Replacing Roof Air Capacitors

[Kev]

This info applies to Dometic roof mounted air conditioner model 600315.421. These are the original roof airs on my 1997 coach. Yours may differ so be certain you confirm part numbers before replacing anything.

Why did I replace these parts? I recently noticed that both the rear A/C fan and compressor were taking longer than usual to start and were a bit noisier.

The first (and least expensive) place to start was the capacitors. Each A/C uses a Start Capacitor, a Run Capacitor and a PTCR (thermistor) device.

The date codes on my parts were telling me that they were probably original (or very close to it). Capacitors don't last forever - they degrade over time. A weak cap causes the device it's supporting (in this case either the fan or compressor) to work harder. Given enough time, a weak cap can lead to premature fan failure or, even worse, compressor failure.


The new parts I used:

Start Capacitor: MARS p/n 11963 (108-130 MFD, 330 VAC)
Run Capacitor: MARS p/n 12713 (25 + 10.0 MFD, 370 VAC)
PTCR Device: Cera-Mite p/n 305C20 (35 Ohm), Dometic p/n 3100876.006


Dometic service manual specifies the following:

Start Capacitor: Dometic p/n 3100236.219 (88-106 MFD, 250 VAC)
Run Capacitor: Dometic p/n 3100248.453 (25 + 10.0 MFD, 370 VAC)
PTCR Device: Dometic p/n 3310731.009 (35 Ohm)


To assist the older compressor upon start-up I chose to use a slightly larger (+22.7%) Start Cap. I spoke with Tecumseh (A/C compressor manufacturer) tech support and was told that using a larger Start Cap (~20%) was ok.
The Run Cap and the PTCR device I used are the exact specs that Dometic recommends.

You don't have to use Dometic parts. In fact, you'll save yourself some money by choosing an equivalent replacement that's not a Dometic (i.e., 3310... or 3100...) part number. For the Start and Run Caps I chose to go with parts made by MARS. However, the PTCR device I used is a Dometic part number.

It's very easy to replace these parts. Make sure you first disconnect power from the air conditioner. After that just remove the A/C shroud, remove the capacitor cover plate, and replace the capacitors and PTCR. If you move slow like me, by the time you get to the roof, remove the covers and gain access to the caps, any voltage will most likely have dissipated. But you might check with a voltmeter just to be safe.

And while you're up there with the shroud removed, it might be a good time to clean and inspect the entire roof A/C unit.

After replacing these parts and putting it back together I noticed a decrease in both the fan and compressor start-up times and less noise overall.

[NoGas]

Excellent info Kev.

For those curious about his front A/C, we have both installed Micro-Air’s Easy Start module (see posts 26-30 HERE). This unit replaces the capacitors and smooths compressor startup, allowing an inverter to power ones AC without using the generator. However that black box isn’t cheap, so on my rear AC a stronger soft capacitor works just fine.

[mpierce]

My center AC quit. Was ready to replace, but decided to try replacing the capacitors. Works just fine now!

[Mallie Lennon]

I would be reluctant to pay $300 for a magic start box. There is nothing magic about a start device. There are only a couple of ways you can engage and disengage a start capacitor. PTC or potential relay.

[badandy]

Originally Posted by Mallie Lennon

I would be reluctant to pay $300 for a magic start box. There is nothing magic about a start device. There are only a couple of ways you can engage and disengage a start capacitor. PTC or potential relay.

Are you referring to a Microair starter? If so you couldn't be more wrong.

[irmagoo]

Originally Posted by Mallie Lennon

I would be reluctant to pay $300 for a magic start box. There is nothing magic about a start device. There are only a couple of ways you can engage and disengage a start capacitor. PTC or potential relay.

hi mallie
this is not what you think it is more like freq. drive as it lowers inrush amp i am not sure of what all is involved but they are the real deal i would think they play with voltage as well as current.. if i ever get one i will figure it out.. but as of now i don't know its magic!!
mike

[Randy Dupree]

Its pure magic until you guys figure it out,as far as i'm concerned.
Myself,i stick with the old school stuff,caps are fun!
I will bring one to my seminar,will it be charged?or not?

[Mallie Lennon]

Originally Posted by irmagoo

hi mallie
this is not what you think it is more like freq. drive as it lowers inrush amp i am not sure of what all is involved but they are the real deal i would think they play with voltage as well as current.. if i ever get one i will figure it out.. but as of now i don't know its magic!!
mike

Thanks Mike, I would like to know.

[iamflagman]

Originally Posted by Randy Dupree

Its pure magic until you guys figure it out,as far as i'm concerned.
Myself,i stick with the old school stuff,caps are fun!
I will bring one to my seminar,will it be charged?or not?

Who are you going to try it on? I won't be near you during that show and tell.

[irmagoo]

Originally Posted by Randy Dupree

Its pure magic until you guys figure it out,as far as i'm concerned.
Myself,i stick with the old school stuff,caps are fun!
I will bring one to my seminar,will it be charged?or not?

i think they are all charged - until ya check-um...been to may radio/ham swap meets!!