[Richard]

I have long been concerned about the short cycling of the front unit on our coach. The unit would run for some period, then turn off. But if the sun were shining on the front of the coach or the passenger side, it would come back on in as short as 30 seconds after shutting off. 

This is very very hard on the compressor since there is not a pressure bleed mechanism in the unit.

I isolated the problem to the thermostat. The temp actually changes that quickly as soon as the unit stops. The tstat SHOULD have a delay on restart built in, but it doesn’t. The Coleman Mach replacement does not have a delay built in. 

I tried insulating the tstat, insulating the tstat temp sensor, leaving the fans on all the time. Nothing I tried really helped. If this were a 24VAC circuit then delay on restart timers are plentiful, but nothing I could find for 12VDC. I cobbled up an adjustable timer with 4 channel relays, but it did not have the program features I wanted, but it worked better than nothing. 

Then I stumbled onto this little 12V timer. https://timers.shop/Multi-Functional-3V-..._p_77.html. There are versions that have internal 10 amp relays also. 

Perfect ! It takes 4 of them, and you have to program it. You can buy the computer adaptor to program, or spend a little more for the WI-FI programmable version. This device is inserted between the thermostat and the basement unit. 

I used Program 36. The 36 program will not delay the first time the unit comes on, but will delay on subsequent restarts. I used 5 minutes as the delay time. EDIT: If you do this, let me know, and I will send the CONFIG file with the program and other setpoints to save you time.

It is FAR easier to program than the Chinese derived instructions you get with the typical relay board. This timer is made in USA and instructions and videos were done by native English speakers. 

I wired it to enable one more feature. I wired an on off switch to the timers. That way, I can turn off the AC units without turning off the Tstat. The Tstat retains it’s setpoints. I turn off the AC to shut down the generator. Rhonda turns the switch back on when she sees the shore power light. It’s minor improvement but it is sweet when you do as many campgrounds per year as we do. The ACs come back on immediately at the setpoint we left in the Tstat. 

I post this because this is an incredibly versatile timer relay for 12VDC use. We have a number of electronic gurus on here that “improve” 12V features on the coach. 

I have no interest in this outfit but it is a really neat product. For the electronics guys it has two built in features that make it way easier than building your own. One, it has built in pull down and pull up resistors to make the trigger inputs behave consistently. Two, it has a built in diode to prevent the circuit you are turning on from spiking the voltage when the circuit turns off. This is a real problem in controlling the basement unit since the tstat is turning relays on the circuit board on and off.

Yeah I know, install mini splits and this problem goes away. I am not ready to go down that path yet.