[Richard]

I have the basement SCS air conditioners driven by the PSG thermostats. In certain climate and sun load situations, the AC units will short cycle. Meaning they come on briefly, shut off quickly as in under a minute, and a minute later this behavior repeats This is VERY VERY bad for the compressors. It forces the compressor to start against a high head pressure. 

I investigated all manner of fixes and solutions. I looked for flaws and shorts in the control boards. I insulated behind the tstat. I checked voltage to the tstat. I built boxes around the tstat that helped but did not gain approval of interior consultant. I mounted a small fan to blow on the tstat, which helped but again was nixed by the aesthetics committee. I did observe the root cause to be the actual temp on the tstat as displayed was changing rapidly downward by a few degrees anytime the AC came on. This was happening on both the front and bedroom units. A replacement tstat of another brand did not help. 

In house based units, there is a common and easily obtained "delay on break" module that is inserted between the tstat and the outside unit that prevents this from happening. Once AC is called for, it keeps it on for a predetermined and adjustable time, before it allows the unit to turn off. The problem is those delay on break modules work on 24VAC. Our tstats are 12 VDC. Trust me, the 24 VAC delay on break modules do not work on 12 VDC. If you want two, I can fix you up. 

I was on the verger of using an Arduino micro computer to build my own thermostat. But I found this https://www.amazon.com/Multifunction-Mod...18DC&psc=1

What is this? It is a four channel, since you have four AC control wires out of the tstat, highly programmable timer module. I have set it up to run the fans and compressors for at least five minutes any time they are activated by the thermostat. I installed it in the cavity behind the tstat. It does not replace the thermostat or the SCS control board, instead it is placed between them. 

If you decide to participate in this malarkey, there are a few things you should note. One, the instructions are translated from Chinese into Chinglish. It took me days to figger out how to program the thing. I will include a link on how to program it. Follow the instructions to a T, or proceded at your peril. https://docs.google.com/document/d/11ZS5...sp=sharing

The next thing to note if you look at the back of the board picture.

       

Is that I soldered some small wire from the 12V input terminal to the center runs on the relays to minimize external wiring. EXCEPT, you can generate an unwanted situation with the two speed fan when using a delay on break logic. If the lo speed is programmed to run for five minutes after it loses the signal from the tstat, AND the tstat ramps from 1 compressor to 2 compressors which automatically turns on the high speed fan, then the fan motor will be getting voltage to both sets of windings. NOT GOOD. The solution is to wire the 12V hot from the Normally Closed side of the HI Speed relay to the center post of the Lo Speed relay. That way, IF the high speed fan comes on, the power to the low speed is interuppted 

Here is the high technology test session under way.    

[folivier]

Another excellent Richard fix. In my '93 one of the thermostats was directly in the air stream for one of the A/C vents. And it never kept a good temperature until I moved the thermostat to a different location.

[hbens]

I am interested in seeing this fix. The first paragraph describes my operation dang near to a T.

[Richard]

I ran into a slight glitch in my approach. Because the relay board is between the Tstat and the control board on the AC unit, turning off the tstat, does not immediately turn off the AC unit. As built the AC unit would continue to run for five minutes.

I didn’t want to wait 5 minutes to shut down the generator, and I wasn’t going to shut it down with a load on it.

The solution is very simple and straight forward. There is a terminal on the board labeled END. If you short that terminal to 12V Positive, the board opens all the relays. I added a surface mounted toggle switch at the tstat to be able to shut the AC unit down quickly.