[Richard]

Today was my day to learn about SCS AC systems with some field diagnosis labs. Most of my previous experience had been with the Dometic split units and their control systems.

Not one but both units decided to play ugly. It may be that one or both had been getting sick but it only surfaced in the 100%RH and high 80's of where we are staying.

On one, the second stage kept tripping the breaker. Some quick work with the meter shows short to ground on the compressor legs. Tom has suggested that an wire between the board and the compressor could be grounded but I'll have to take it out to check that.

The more interesting and educational problem was the other unit. Symptoms.....no low speed internal fan on any setting. High speed fan comes on and stays on for a few minutes than shuts off only to restart on it's on after ten minutes or so. Also coils were found iced up.

There is a decent trouble shooting guide provided in the SCS manual, and believe it or not, I actually used it. Using the guide I determined that the Tstat was sending the proper signal to the circuit board, but the blower motor was not getting 120 volts on only low setting. The motor would get 120 volts on the high setting. The motor would pull 6 amps or so on high. Too high.

I used the meter to see the voltages. It was pretty straightforward looking at the picture of the circuit board in the literature. The motor did show appropriate ohms between legs, and was not shorted to ground. Hmmmmmmm, is the problem board or motor? The answer is both. To test the motor, I used the 120 volt supplied to the unit and hot wired it to the motor. When wired to the low speed leg, it runs fine, runs without stopping, and pulls about 2.5 amps. When wired to the high speed side, it stops after a few minutes, smells that electrical smell, and restarts after sitting for a while. Ok, so the motor has a problem with the high windings or possibly bearings. But still the circuit board is not sending 120 volts to the low windings. A quick examination didn't show any visual or electrical evidence of open circuits all the way back to a relay that must make to power the low side. So I suspect a bad relay. Hmmmmmm, I don't have one today, and Rhonda is hot. The unit won't run on high cause of the motor, and the unit won't run on low because of the board.

So I jumpered off the low side supply to the condenser fan to the low side of the evap fan. Ta Da ! It only works for low (one compressor operation) but it's AC until I can get the board fixed. I do believe the intermittent fan behavior led to the icing since the compressor and condenser fan were running ( and generating COLD) even though the evaporator fan was not running. The amperage draw on the compressor does not indicate an excessively low charge.

So the take aways that may help others is to use the 120 power right there at the board to temporarily wire any of the three motors, compressor, evaporator, or condenser to isolate the problem to the motor or the control circuitry. Use proper precautions and kill the breakers plus confirm zero voltage before touching the wiring. Also use a safe jumper setup. It's easy to monitor the amperage this way by clamping the meter around the jumper. Of course I didn't try this with the sick compressor because I had already confirmed that it was shorted to ground.