[Richard]

http://old.rvcomfort.com/rvp/pdf_documen...mostat.pdf
The manual for that thermostat does not show that both compressors should be on in lo mode. So, I don't quite understand pins 5 and 6 both being hot in Low mode.

But if the setpoint in auto is 2 degrees higher than actual, the manual indicates both compressors will come on.

[Chester Stone]

Richard, I was surprised by that also so I made the same check on the front AC and found the same condition. The front unit operates correctly so I concluded that the high/low system must shut down #2 compressor when set on low.

Another unexpected surprise today. The rear AC started operating on low, that is the low speed fan and #1 compressor. On high, the fan went to a faster speed and #2 compressor started along with #1. This is the first time that I can recall it ever operating at all on low.

On both the front and rear units, I checked the volts going from the board to the compressors. It was 122 volts in all cases, even when I reversed the compressor connections. On low, compressor #1 is 122, #2 -0-, and on high both are at 122. However, there was something I did not expect. The common for compressor #2 is not connected to the bundled common which #1 is connected to. I think this tells me that when I install the heavy duty relays, I should keep the common for #2 separate.

As I have stated, the front AC operates normally. On low, #1 draws 8 amps, #2 -0-, and on high both draw 8 amps. Not so for the back AC. With compressor connected normally as they were originally installed, #1 draws 4 amps on low and 5 amps. on high with #2 drawing 7 amps. When I reverse the compressor connections, the amps are also reversed. On high or low, #1 draws 7 amps, and #2 draws 4 amps.

Now as to cooling, with the coach at 65 degrees inside, the AC exhaust temp was 57 degrees with only #1 operating at 4 amps. But when I reversed the compressor connections, there seemed to be no cooling even though the compressor was drawing 7 amps.

With 122 volts at the compressor connections under all circumstances, compressor #1 draws 4 amps normally and 7 when the connections are reversed, could something in the board cause the drop in amps? Looks like it is time to install the front board in the rear AC of ship the board off to Flight Systems, Inc. for service.

Richard, I feel guilty that you are spending so much time on this, but you have taught me a lot.

[Richard]

Interesting
When you say common, do you mean neutral (white) or ground (green)?

The compressor connections have both hot and neutral
Going through the board. So I don't really understand what you described

[RussWhite]

As I recall, the logic that prevents the 2nd compressor from running in low is not in the thermostat. Rather it is in the circuit board at the A/C unit. It just knows the T/S is on low, so blocks the signal to run the 2nd compressor. Russ

[Chester Stone]

Richard, the commons I am referring to are the white wires going to the compressor connections. The white to the #1 compressor is connected to a bundle of whites including the incoming power. While there is a green ground wire at the thermostat that connects to blue, I have found no green at the board.

Looks like Russ is correct even though I do not understand why.

Later this afternoon I ran the rear compressor in low for a longer period and while the fan worded, I doubt it was cooling. I will run again in the morning and if it is not cooling, it's time to swap out the boards between #1 and #2.

[Richard]

I am probably dead wrong.

I can't help but think, when you hear hoof beats thinks horses. No cooling and low amperage on a compressor would be classic low or no gas in the system.

But, you have confirmed numerous times, that the low amperage follows the wiring on the board, not the wiring to the compressors.
So my idea doesn't make sense.

The second situation where the fans started working on low is also puzzling.

That neutral wired liked that, is very interesting. And different than the SCS units I have had my hands on. I might take the time to unfurl all that wiring tomorrow when switching boards and truly understand what goes where.

You really cooked up a good one here !

[Richard]

Next time I'm with an electrical engineer, I am going to get some tutoring relative to the control schematic on Page 5 on the installation manual.

The answers on how the board works are in that schematic, I just can't noodle it out.

[Richard]

I haven't figured out the answer to Chester's issue, but I enlarged the control schematic to try to noodle it out.

Using colored markers, I discovered that indeed on the LOW setting Compressor 2 is prevented from starting even if a signal is sent to start it. Relay 8 is activated when the fan is on LOW. It took a magnifying glass to discern Relay 8 is normally closed. Relay 8 interrupts the signal from the thermostat to power Comp 2. When the relay is activated the trigger to close the power relays to Comp 2 is interrupted.

In studying the control schematic, I can see that problems in the logic would occur IF the neutral for the outdoor fan is wired around the circuit board. Relay 2 switches the common when going from LOW to HIGH. I am really keen to examine the wiring now from the board to the motors, all four of them. All of the commons should originate from the board. If they don't I would fix that before doing any more diagnostics.

My head hurts.

I'll draw more careful sketches in color tomorrow and post them.

This has been a GREAT learning exercise for me.

[RussWhite]

Richard,

I should have driven over to see you on the way down to the Keys. I could easily have walked you through that schematic. But, I really think if you just relax and make some notes on what each relay does using the name given on the schematic, you will get it.

As we discussed so long ago, the only real problem is that the circuit board was just undersized for the current of the compressors. Going to external relays for the compressors and fans will result in a circuit board that will outlast us. Let the board just do the logic and let the external heavy duty relays carry all the load. Problems over! Russ

[Chester Stone]

I am so sorry, but I think I have wasted a lot of your time since I have been looking at this incorrectly. I looks like there were two problems that got me confused. The original problem is that nothing worked with the thermostat set to low. With the lines going to the compressors reversed, that problem went away for some unknown reason. Then I got confused between the leads to the compressors and the connectors. I was checking the amperage at the connectors which changed when the wires were reversed. Thus the low 4 amps moved from #1 "connector" to #2 when compressor wires were reversed. Duh! It's the compressor I am switching not the power source. Thus Richard is right, a compressor system problem not the board. Probably low on gas, but testing the system will tell.

As to unconnected commons, this is logical since the two compressors are on different 110 circuits at the breaker box.

Again, sorry.