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Old 06-18-2021, 06:53 PM   #1
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Onan generator only runs 1 stage of basement A/C

I've been scratching my head for a while. Have a 2000 Itasca horizon 36 LD. It has an Onan Marquis 6500 LP generator. The basement Coleman Mach AC appears to operate just fine, showing 24 amps, while plugged into 30 amp shore power. However, if I'm running the generator it only appears to run one stage, showing 16 amps.
Any ideas what would cause this issue?
Motorhome is new to me 3 years ago. I'm unsure if this is a new thing this year, or something that I've been experiencing, but not noticing, since I bought it.
I've been told that maybe the generator isn't cycling right, and not providing the power at the right Hertz rating.
The motorhome has a factory intellitech EMS, and Todd engineering power switch model PS245L.
I'm not an electrician, but part of me wonders if the power switch would be a more likely culprit. Any ideas on what I could test, to find out what my issue is?
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Old 06-18-2021, 07:45 PM   #2
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There are 2 circuit breakers on your generator. Probably one of them is tripped.

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Old 06-18-2021, 07:48 PM   #3
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I agree with Richard; plus, only #1 compressor runs when the actual temperature and set-point are within 1-2į, so set your 'stat 5į lower than inside temperature to check that out.
repair manual: http://www.rvrepairmanual.com/manuals/OnanNHMmanual.pdf
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Old 06-18-2021, 07:55 PM   #4
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I suspect you are correct in your inclination that the Todd Engineering Power Switch is faulty. While the most common failure is an inexpensive capacitor, it has become increasingly difficult to obtain repairs for these obsolete units. Some folks obtain rebuilt boards for these units, but my experience is that they are back looking for replacement of the entire unit within 6 months.



My suggestion is you consider replacement with an ESCO model LPT50BRD It is a fifty amp unit with much more robust relays, is from a current manufacturer, and supported. Price should be under $350, and installation is quite straightforward.
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Old 06-18-2021, 08:44 PM   #5
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Originally Posted by Ray,IN View Post
I agree with Richard; plus, only #1 compressor runs when the actual temperature and set-point are within 1-2į, so set your 'stat 5į lower than inside temperature to check that out.
repair manual: http://www.rvrepairmanual.com/manuals/OnanNHMmanual.pdf
Iíd agree with the most likely being one of thr breakers on the gen being tripped. They have also been known to fail. The only way I know to check them is take it out and test with an vom meter But outside that, does everything else work when running the generator? A generator is just like being plugged into a 50 amp pedestal. Two lines of 120 coming into the distribution panel. When you plug into 30 amp with a dogbone, your just taking 1 30 amp circuit and applying it to both legs in the distribution panel.

Your intellect ems has lights on it to show which circuits have power to them. They are the high load items that the load shedding shuts down if you try to pull more power than you have available.

When you are running the generator all the lights should be lit. Water heater, ac comp 2 fan speed and refrigerator. They also should all be on when plugged into 30 amp but say you are running both ac units and try the microwave it will shut down comp 2 or water heater or both.

Whe I run my inverter, and no shore power or generator only the refrigerator one is lit, because that is alll the power available.

After checking the breaker on the generator, if itís not tripped, the next step I would take is to take the cover off the main circuit breaker panel. You have a double 50 amp breaker but they are tied together so they are both either off or on. Test the voltage to the output of each leg. From the screw on the bottom of the breaker to the ground bonding bar. This is with the generator running. If one has no or low voltage you are loosing power between the generator and the main panel.

If you have power to both, 120 volts itís probably the elms. If you donít have power the next place I would look is the ats transfer switch. Take the cover off and measure both the inbound voltage and the outbound. It is possible that one contact on the ats has failed. If you have power on the income but not the out the ats has failed.

If you have power with the generator running, both in and out it almost has to be the ems panel. But while you have it open, the intellect ems is right in the same distribution box you just took the cover off. Measure the voltage to all the terminals. Only two should have 120 volts when operating properly.

The other thing, just to make sure, donít trust the amp meter on the display. Put a clamp type amp meter over each line and make sure you are getting the right readings.

Also, you can check while connected to shore power. Turn the breaker off for compressor 2 and read the load. Then turn it on. Measure again. I can actually hear my second compressor start when I flip the breaker.
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Old 06-18-2021, 08:46 PM   #6
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Originally Posted by Oregonpharme View Post
I suspect you are correct in your inclination that the Todd Engineering Power Switch is faulty. While the most common failure is an inexpensive capacitor, it has become increasingly difficult to obtain repairs for these obsolete units. Some folks obtain rebuilt boards for these units, but my experience is that they are back looking for replacement of the entire unit within 6 months.



