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Dometic Duo Therm rear AC unit making a loud tapping noise - Reverser coil?


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My rear (bedroom) AC is acting up. It's a Dometic Duo-Therm (13.5K Heat pump) with the 5 button thermostat. I was camping for a week in TN, and it was running all day. One evening I was outside and heard a loud tapping or knocking noise form the ground. I traced the noise to the rear AC unit. I climbed up on the roof and removed the AC cover. It sounds like it is coming form the right rear corner area (which I later identified as the "Reverser Valve" (area) by looking at the parts manual to see what was in the right rear corner of the AC unit.

Back inside the Coach the thermostat would not work so I could not shut off zone 3. The mode button was not functioning (and the screen looked dim) so I could not go through the various AC modes and turn off the unit. Then I switched the main on/off switch at the base of the thermostat off and then back on and the thermostat was working again. So whatever the issue is with the tapping noise it is affecting the thermostat too.

I just ran the AC in Fan only mode that night. Next day I put AC back into cool mode and it ran all day with no issues/noises. But the next day that tapping noise was back after running fine all day. Back home now, I ran the AC unit on generator all day and it ran fine. I never tried running it in heat pump mode.

I read several posts and watched some You Tube videos on troubleshooting the "Reversing Valve" (that is the part that switches (reverses) the AC to go from cooling to heat pump. Nothing in the troubleshooting posts or videos mentioned any tapping noises. So I'm not sure if the reversing solenoid is going bad or something in the control side. Anyways, I'm going to take a chance and just replace the reversing valve solenoid. It's on backorder everywhere - but I have my order in. It's about the cheapest part on the unit and about the only thing I can do without special tools. 

I'll report back if it fixes it. If anyone has had this tapping/knocking noise and diagnosed it, let me know.

Edited by CountryB
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I'd say check if there is a constant power to the solenoid while you hear the clicking. If yes, you are likely right. If the coil clicking correspond to its power interruptions, no sense to replace the coil as it might be a problem with the board, connections due to vibrations or the thermostat itself.

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I have not been able to get the AC to act up when I'm ready for testing it. I'll try and run it another 8 hours or so - maybe I can catch it then. Or it could have acted up due to cooler nights in TN (I'm back in FL now).

I'm sure if I cant get to fail now while I'm home, it will fail on my next camping trip...

BTW - I ordered the Reversing solenoid ($50). I'm just taking a guess - if I'm wrong, at least I eliminated one thing. Since this is an intermittent problem it may take a while before it acts up with me watching. 

Interestingly, the troubleshooting manual says do not use a voltmeter for testing - use a 120V light bulb.

Great so now I have to make this testing tool. What could possibly go wrong with that? 

Copy/paste from the manual (below): 

a.  Roof Top Air Conditioners

The operation of the AC control box can be checked at the 6-pin plug connection.  Disconnect the 
unit and use a 115 volt light bulb to check from terminal 5 (the white or common wire) to:
Terminal 1 (blue) is the compressor Terminal 2 (black) is high speed fan Terminal 3 (yellow) is 
medium speed fan Terminal 4 (red) is low speed fan Terminal 6 (green/yellow) chassis ground
If the circuit is completed the light bulb will illuminate.
Note:  Do not use a voltmeter to do the above tests as it will
give erroneous readings.


Dometic Duo-Therm wiring schematic.jpg

Edited by CountryB
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