[Chester Stone]

I have stumbled onto something that does not look correct and may be the problem or at least one problem.  I have worked today to prepare two schematics in color.  One is the wiring for the front A/C which seems to work perfectly.  I used the wiring directions supplied by Newell when I replaced both thermostats a few years ago.  The other attached picture is the schematic for the rear A/C which shows a small but perhaps significant difference.  On the front schematic picture, note that pin #4 on the control panel is blue, which is 12 volt negative ground.  On the rear schematic picture, pin #4 is a white wire.  Every control panel schematic that I have found shows this pin as blue.  At the rear thermostat location where the thermostat wires connect to the coach bundle of wires, there are several wires that are not used, are cut short and white is one of them.   I reason that the rear A/C has not operated properly with two compressors since it left the factory.  I will try to verify that the short, cut white wire at the thermostat is the same one attached to pin 4.  If so, I will connect it to the blue thermostat wire group.  Thoughts please.
       

[Richard]

Can you clarify a couple of points.

Schematic number two was done by you or Newell?

Are you saying that pin 4 on the control board is connected to the white wire?

Are you saying that the white wire is not connected to the thermostat?

The blue wire as you have said is 12 neg. Is the blue wire or green wire hooked to the thermostat in any way? In other words if the tstat is not getting 12 neg, a ground, then how is it functioning. We know 12 pos comes from the heavy red wire.

The white wire according to the RV comfort thermostat manual is for Heat, the white wire coming from the Tstat should trigger the Aquahot rear zone I think, but not sure.

[bikestuff]

Chester,

In the category of for what its worth.....

Like Richard says, there will be differences between the front and back wiring because of the heater.  

I have included a quick sketch of my wiring.  (Thermostat on right and wires going into the wall on left).

The blue wire that is in the grey cable is connected to green and to a bigger blue wire.

My thermostat does not have a High / Lo control so my Red wire is capped.

Hope this helps!

bill

[Richard]

I am always hesitant to recommend to connect wiring based on color because of what Bill said.

I think you could easily disconnect the thermostat connection to the control board, and check it with a voltmeter, before you took a chance based solely on color.

From your previous posts, you already know the scheme of wiring. There is a nice chart in the manual that shows you exactly what pins will be hot at the various thermostat settings. Look at the logic table on page 5 for the specifics.

[Chester Stone]

When I installed new thermostats several years ago, Newell supplied the wiring instructions as pictured below.  I wired the front and back thermostats based on these instructions, with the front working perfectly and the rear having the problems which I have detailed.  I prepared the color schematics posted yesterday based on Newell's diagram and the way I connected the wires.

I am going to do following today, and then I will get back to you with my findings and answer the questions you have asked.  I will first determine if the white wire at the rear pin #4 is 12v negative.  If yes, then I need to know if it is spliced somewhere in the coach to the blue wires at the thermostat (continuity).  If yes, then I am back to square one and will need to swap control boards which takes some time.  Second, I will verify that the front thermostat is wired as depicted in my schematic.

I cannot thank you gurus enough for the time and effort you have put in to help me solve this issue.
   

[Chester Stone]

Well, I am out of luck. The white wire in #4 pin is grounded, and so is the blue and large green wires at the thermostat. I checked continuity, but since they are all grounded, there is continuity. I also checked the front thermostat wiring, which runs as it should, and both thermostats are wired the same. Since both rear compressors run at 8 amps when connected to the number 2 compressor connection, I can only conclude that any problems are in the circuit board. I think I will call Flight Systems on Monday to verity that they still rebuild CSC boards. On the internet, I note that the rebuild price has gone up from $85 four years ago to $195. I may switch boards next week before sending it off for repair. Any other thoughts? Again, thanks guys.

[Richard]

you might think about a couple of things.

One, if there is a high resistance on the board between the incoming power and the outgoing to the compressor, not only would it result in a low amperage to the compressor, but the voltage going to the compressor would be reduced when the compressor was running. If you checked voltage at the compressor leads but without the compressor running, it would read the same as the incoming. Only under load would you detect a reduced voltage. If you look carefully at the board where the compressor plug is attached you will see two little gold colored dots. If you can get your probes on those dots, they are there for checking the board. Otherwise, you might can get your probes into the splices to measure voltage. If you do have a voltage drop on the board, the first place I would look is on the back to see if the circuitry is burned or compromised. The second likely culprit would be the compressor relay.

If the above test is inconclusive or not possible, I would switch the boards and see what happens.

[Chester Stone]

Richard, I will do what you suggest and check the voltage with the compressors running in their original configuration and with the leads switched.

After thinking about this issue over night, I decided to check the voltage output at the thermostat to the board under high and low settings. Then I will check to verify that the output from the therm is being received at the pins on the board. I think these should all be 12v readings. It seems to me that this should be easy to do and should tell me if there is a problem with the therm or wiring to the board. I will also verify that 12v power is coming into the compressor 1 section of the board. I should have thought of these checks before, but with your help I am learning as I go.

In an earlier post I pointed out that when the therm is on high, the amp draw by compressor 2 is about 8 amps with compressor one drawing 4 amps. Recently, with the therm set on high, I disconnected the wires to compressor 2, and compressor one drew no amps. Strange. This led me to question the output from the thermostat. The above voltage test should also be interesting.

[Richard]

It could be one of two things.

One, low speed blower signal from the tstat is not getting to the board. This may be consistent with the very first post in this thread. NOTHING happens on low. The circuit board has some logic in it, that I am not savvy enough to see in the schematic, that prevents the compressor from starting if the fans are not running. It's logic that prevents the compressor from burning up in case the fans don't come on.

Two, the circuit board logic is all flummoxed because of a failure in the board.

I really like your idea of measuring the voltage at the board coming from the tstat. That verifies both tstat and wiring continuity.

Use that logic table mentioned above to know which pins should be hot under the lo and hi settings. You might also check blower only settings on the tstat. Don't forget, there is a three minute or greater delay before anything happens. And there must be a two degree or higher differential to kick high mode on.

You will soon be the guru of SCS control boards.

[Chester Stone]

The thermostats seem to be OK. On both the front and rear units, pin 1 shows 13 volts when on low, nothing on pin 2, and the reverse when set to high. Interestingly, pins 5 and 6 both show 13 volts on both low and high. I guess the fans do the switching of current to the compressors. Today I will test for volts going to the compressors while running on high and low as configured by the factory and with the compressor leads switched.