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justplumducky 08-31-2011 11:20 PM

Miller Package Unit A/C stopped working.
Not certain of the model number, but certain about the serial number:

Model: MSC-3GE-C2 (am fairly certain of all characters but the "E" - could be an "F").
Serial: MK0387-01039 (certain of this number).

Miller Package Unit A/C (that's what's on top of the cabinet: "Miller") - could only make out Division of Lear Siegler, Holland, Michigan. This mobile home park was built in late 50's. This unit could be quite old. There is a readable date of Mar. '87 - have no idea what else it says on that line of data plate, before or after that date. Electrical diagram was no help - unreadable ( what was left ot it).

This unit stopped working, not sure if the blower was still running or not when compressor and condenser fan stopped. There is 240vac at the outside disconnect next to the package unit cabinet, and when I push contactor in, compressor hums/groans, but no start. Cond. fan does nothing. Didn't check power at contactor yet (primary or secondary). When I heard the compressor groaning, I just assumed there was 240vac getting to it - should've checked - will tomorrow.

There are three capacitors. Two smaller ones are both 7.5uf/370vac. The third one (Dual Run cap only being used on the HERM side) has no markings on it anymore. Its size is 5" tall, 3-1/2" wide, about 1-7/8" thick (as mounted/positioned on the electrical compartment vertical wall, with terminals on top).

The Dual Run capacitor is only being used on the HERM side, which I know is for the compressor. It measured (common to HERM) 37.2uF . If the tolerance range 5% (is that correct, 5% ?), and this is a 40uF capacitor, then 40-5%=38uF. It measures just under at 37.2uF. Would that be enough to make it not start that compressor? Or could this be a 35uF capacitor and it's just a bit stronger than its rating, or is that a bit too high of a measurement (37.2uF?) for a 35uF capacitor?

Would it hurt anything if it were supposed to be 35uF and I put in a 40uF?

The capacitor that goes to the cond. FAN is a 7.5/370v, but measures 6.85uF. 7.5uF less 5%=7.13uF. As in above, that's enough to not run the FAN? ( this FAN is not locked up, nor is it running).

Not that it matters as far as the cond. fan and compressor not running, but...The other 7.5uF capacitor for the blower measures 7.6uF Never did check for air from the vents yet (don't remember hearing anything though from that side of the A/C cabinet though (I'll check it out the morning).

None of them are dome-shaped at all, on top or bottom, nor leaking.

justplumducky 09-01-2011 07:59 PM

Went back today and watched exactly what was happening (which I should have done yesterday). Package unit would run for 5 or 10 minutes, then condenser fan would come to a halt. Its motor would hum loudly and compressor stopped running.

First I changed all three capacitors, but no help, same thing happened. Installed new cond. fan motor - no more shutdown. Cond. fan ran without stopping, but compressor wouldn't run till I put the new capacitor back in (for compressor - I had taken them back out after I realized the caps weren't the problem - it was a bad cond. fan motor).

Okay, so new cond. fan motor and new capacitor (40uF/440 vac - they didn't have a 370vac model) put it back in the pink. Plenty of condensation on the suction line after a 15 minutes and the line was cold. Amps draw on one leg of compressor was only 12 amps (maybe that's ok?) and 5 amps on the other leg, but the house cooled down to "comfortable" pretty quick, which me thinks was pretty good considering the heat here today, It was in the hottest part of the day when I got it turned back on, when the home was really hot inside.

beenthere 09-02-2011 09:02 PM

Glad to hear you got it working.

justplumducky 09-02-2011 09:13 PM

Thx for your reply beenthere.

I measured the amp draw on the compressor legs. One leg was 12 Amps, the other was only 5 Amps. Is this normal or do I have a problem in that circuit somewhere? (the one measuring only 5 Amps?

beenthere 09-02-2011 09:18 PM

The one only measuring 5 was the run winding, its fed off of the start windings EMF through the capacitor, so its normal to it to be much lower.

justplumducky 09-02-2011 09:30 PM

I have done an ohms check once before on a compressor, so I understand that the difference is probably due to the difference resistance values of the start and run windings, but not getting a clear picture in my head. Not a problem - I can look for a diagram online that might help me. I understand that basically that one of those legs going to compressor comes directly from the 40uF capacitor - I'll check it out some more. For now, good to know that it's normal to have the difference. Maybe if I re-read the article I saved a while back about PSC motors. Thx much for your help.

beenthere 09-03-2011 04:12 AM

Your welcome.

Its not the difference in the windings resistance. Its the electrical force generated by the start windings. It produces a higher voltage then what your house current is. If you measure across the capacitor while the compressor is running, yo will read a voltage in the 300 to 440 volt range.

Master of Cold 09-03-2011 08:29 AM

If you take your amp clamp and measure both the start and run wires the same time, you should have about the same reading as the common.

justplumducky 09-03-2011 06:29 PM

Thank you Master~ Didn't know you could do that (two wires at the same time). Is there a tolerance percentage, related to the data plate compressor amps, for a twenty year old compressor?

Master of Cold 09-03-2011 06:53 PM

Im sure there is some chart somewhere, but your amp draw is going to vary due to a number of factors, including voltage and temperature. As long as your under the RLA rating you should be fine.

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