Repair of Dometic RV Air Conditioner Control Box

 

Our 2015 Forest River Georgetown motorhome had a failure of one of its Dometic 13,500 BTU air conditioners.  I replaced it, and all seemed well for a year.  Then the fan started doing odd things.  The fan would stop with the A/C running, change speeds, then start again.  It would do this multiple times per hour.  And sometimes, the fan would stay on and not cycle off for hours, while the other A/C unit was operating normally.  These are some of the symptoms the old A/C had before it died.  The only common part between the old A/C, and the new one, is the control box.  I decided to open it up and have a look. 

I'm a retired aerospace engineering tech, so this type of work is very familiar to me.  I share this info so you can see what's involved, and you can decide if you're willing to DIY.  Usual precautions apply.  Disconnect all power, including 12VDC.  It's not a difficult or time consuming fix once you know how.


From inside the RV, the air filter cover plate was removed, exposing the control box, mounted just below the roof mounted A/C unit.

Once all the screws were removed, I took multiple pictures of the wiring to make reassembly easy.  I could see some corrosion on the board, so I carefully removed it.

The underside of the board was fine.  I took pictures of the P/Ns in case it needed replacement.

Removing corrosion is actually very easy.  Take a cheap "acid brush", and cut the bristles down to a 1/4".  Put a little 99% isopropyl alcohol in a small cup.  Dip the brush into the alcohol, and scrub the circuit board any place corrosion is visible.  Continue dipping in alcohol and scrubbing until it's clean.  Wipe away residual alcohol and let air dry.

This is the before cleaning pic.

After cleaning, it's like new again.
 
The entire job, from disassembly, troubleshooting, repairing, reassembly, and testing... took about an hour.  It could have been done at a campground with some basic tools, an acid brush (available at hardware stores), and isopropyl alcohol (available at any pharmacy).  This completely fixed all the issues.  The fan motor now cycles properly, and the unit is cooling properly. 

My theory of the cause is based on experience with similar failures in radio equipment.  The board was completely dry, with no evidence of water staining.  I have found manufacturers sometimes use the wrong flux when soldering.  The flight hardware requirement we had at work was to only use RMA Flux.  That's Rosin Mildly Activated.  After soldering, it can be cleaned as described above, but the residual flux is NOT corrosive.  RA (Rosin Activated) flux works on dirty components, but if not cleaned thoroughly, it causes corrosion later, just like what's pictured.  Maybe this is what they used, and someone didn't follow through.  Maybe it only affected a few A/C units, maybe many.  I have no way of knowing, but if your A/C fan starts doing weird stuff, now you know where to start looking.

These are the products I used:

 

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Last updated 01/22/22   All rights reserved.