When Air Conditioners Work Too Well

mark – 21 October, 2014

One morning we were surprised to wake up to a brisk 59 degree bedroom. Given that the thermostat was set on 73, obviously something was wrong. Our motorhome has dual ducted central thermostat controlled air conditioners. Although people are quick to call a repair man, issues with these units are typically easy to diagnose. Before we get into it, one thing you should know is that these units are factory filled and sealed with no way to check or fill the “freon” (also properly known as refrigerant) level.

If you are having cooling problems you have basically 3 options - clean the filter, clean the coils (inside and out) and use a fin straightener on the outside coils.

The way our particular air conditioner works is that the thermostat on the wall receives +12V from the coach battery system and sends it out via various wires. I know this because I changed our coleman mach thermostat to household digital thermostats some time ago in order to better regulate the temperature. The rest of the operation can be derived through basic troubleshooting. When the thermostat on the wall engages a click will be heard at the wall, the relay in the thermostat. A few milliseconds later a relay in the roof unit can be heard clicking. If AC power is being delivered to the unit it will turn on.

In our case the second click at the unit was not working for the compressor. Only for the fan. Deduction tells us that the compressor relay seized shut. I opened the unit and found a board with 3 relays. I ordered all 3 (actually 2 sets, for the front in case that ever went) and when they arrived I went about replacing it.

Replacing it is a matter of removing the board, keeping track of everything connected, desoldering the offending relay and then soldering in the replacement. This is easier said than done! In our case, the legs of the relay fit into the holes very snugly and as a result when desoldering and pulling the entire via came out. The via is a metal tube that connects the top to the bottom of the circuit board.

Board with one of the non-offending relays removed

The result is that a circuit connection that was made before is no longer made. You have to trace these connections manually and add wires to the board to make the connections again. Once I figured everything out, soldered on new wires to repair the damage we were back in business. This board is sold for a lot of money by itself but I replaced all of the relays for around $10.

Last modified: 21 October, 2014
Created: 16 August, 2013