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Old 04-01-2012, 04:52 PM   #1
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Roof top AC compressor kicks on, runs for 3 seconds then kicks off

I have a 2000 model 5th wheel that has a roof top air conditioner, I believe its a coleman mach 3 (I cant find a model for it anywhere, the thermostat just says "Coleman Mach" and looking at pictures of other mach 3 units, it looks just like mine). The fan will blow air just fine and appears to be working perfectly, but the compressor will not stay running. When I change the thermostat to cool, the compressor will either kick on or try to kick on and run for about 3 seconds then quit. 25-30 seconds later it will kick on/try to kick on, run for about 3 seconds and quit.

When I first tried it a few weeks ago, the compressor wouldn't even try to kick on at all, but I replaced the start capacitor and that made it actually kick on and run the way it is now. I have also replaced the run capacitor but that didn't change anything. I've checked the voltage running to the roof unit and it never drops below 120-122 volts. A few weeks ago I also checked the voltage running to the compressor when it tries to kick on, I forget the exact voltages running to it, but I think the lead from the run capacitor was dropping to like 50v (I'll check again if this should be looked into).

Is there anything else that I can try? The only other things I've come up with are getting a new potential start relay or getting a 'super boost' to get the compressor going.
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Old 04-01-2012, 05:05 PM   #2
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Perhaps it needs to be recharged
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Old 04-01-2012, 07:01 PM   #3
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Recharge is the least likely issue. You need to do some trouble shooting and see what is the item that is cycling the compressor off. There is a Klixon (thermal element ) mounted on the compressor. If the compressor is drawing excess current it will trip the Klixon and when it cools it will let the compressor start until the element get hot again.

Also, using a clamp-on amp meter, check the compressor amps when running. Also check the voltage at the A/C unit.

You will have two sets of current on the data plate. One for the whole unit and one fro the compressor only. You need to make sure you are measuring the correct current.

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Old 04-01-2012, 09:45 PM   #4
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Automotive and home AC systems behave like that when the freon is low. I have zero experience with RV AC units, but if they're anything like the others, using a manifold guage set to check the freon level is super easy.
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Old 04-02-2012, 05:34 AM   #5
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Use a clamp on amp meter to determine the amperage of start up (or attempt). If you see something like 75 amps you may have locked rotor. You can also check running amperage and see if the klixon is the problem (tripping out under it rating), Good luck, stay safe and keep us posted.
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Old 04-02-2012, 09:47 AM   #6
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RV air conditioners have no refrigerant connections for the high and low side. There is no way to connect gauges to check the system.
A good AC technician can cut into the system and add ports to install freon if the problem is due to a freon leak that can be repaired.
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Old 04-02-2012, 09:54 AM   #7
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I will add one other possible issue, the compressor could be partially siezed, overheating dn doing the thermal cut out thing when it starts,, This may be due to a bad start capacitor but is more likely due to an internal issue and will require replacement of the unit.. Just so you are prepared.

But the other folks may be right too, That would, in fact, be my first suspect. (THat they are right) I'm just giveing you worst case.
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compressor, roof

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