[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.

[Jack Houpe]

Looks like your determined to keep those SCS units so your fix will most certainly save your compressors and all the components feeding them, good info. I will be in Cedar Key this Feb-Mar for an extra hand if you change your mind, we cruised into the campground with the front 2 ton units blowing cold the only problem is the wifi control doesn't work when driving down the road as its starlink and I can only control one of the evaporators with my remote control from the drivers seat, gets chilly if you set it wrong. Dig, rub  Kathleen follows pulling our jon boat. 

While you had your front suspension tore off the coach we at the rally started brain storming some options for the old dometic split units that are failing, I have a 1 ton 110vac inverter compressor I got from my son years past its perfect but his evaporator got plugged up with the truck bed material he sprays at his business. So Steve M and I considered removing one of his old evaporators in the back of his Prevost so I can experiment with trying to just replace the compressors of the dometic unit and leave the evaporator. If it works the older coaches will be brought up to modern state pretty darn easy, if it doesn't then nothing lost.

[Gnawrocki1]

Richard, this does solve the restart issue for the compressor and should help with the life.  Although I have not noticed that mine cycles at the rate you stated, I can imagine there will be conditions like you state that it would happen.

Do you happen to have a wiring diagram for the 4 time delay relays + on/off switch?

Do you know that when the power is turned of does the timers reset?

Thanks for your research!

Glen

[HoosierDaddy]

"While you had your front suspension tore off the coach we at the rally started brain storming some options for the old dometic split units that are failing, I have a 1 ton 110vac inverter compressor I got from my son years past its perfect but his evaporator got plugged up with the truck bed material he sprays at his business. So Steve M and I considered removing one of his old evaporators in the back of his Prevost so I can experiment with trying to just replace the compressors of the dometic unit and leave the evaporator. If it works the older coaches will be brought up to modern state pretty darn easy, if it doesn't then nothing lost."

Jack, I am very interested in your findings from this research....

[Jack Houpe]

"While you had your front suspension tore off the coach we at the rally started brain storming some options for the old dometic split units that are failing, I have a 1 ton 110vac inverter compressor I got from my son years past its perfect but his evaporator got plugged up with the truck bed material he sprays at his business. So Steve M and I considered removing one of his old evaporators in the back of his Prevost so I can experiment with trying to just replace the compressors of the dometic unit and leave the evaporator. If it works the older coaches will be brought up to modern state pretty darn easy, if it doesn't then nothing lost."

Jack, I am very interested in your findings from this research....

It will be after the first of the year, Gordon called yesterday and found the dometic evaporator. I'll start a thread on it. 

[Richard]

Glen,

Drafting was not a subject I excelled in. Here is my feeble attempt. Only the timer relay power is switched on and off. When the relay power is switched off and on, the timer does reset. However, using PRGM 36 in the timer setup results in the relay closing immediately upon repower IF the trigger is present. Only after the first trigger initiation does the delay come into play on the second occurrence of the trigger. That way, I turn off the AC’s when entering a campground, then turn off the generator. When plugging in, Rhonda switches the relays on, and the AC comes on immediately.

SCS timer:relay schematic.pdf Size: 1.03 MB  Downloads: 10

The blue trigger wire on the timer/relay is NOT used in this application.
The white wire on the timer/relay is the data port

The white wire on the thermostat is for the Aquahot heat and is not shown on the schematic.

DO NOT hook the white wire on the thermostat to the white wire on the timer/ relay.

EDIT: One more thing, I made up a little panel to hold the timer/relays. I used terminal strips for both the in triggers from the tstat and the outgoing signals from the relays. That way, if the circuitry goes haywire, or I sell the coach. I can connect the tstat wires back to the original wiring in seconds.

[ccjohnson]

On the Dometic 90's era compressor; Newell was using a replacement compressor from Johnstone. It was pretty much drop in unit once the drier filter that had been plugged with solder was replaced.

[HoosierDaddy]

I have installed two of these.