My suggestion is you consider replacement with an ESCO model LPT50BRD It is a fifty amp unit with much more robust relays, is from a current manufacturer, and supported. Price should be under $350, and installation is quite straightforward.

Huh?
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Old 06-21-2021, 02:26 PM   #7
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I’d agree with the most likely being one of thr breakers on the gen being tripped. They have also been known to fail. .....
So I haven't yet had a chance to run through all the tests you gave me. Thank you for everything so far.
I have confirmed both breakers on the generator are on. I also have the thermostat set at a 20 degree difference because I knew there was a temp difference needed, but had no clue it was 3 degrees.
I checked voltage at the transfer switch, and it does appear I have 120 on all inbound lines from the shore power line and generator lines. (I did check them independently. ) I can't really tell where the outbound lines are, but still looking. I need to get an amp meter to check the lines as well.
I did notice that the 2nd stage attempts to start with the generator, but the 2nd stage immediately shuts off. After it stops, my amps dropped to 10, and don't go back up to the typical 14 that show when the first stage is running prior to the second trying to start. If that makes sense.
While on generator, all of the EMS lights stay on, even when the 2nd stage shuts off. I did try on ShorePower, and overloaded the power, and the EMS shut the water heater, and AC compressors off.
When I'm only on battery power, I have no lights on my EMS.
I am able to run the first stage AC, water heater, and microwave(34amps) from the generator at the same time.
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Old 06-21-2021, 07:16 PM   #8
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Your last description sounds like the either the start and/or run capacitor is bad.
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Old 06-21-2021, 07:31 PM   #9
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Your last description sounds like the either the start and/or run capacitor is bad.
On the 2nd stage of the AC?
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Old 06-21-2021, 11:38 PM   #10
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On the 2nd stage of the AC?
Yes, #2 compressor.
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Old 06-30-2021, 06:28 PM   #11
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I replaced all five relays on the AC. I figured I would hopefully solve any future problems by replacing them all.
However, I'm still having the same problem. The second stage will not start while on generator power.

Wall mounted amp meter shows the following values:

2 amps - inverter plus miscellaneous draws
5 amps - inverter plus AC fan only
13-14 amps - AC plus first compressor
19-21 amps - AC plus first compressor and second compressor

The amps drop to 10 amps after the second compressor stalls out. Any ideas?
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Old 07-01-2021, 05:36 AM   #12
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I would go after both the capacitors 2nd stage comp, does the fan seem to slow down/drop a speed after 2nd stage fails to start. I would also be very observant for spade connectors that are loose or burnt while in there. If 2nd comp. is still starting harder than first, if caps get it going, consider adding a hard start cap like Suppco SPP6.
If I'm on same page of model your working on the start cap is more like a hard start cap with PCTR that may have failed .
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Old 07-01-2021, 06:13 AM   #13
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Originally Posted by FiveOhRanger View Post
I replaced all five relays on the AC. I figured I would hopefully solve any future problems by replacing them all.

However, I'm still having the same problem. The second stage will not start while on generator power.



Wall mounted amp meter shows the following values:



2 amps - inverter plus miscellaneous draws

5 amps - inverter plus AC fan only

13-14 amps - AC plus first compressor

19-21 amps - AC plus first compressor and second compressor



The amps drop to 10 amps after the second compressor stalls out. Any ideas?


Iím guessing that these amperages are the running amps. Your starting amperage for the second stage may be higher than your generator can supply. The control circuit may shut it down because itís not getting started.

A hard start kit might get the compressor started but only if you have enough amperage available.

Does the second stage start normally on shore power?

Is the amperage for the first stage within spec?

Not sure if you could lock out the first stage and see if you could get the second stage to start on generator.

A meter that can show inrush current and the frequency of the power could tell you more.
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Old 07-01-2021, 06:42 AM   #14
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I had a similar problem with my basement AC a few years ago.

The EMS was preventing the 2nd compressor from starting. If you look in the electrical box on the AC unit, you'll see the two controls for compressor 1 and 2. No. 2 will have some small wires that divert out of the box before some wires come back in. Those go to the EMS. I simply unplugged the EMS diverter and plugged right into NO. 2 to bypass the EMS. Worked like a charm.

Last summer, I rebuild the whole basement AC. See my saga here:
https://www.irv2.com/forums/f258/sag...-c-496195.html
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