kohm. But the a/c circuit showed a reading of .5 mohm. Since my meager understanding is that since a mohm is 1000 times a kohm, it seems like the a/c circuit is 1000 times or more as "short circuited" as any other AC circuit. (a/c = air conditioner; AC = alternating current). Is this normal? Is this the reason that, when the a/c starts up on generator, the generator display states "short circuit!" and the generator shuts down? And if this is a short circuit, why doesn't a circuit breaker trip when the a/c runs on shore power?

I am really out of my depth here; any advice would be appreciated.
Since I don't know what you really did I'll just throw out some standards:

Never measure resistance on a live circuit - all power has to be removed.
When measuring resistance insure there is only 1 path for the meter current to travel. This can be easily be done by lifting 1 lead to insure it is open.
0 ohms is a short. Shorts cause lots of current flow.
MegaOhms to infinity ohms is an open circuit (almost no current flow)

If you open the circuit breaker to the A/C and have the thermostat OFF (It's not trying to turn the A/C on) you should have megaohms to infinity resistance (your meter may show out of range or a flashing 1) between the hot (black lead) and both neutral and ground (2 separate measurements).

This would also be true for any other circuit if the load was disconnected.
For example - if you opened the breaker that supplied nothing but 120 VAC plug outlets, and had nothing plugged into any of the outlets, you would expect a very high resistance between hot and neutral.

BUT - if you opened the breaker to the converter and tried the same measurement you would like get a pretty low resistance reading as the converter is connected on the other end of the wires and you would be measuring the resistance of the converter's circuitry.

I hope that helps.
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Old 10-11-2020, 10:53 PM   #7
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When I used the ohmmeter, no current, either shore power or generator, was on. Also, the AC master and all circuit breakers were off. I was trying to see if there was an observable short between the black/hot wire and the white/neutral with no power in the system, and was frankly surprised to see any resistance readings at all, but let's face it, I really don't have a handle on this.

Sounds from what you're saying that, if anything, there was less resistance in the a/c circuit than the other circuits. But maybe I'm barking up the wrong tree.

Cummins Onan manual says when you get the "short circuit!" message, determine which appliance is at fault and repair/replace it. Seems like I've determined that, as the a/c shuts off the generator as soon as it comes on. But how can the a/c have a short when it's on generator power, and not on shore power?
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Old 10-12-2020, 12:09 AM   #8
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Quote:
Originally Posted by steve2777 View Post
If I tested between the neutral bus and each circuit breaker's black wire contact screw, every circuit EXCEPT the a/c circuit showed a reading of between .05 and .5 kohm.
You are measuring parallel paths through loads at the other end of the circuit:
50 to 500 ohms.

Quote:
Originally Posted by steve2777 View Post
But the a/c circuit showed a reading of .5 mohm.
That would be 500,000 ohms and is probably a good reading.

Quote:
Originally Posted by steve2777 View Post
Sounds from what you're saying that, if anything, there was less resistance in the a/c circuit than the other circuits. But maybe I'm barking up the wrong tree.
No, there's MORE resistance in the A/C circuit than what you measured in the other circuits.

If the repair shop did nothing else but replace the A/C, and the new A/C is wired correctly, then you have a bigger wiring problem somewhere, like maybe an open ground to SP.

Your neutral and ground buss should NOT be bonded in the RV. You can test this with your ohmmeter.

They are bonded in the house's main control panel, or in the RV parks main distribution panel. They are also bonded in your RV's generator.

So if you have a problem in the wiring to your A/C - say, a screw is shorting the hot lead to the ground cable, but you have an open ground connection in your SP cable, the A/C would still run normally but you may also be energizing the ground of your RV - a "hot skin" condition.

But when switched to the generator which does bond the ground and neutral, that same screw creates a short circuit and overloads the generator. The fact that it does not happen until you turn the A/C on makes it seem like the short is in the A/C itself and not the wiring to it.

At this point I think you are beyond the non-electrician handyman DIY stage and need onsite electrical expertise troubleshooting. A good mobile RV tech or electrician should be able to help troubleshoot this issue.

You need to verify the wiring from your RV's main panel, back through the ATS to the end of the SP cord is good. This is most important as "hot skin"/open ground is a dangerous thing.

