Dometic Duo Therm. Bad capacitor?

[PancakeBill]

Our front AC is not cooling, fan starts up and runs fine, bot compressor never starts up.  Quick thought and easy to get to fix, is the start capacitor.  Found one on an RV parts site, little pricey, but not horrible.  What do you think the chances are of it being that part?  Turn on to cool, pick a low temp, fan starts right up but no chug for compressor and only blows air.  Rear AC working fine.  Considering possibility of the Comfort Control center being bad, had that issue in a different coach, I think it is only a matter of switching the cables on the unit. 

Any ideas?  Before I order a capacitor kit?

 

[Henry J Fate]

Before you order any parts it is good practice to visually inspect the unit. Look closely for any signs of excessive heat, unhealthy wires etc. If all looks good, have someone start the unit while you are at the unit. Check to see if the compressor is attempting to start or any other clues to what is going on.

A more technical approach would be to have a multimeter handy and check for the existence of the needed compressor voltage.

I have not looked at the schematic for your specific unit but there is an overload protection device probably mounted on the exterior of the compressor. Also be sure the breaker to the unit is on and in good order but it would seem to be ok as the fan is coming on.

 

[PancakeBill]

I was up top, no sense of the compressor attempting to start.  Looked at cap, it looked OK, not bulging.  Did not have my meter but didn't really know what to check.

 

[Lou Schneider]

Remove the wires from the capacitor and short out the capacitor to discharge any voltage.  Then you can test it with your ohmmeter.

A good capacitor will start out at 0 ohms and then climb to a high resistance.  A shorted capacitor will stay at 0 ohms, an open capacitor will read inifinity.

 

[Henry J Fate]

Start capacitors are typically wired in parallel with the corresponding incoming operating voltage. There should be an indication of the compressor trying to start even if the start cap is bad.

If the capacitor tests good, use your meter to check for voltage to the compressor at start up.

It is also helpful to find a schematic and wiring diagram to help guide you through the troubleshooting process. I believe those units have a run capacitor and a start capacitor and both could be combined into one body.

If you have the time, post the model number and the date of manufacture.

 

[Gary RV_Wizard]

Typically a bad capacitor results in the compressor groaning for a second or two as it tries to start but fails.  If there is no "grunt" of the compressor attempting to start, odds are that the start capacitor is NOT the problem.

Since you have two a/c units, you can swap the capacitor and run device from the second a/c to determine if that has any effect.

 

[PancakeBill]

It could be a thermostat.  Hope I can get to the bottom of it tomorrow.

 

[Gary RV_Wizard]

A thermostat?  What or where?  The wall thermostat must have worked if the fan started; everything after that is controlled by the a/c circuit board. Did you mean some other thermostat internal to the a/c unit?

 

[PancakeBill]

Had another AC that ended up being bad thermostat.