Roof A/C out at 104F (of course)

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randy1

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Jul 5, 2007
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Working on the coach in preparation for Yosemite trip and A/C was working fine then suddenly the generator RPMS soared (probably to 2500 from 1800)
and unit quit cooling. I adjusted the idle back down, but compressor would not kick on. Is this a symptom of a worn out start capacitor or maybe
something more?
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
Chicken or egg. Did the RPM surge take out the roof AC or did the roof AC fail the the loss of load have the generator overspeed? In any event
sounds like the governor needs to be lubricated before proceeding
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
The genset never bogged down much when the A/C compressor engaged and had been running at a constant 1800 RPM putting out 116v. Then is revved up to
probably 2500 RPM and stayed there until I lowered the idle.
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
> Working on the coach in preparation for Yosemite trip and A/C was working fine then suddenly the generator RPMS soared (probably to 2500 from
> 1800) and unit quit cooling. I adjusted the idle back down, but compressor would not kick on. Is this a symptom of a worn out start capacitor or
> maybe something more?

What do you suppose 166 volts at 83Hz did to the AC unit?
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Every day I become more convinced that I am the only person left on the planet that recognizes nonsense for what it is."
 
The Onan has no internal voltage regulator?
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
> The Onan has no internal voltage regulator?

Of sorts. But voltage AND frequency are dependent on speed (RPMs). Check the specs for no load and full load RPMs, voltage and frequency. RPMs go up,
voltage and frequency go up. RPMs go down, voltage and frequency go down.

The power surge almost certainly released the magical smoke from some part of the roof AC unit.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Every day I become more convinced that I am the only person left on the planet that recognizes nonsense for what it is."
 
The Onan "voltage regulator" is the engine speed governor. This is not an
electronic system--the voltage and frequency is directly controlled by the
engine speed. That's why it makes such a nice sine wave, and why it
doesn't need the big dedicated inverter that my Generac does.

Rick "mechanically regulated systems are pretty reliable but need regular
inspection and service" Denney

> The Onan has no internal voltage regulator?
>

--
'73 230 "Jaws"
Northern Virginia
rick at rickdenney dot com
 
Since the fan still runs, is it worth it to try a start capacitor or just jump to replacing the A/C?
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
> Since the fan still runs, is it worth it to try a start capacitor or just jump to replacing the A/C?

Randy,

For the cost, it sure is worth the try. While that could be the problem, I kind of doubt it. The good thing is that the sealed compressor motor is
not likely to be hurt because is can't draw much current at the high frequency even with the expected higher voltage.

I don't know which manufacturer your unit is, but most that I have had open had a cut-out in the little box on the side that is held closed by a wire
clip. That cutout is both temperature and current sensitive. Some also have a manual reset on that sensor (makes sense in a roof top unit with ~87
screws to access). There is not a high likelihood that the pump (compressor) is damaged, but the surrounding devices sure could be.

Wishing you luck

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Still Loving OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
It's called a 'volts per hertz' system, a design used in pretty much all the non-inverter small gensets. Spin it at the correct speed (1800 for a
four pole alternator, 3600 for a two) and you get 60Hz. Consequently, engine speed government is essential. Overspeed will result in overvoltage,
which will toast equipment if it's much over, and over frequency which will help toas electric motors. Baaad juju all around.

--johnny
--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased
 
The "maintenance" is as simple as drop of oil on each of the linkage parts. Duane Simmons would pull the oil cap dipstick assembly and use that as an
oil drop applicator. Saved standing up and going to get the oiler when you were already seated on your work stool at the Onan. Pop the rod end and oil
and replace per his video.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Went out the next morning and it started right up. I replaced start capacitor anyhow A/C ran for about 45 minutes and put out 60 degree air at 98 OAT.
However, once I cycled it off it would not start back up. It's cooling now, so I'll see what happens.
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
> Went out the next morning and it started right up. I replaced start capacitor anyhow A/C ran for about 45 minutes and put out 60 degree air at 98
> OAT. However, once I cycled it off it would not start back up. It's cooling now, so I'll see what happens.

The time delay relay will keep the compressor off for a minute or so after it stops. It can't start against high pressure refrigerant so it needs to
stay off until the pressure bleeds down. You can't just turn it off and expect it to come back on immediately.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Every day I become more convinced that I am the only person left on the planet that recognizes nonsense for what it is."
 
So will the pressure bleed down due to the time delay and re engage the compressor if I just leave the switch on and wait?
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
Yes. If the time delay relay is still good. There was a writeup on how to bypass.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
> So will the pressure bleed down due to the time delay and re engage the compressor if I just leave the switch on and wait?

That's the purpose of the time delay relay, for inevitable momentary power outages. But if someone has removed the time delay relay, leaving the AC on
will bake the compressor in its own juice.

If the delay is much more than a minute or two, or doesn't come back on at all, there might be a problem with something else overheating and keeping
it off.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Every day I become more convinced that I am the only person left on the planet that recognizes nonsense for what it is."
 
> Overspeed will result in overvoltage, which will toast equipment if it's much over, and over frequency which will help toas electric motors.

Overspeed results in an over-frequency condition that is linearly proportional to the amount of overspeed but the excitation system has a limiting device - the thyristor - which puts an upper bound on excitation and thus an upper bound on output voltage. It is this device which prevents output voltage from soaring in a no-load situation but it also serves to limit maximum output voltage in any operational circumstance.

Many motors will tolerate an over-frequency condition but may draw increased current or deliver decreased speed due to flux saturation of their magnetic components. I have regularly operated 60Hz rated motors at 80Hz in CNC positioning applications but each motor design has differing magnetic characteristics so this statement cannot be extrapolated to the A/C compressor and fan.

Did the OP ever report back as to whether the A/C restarted after the short-cycle lockout timer expired?

—Jim

1977 Eleganza
1977 Royale
Hamilton, OH
 
Yep A/C working, but Onan surging.
--
Randy
1973 26' Painted Desert
Ahwatukee (Phoenix) AZ
 
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