You need to verify the wiring from the RV's main control panel to the A/C, through the controller and relays, to the fan motor and compressor is good as well.

Good luck and let us know what you find out.

All connections in the ATS need to be verified correct and good as well.
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Old 10-12-2020, 12:10 AM   #9
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Quote:
Originally Posted by steve2777 View Post
When I used the ohmmeter, no current, either shore power or generator, was on. Also, the AC master and all circuit breakers were off. I was trying to see if there was an observable short between the black/hot wire and the white/neutral with no power in the system, and was frankly surprised to see any resistance readings at all, but let's face it, I really don't have a handle on this.

Sounds from what you're saying that, if anything, there was less resistance in the a/c circuit than the other circuits. But maybe I'm barking up the wrong tree.

Cummins Onan manual says when you get the "short circuit!" message, determine which appliance is at fault and repair/replace it. Seems like I've determined that, as the a/c shuts off the generator as soon as it comes on. But how can the a/c have a short when it's on generator power, and not on shore power?
It can't. My money is on the transfer switch or generator wireing.
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Old 10-12-2020, 12:15 AM   #10
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Thanks! I'll get busy.
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Old 10-12-2020, 04:27 AM   #11
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Time to look at your problem from a different direction

Why was your air conditioner replaced?

I ask because our generator, shore power, air was acting just like yours.

Everything worked in any campground.
Everything but air worked on gen.
Many tests and a lot of forum searches gave me several things to try.
1. Run seafoam in the mh gas tank to clean the gen carb = helped a little gen would keep running when air started, 2 or 3 times out of a dozen tests.
2. Replace gen carb = total failure, gen would barely run but not good enough to make any elec
3. Replace the start capacitor with a hard start capacitor = success no more gen/air problems.

Here are some threads that will keep you busy
https://www.thorforums.com/forums/f1...ator-8140.html

https://www.thorforums.com/forums/f2...pp6-21367.html
The capacitor I got.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
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Old 10-12-2020, 11:19 AM   #12
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Our generator is diesel, so no carb. Old a/c was replaced because inadequate cooling, but the generator ran it fine, and gen has about 3 times the watt capacity that the a/c draws. It runs everything else, even up to a full 30 amps. When the a/c is started, it doesn't struggle, it just shuts down, with a short circuit warning. So probably a different situation. I think 16ace27 may be on the right track. Thanks.
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Old 10-12-2020, 04:57 PM   #13
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Eureka !!

I've found the problem! The hot wire to a water heater was patched into the same circuit breaker as the a/c. Heater operated normally on both shore power and generator. When I removed it from the breaker, the generator operated the a/c without a problem. Now I just need to determine whether the heater itself, or the wiring leading to it, is defective.

Thanks to everyone who shared their ideas !
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Old 10-12-2020, 05:09 PM   #14
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Glad to hear that the mystery has been solved!
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Old 10-12-2020, 10:24 PM   #15
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Quote:
Originally Posted by steve2777 View Post
I've found the problem! The hot wire to a water heater was patched into the same circuit breaker as the a/c. Heater operated normally on both shore power and generator. When I removed it from the breaker, the generator operated the a/c without a problem. Now I just need to determine whether the heater itself, or the wiring leading to it, is defective.

Thanks to everyone who shared their ideas !
Well, that's not right. The A/C should be on its own circuit breaker and the water heater, which is also a heavy load, really shouldn't be sharing a breaker either, unless the other circuit is very lightly loaded.
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Old 06-14-2021, 05:51 PM   #16
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I thought I had found the water heater to be the culprit, but beginning this summer the problem reappeared: every time I tried to start the AC on generator, the generator stalled and displayed "short circuit"

Well, some users over on IRV2 forum hit the nail on the head! A QD3200 generator will only RARELY start a 15k air conditioner because of the 1/6 second amperage surge of more than 3500 watts. I decided to install an EasyStart soft start box to see if it would allow the generator to start the AC, and it worked perfectly! Smooths out the compressor startup surge so the generator can handle it. I'm baffled that Thor, beginning in 2019, sold coaches with 15K AC's, and the QD3200 generator. The only thing I can think of is that they must have equipped them with a soft starter, or customers would be in mass revolt.
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