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Old 07-31-2021, 01:54 PM   #1
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Unhappy A/C works on shoreline, but not generator

Onan QG 4000 onboard generator has power to everything , including the fan on the AC ( Coleman Mach 15 ). But on AC setting at thermostat, still only fan blowing. AC works when plugged into shore power. 30 amp. Flipped breaker switch on generator from off to on to off to on to make sure. Circuit breaker in motorhome turned off and on and off and on to make sure. Fuse in thermostat is good. 2015 Thor Windsport 27K.
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Old 07-31-2021, 02:02 PM   #2
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Nobody knows what kind of rig you have, class A class C tent or what. they all work different. Some class C motorhomes need you to plug the power cord into an outlet inside the electrical bay when using generator. posting a pic of your thermostat would also help.
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Old 07-31-2021, 02:09 PM   #3
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Originally Posted by Phred D View Post
Onan QG 4000 onboard generator has power to everything , including the fan on the AC ( Coleman Mach 15 ). But on AC setting at thermostat, still only fan blowing. AC works when plugged into shore power. 30 amp. Flipped breaker switch on generator from off to on to off to on to make sure. Circuit breaker in motorhome turned off and on and off and on to make sure. Fuse in thermostat is good. 2015 Thor Windsport 27K.
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One item to consider that you may already be aware of, when you switch to the generator and if the A/C was recently running on shore power, that most A/C's have a time delay of about ~5 minutes before the compressor will turn back on. If the A/C fan is running that would indicate that the A/C is getting power. ~CA
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Old 08-01-2021, 08:24 AM   #4
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I thought about the time delay. I let it run on shore power for 1/2 hour and the compressor was working. Switched to generator, while still plugged in, then disconnected shore power, and let the generator run for 1 hour. The compressor was not working for that time. Switched back to shore power, and no problems.
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Old 08-01-2021, 08:33 AM   #5
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Nobody knows what kind of rig you have, class A class C tent or what. they all work different. Some class C motorhomes need you to plug the power cord into an outlet inside the electrical bay when using generator. posting a pic of your thermostat would also help.
2014 Thor Windsport 27K. Class A. Coleman Mach 15 AC. No heat. But thermostat has a setting for heat.
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Old 08-01-2021, 08:53 AM   #6
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I would look at the transfer switch but since you only have 30 amp and the fan works I'm not sure that could be a problem. If you run the gen and verify that the power panel has 120 VAC at the Air conditioner breaker than that might mean it's more control related. I would probably troubleshoot with the shore power disconnected just to make things a little simpler.
Do you have any type of automatic shedding that could be going on?
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Old 08-01-2021, 09:27 AM   #7
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Shut down AC, pull power cord, wait 10 minutes, start generator, wait 1 minute, turn on AC and see what happens.

If generator struggles and then recovers with no cooling air, it may be a generator issue.
If the generator is running lean ( dirty carburator ), the RPMs drop, then volts will drop, amps will climb and the AC compressor thermal breaker may trip.
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Old 08-01-2021, 09:34 AM   #8
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Originally Posted by Phred D View Post
I thought about the time delay. I let it run on shore power for 1/2 hour and the compressor was working. Switched to generator, while still plugged in, then disconnected shore power, and let the generator run for 1 hour. The compressor was not working for that time. Switched back to shore power, and no problems.
What your initial post described doesn't make a whole lot of sense to me. Both sources (shore and genset) should be providing around 120 volts ac. Only thing I can think is that the genset voltage and/or frequency isn't correct. Have you verified what it's putting out?

I'd try for a fresh start. Turn the a/c off and let it sit for at least 10 minutes. It shouldn't matter but I'd go ahead and disconnect from shore power first before starting the genset. Let it run for a minute or two then put the thermostat into cooling mode. There may still be a delay, depends on your system, but hopefully the a/c will get to cooling. I'm not sure what the next troubleshooting step might be if it doesn't.
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Old 08-01-2021, 01:00 PM   #9
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What your initial post described doesn't make a whole lot of sense to me. Both sources (shore and genset) should be providing around 120 volts ac. Only thing I can think is that the genset voltage and/or frequency isn't correct. Have you verified what it's putting out?

I'd try for a fresh start. Turn the a/c off and let it sit for at least 10 minutes. It shouldn't matter but I'd go ahead and disconnect from shore power first before starting the genset. Let it run for a minute or two then put the thermostat into cooling mode. There may still be a delay, depends on your system, but hopefully the a/c will get to cooling. I'm not sure what the next troubleshooting step might be if it doesn't.
How do I verify what the voltage and or frequency is ? and how do I fix that?
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Old 08-01-2021, 01:05 PM   #10
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Originally Posted by twinboat View Post
Shut down AC, pull power cord, wait 10 minutes, start generator, wait 1 minute, turn on AC and see what happens.

If generator struggles and then recovers with no cooling air, it may be a generator issue.
If the generator is running lean ( dirty carburator ), the RPMs drop, then volts will drop, amps will climb and the AC compressor thermal breaker may trip.
The generator does slow down for 1 second when the fan in the A/C kicks in, but then powers up to full speed immediately. When the A/C was working on the generator previously, you could hear the generator working. On a different note, I can hear the click at the thermostat for the various settings. The thermostat is obviously working, because it works on shore power.
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Old 08-01-2021, 01:20 PM   #11
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How do I verify what the voltage and or frequency is ? and how do I fix that?
https://www.amazon.com/AstroAI-Multi...40&s=hi&sr=1-6

Fixing it is specific to first determining the cause of the problem which is why you need a meter to start with. Once you have that, then you are in a better position to determine the cause of any detected problem. There are many possibilities to the cause, could be that the generator just needs a tune-up or it could be something more concerning... or it may be some other issue than the generator but you need to start with some testing, therefore this meter could be your friend. ~CA

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Old 08-01-2021, 02:20 PM   #12
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Most likely your thermostat runs off of 12VDC
sends a signal to your control board on the ac unit

You need a voltmeter to test your transfer switch to see if genny is outputting 120vac on tra nsfer switch input and then is it transferring it accross to output of switch

Do this with no shore power
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Old 08-01-2021, 04:12 PM   #13
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You can check and watch voltage drop from any outlet while the generator is running.
No need to open the transfer switch.

Buy a Kill A Watt meter and plug it in, inside.

Won't measure amps of AC but will show volts and Htz of generator.Click image for larger version

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Old 09-01-2021, 12:55 PM   #14
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Nobody knows what kind of rig you have, class A class C tent or what. they all work different. Some class C motorhomes need you to plug the power cord into an outlet inside the electrical bay when using generator. posting a pic of your thermostat would also help.
Thanks for your input. It turns out the problem was with my Start Capacitor. Replaced it and the Run capacitor and that fixed the problem.
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