Roof-top A/C issues

Piedmontexpress · March 15, 2015

My rear Dometic 59516 15,000btu A/C unit popped the breaker last year. Now that the snow is off the roof I investigated and found a blown motor starter cap. Foil everywhere! It actually put neutral to ground....scary. Anyway....I don't have schematics for testing the motor in case it is shorted. Anyone know the chances it may just be an old (2004) Mountain Aire starter. I know I need to go buy a crystal ball. The motor is not locked up BTW.

Any help would be appreciated!

dsimpson · March 15, 2015

I have had to replace my start capacitor twice. A new capacitor shouldn't cost much more than $15. Try it and see what happens. A Google search should turn up the wiring diagrams.

Kirk W · March 16, 2015

I agree with both other posts, but I do suggest that you be well clear of the unit when starting it just to be safe. It is not probable that you will have the same experience as an electrolytic capacitor blowing up is usually from an internal short but there are a few other things which could cause it so use caution.

Your ac motor windings will read nearly 0 ohms if you check them with an ohm meter since it has no impedance to dc current of the meter. You could check from the power leads to ground or to the case of the motor to be sure that it hasn't cooked the windings.

Jimalberta · March 16, 2015

If you know how to use a multimeter do a quick check of the windings. Put your meter on ohms and do the following. Disconnect the RV from all shore power and do not run the gen to make sure there is no AC power. Disconnect the 3 wires from the compresser terminals and mark them so they go back on the same terminals when you hook them back up.

Then using the multimeter measure from Each terminal to ground there should be no reading..... If you have a reading of any sort your compressor is shorting to ground and is toast. If you have no reading to ground proceed.

Measure from R-C and record your findings..... this is your run winding, then measure from S-C and record...this is your start winding.

Finally measure from R-S and record ....this is both windings and this measurement should be the sum of the first 2 windings.

If this checks out you should be good to go. Replace the blown capacitor, rewire the compresser and give it a try. As stated above stand back aways...lol

For these kinds of tests I prefer to use a good old fashioned analog meter so I can watch a needle....some of me is old school. However a digital meter will work.

Kirk W · March 16, 2015

Then using the multimeter measure from Each terminal to ground there should be no reading....0 ohms. If you have a reading of any sort your compressor is shorting to ground and is toast. Of you have 0 ohms to ground proceed.

Don't you mean that the reading to ground should be infinity? It looks to me like 0 ohms indicates a direct short to ground? I'm not an a/c tech but have been a service tech in other things for 40 years and have always considered 0 ohms to ground to be a bad thing!

Jimalberta · March 16, 2015

Kirk you are right My bad...I have corrected my post.

Piedmontexpress · March 16, 2015

Thank you all! I did some more checking and found a service manual at Bryantrv. It gives the color codes and with your great help I should be able to check it out. Yes I have two meters a Fluke and a Simpson.

Thanks for the safety advice! I won't yell down to my wife and say "TRY IT"

Bill-

Piedmontexpress · March 17, 2015

Compressor checks good! Now finding the cap! 4 local RV joints don't have it. Amazon doesn't list it. I guess I'll have to order it.

Again thanks for your help!

Bill-

Alie&Jim's Carrilite · March 17, 2015

Type the number on the cap into google and see if there is a cross reference. On an older 5'er we had, the 16yr old AC quit. Bad starter capacitor and no one had the same model. Quick cross reference of numbers and my local store had one.

Jimalberta · March 17, 2015

Check with the local HVAC people. I used to carry a universal kit in my service vehicle...could pretty much match any cap.

Why are people so reluctant to deal with the real experts....RV people dont know squat about ACs.

Piedmontexpress · March 17, 2015

Belive me if the compressor was toast I would weigh my cost options which is high anyway you look at it! I read a few posts where some folks did try their local HVAC to no avail.

I will try to get the numbers. I do know its a 52 or 53 microfarad. I don't have it with me at the moment.

Jimalberta · March 17, 2015

Ok...this is what I would do. Google or search the yellow pages for a Totaline Store. This is the wholesale outlet that the HVAC guys go to for their parts. Go down there and they should have the part you need. If they refuse to sell to you...sometimes they will only sell wholesale to a contractor.....just ask a contractor that will be there picking up his parts if you can buy it from them and most likely they will be happy to do so.

Take the old cap with you or write all the numbers down but mostly you need the microfarad size and the vac numbers. Plus the physical size if space is tight.