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Old 09-18-2007, 08:11 AM   #1
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AC Taking hours to start up

Last June our forward a/c wouldn't work at all. We took it in, and they replaced a circuit board.

We took it out a few weeks later, and it didn't start up for about 3 hours. At least the fan didn't start, my wife said she could hear the compressor working. I called the repair shop, and they said that it could be the batteries since they were going on 4 years old - I replaced them.

About a week after replacing the batteries we were out again, and it took 30 minutes for the a/c to start.

I thought it may have been that the batteries did not have a full charge, so this time I put a trickle charge on for a week before taking it out again this last weekend. This time it took over 24 hours to start up.

Now, once the a/c is on it works just like it should. On my way home this weekend, I took it straight to the repair shop. They called yesterday afternoon to tell me that they hooked the trailer up to 50 amp service and the a/c kicked on with out any delay. I asked them to let it sit for 3-4 days and try it again to see if the problem would occur so they could trouble shoot the problem.

i'm just mystified, and need some suggestions on what the problem could be. The rear a/c works just fine; it's just the front a/c that's having a problem. At all three locations the voltage has been in the 119 - 124 range, so it's getting good power.


_________Tom
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Old 09-18-2007, 08:28 AM   #2
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If the repair shop told you that the AC wasn't starting because the batteries were low, I'd find another repair shop. The two are completely unrelated. What type of AC do you have?
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Old 09-18-2007, 08:47 AM   #3
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Quote:
Originally Posted by HiHoAgRV
If the repair shop told you that the AC wasn't starting because the batteries were low, I'd find another repair shop. The two are completely unrelated. What type of AC do you have?
They said that there is a 110v board for the fan, and that with the rear a/c and refrigerator running low batteries may have been the cause.

The front a/c is a Dometic 15,000 btu heat pump.
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Old 09-18-2007, 09:37 AM   #4
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Quote:
Originally Posted by HiHoAgRV
If the repair shop told you that the AC wasn't starting because the batteries were low, I'd find another repair shop. The two are completely unrelated. What type of AC do you have?
Actually, these roof-top units need BOTH 120VAC and 12VDC to operate. The 120VAC rovides the power to run the blower motor and compressor, but the PC boards in these units use 12VDC as the control voltage, so they may have been correct, as they will not operate in a DC environment of less than 11.5VDC.

It sounds like you either have a bad relay board in the upper unit, a short within the blower motor itself (sometimes refered to as a 'flying short'), or possibly a thermostat problem. It's really hard to diagnose without being there to see and take readings, but I would consider a different repair shop.

YOu also coould have a weak, but not fried, start capacitor and PTCR which is taking a long time to build up sufficient charge to allow the motor to start. These can be tested to see if they are with-in the spec printed on the outside of the capacitor.

I would call Dometic's customer line, or check their web site and find a different shop that is certified in roof A/C units.
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Old 09-18-2007, 09:44 AM   #5
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Quote:
Originally Posted by lewster
Actually, these roof-top units need BOTH 120VAC and 12VDC to operate.


Obviously a much newer unit than my old junk...
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Old 09-18-2007, 11:08 AM   #6
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Probably the blower or blower motor control

Quote:
Originally Posted by lewster
It sounds like you either have a bad relay board in the upper unit, a short within the blower motor itself (sometimes refered to as a 'flying short'), or possibly a thermostat problem...YOu also coould have a weak, but not fried, start capacitor and PTCR which is taking a long time to build up sufficient charge to allow the motor to start...I would call Dometic's customer line, or check their web site and find a different shop that is certified in roof A/C units.
A new Bambi had a similar problem at TAHI a couple of months ago - the resident electrician removed the top cowel and found a wasp nest on the fan shaft - this created just enough friction to not permit the fan to turn. A possible explanation of the delay start could be that the comperssor started, was shut down by some safety system and attempted a restart after a certain amount of time, and on a subsequent restart the fan started to turn.

On your unit it could also be that the fan motor is just not starting for other reasons.

At any rate, checking the fan blower for ease of rotation (or self starting) is a simple enough excercise. The fact that the compressor will start, then goes off, but the blower does not work, points to a problem centered around the blower motor or whatever controls the blower motor operation.
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Old 09-18-2007, 12:45 PM   #7
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Dennis,

No wasp nests in the fan cage. I've talked with the repair people, and they hooked it up to 50 amp service and the a/c started right up. They have agreed that I should pick the unit up and let it sit for a week or two and then bring it in to see if it starts up then. They can't do anything until they can replicate the problem. Just glad I'm not taking it out again until mid-October.

_________tom
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