A/C and Electrical gurus- need some guidance

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Cuerno Largo

Cuerno Largo

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Apr 20, 2004
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I have a 5th wheel RV that I will be living in soon. It has two A/C units. The main unit had a thermostat and I already replaced it with a programmable stat.

The second unit is a Coleman Polar Mach III. It does not have a thermostat, but rather a dial to twist to a "temp" that turns the compressor on and off while the fan runs the entire time.

I would like to install a programmable thermostat to this A/C unit so I can better control temps in the trailer.

I have seen some vague instructions online that I don't necessarily trust.

I guess there are 2 possible scenarios that I would be OK with.

1) Leave the fan running constantly and use the programmable thermostat to turn the compressor on and off. This would be OK, but ideally I would like.....

2) for the fan and compressor to turn on only when the A/C is needed.

Anybody have specific instructions on how to accomplish either scenario?
:cheers:
 
are the controls onboard the a/c in question line voltage or low voltage? If they are low voltage, then the dial switch could be replaced by a standard stat. If the controls are line voltage(120/240), then it will require a bit more ingenuity or a full voltage type of t stat.
 
I am not sure, but I think low voltage as people have not been using line voltage thermostats from what I can gather. They are using battery powered programmable stats like Hunter makes.

Here is a cut and paste from another site that I found. I do not have a furnace for this unit so I would exclude that part. There seem to be a hundred ways to do this but this seems to make the most sense to me so far. Edit: Looks like he is using line voltage stat?

(A/C = air conditioner, AC = 110 volts)

My A/C fan switch has four wires, a black hot AC input from the breaker, a purple AC output to the compressor, a red AC output to lo-fan winding, and a black AC output to hi-fan winding.

The original A/C thermostat has two white wires, a white one from the compressor AC neutral, and a white one to AC neutral.

With the fan switch in either hi or lo position, no power is supplied to the purple wire, so no compressor "enable".

With the fan in either lo cool or hi cool position, the purple wire is AC Hot, and goes to the compressor ("compressor enabled"), but the compressor won't turn on until the thermostat says so, because it's looking for an AC neutral, which it finds through the original thermostat.

Someone mentioned earlier that an A/C that they were familiar with required one winding energized for lo-fan and both windings energized for hi-fan. I don't know this from experience, but it wouldn't be a problem either as a DPDT switch can be used as a hi-lo fan switch instead of the SPDT switch that I used.

All of my connections to the original wiring in the A/C used crimp on male spade terminals to plug into the existing female spade terminals, so nothing I did is a permanent change, and can be restored back to factory config in a few minutes by replugging the existing connections. I did choose to physically remove the fan and thermostat controls, and cover the holes and plates to tidy it up, which only involved 5 screws. (This also prevents someone from twisting the knobs and getting no results.)

The only thing I had to add, other than the three SS relays and the digital thermostat itself, was a 22 gauge 3-conductor cable from the digital thermostat location to the SS relay location in the A/C through the existing AC supply conduit in the ceiling.

When completed there are five wires going to the new digital thermostat, two from the furnace, and three from the SS relays in the A/C. Wiring it up could pretty much be described verbally without a schematic.
 
He is using three relays, which may be controlled by the stat. The t-stat is easy enough, but if your not familiar with electrical components, then the relays could be a tad overwhelming.

How about some pics of what you got?

Identifying the wire size for t-stat control may identify if it is line/low voltage.





Plethora!
 
pics

ac1.webp
ac2.webp
ac3.webp
ac1.webp
ac2.webp
ac3.webp
 
I was hoping for pics of the electrical components, any chance you want to email coleman for a wiring diagram/schematic? I did a quick google search and didnt find anything.

I guess to be more specific it would help to see whats there (electrically) for the fan and for the compressor, and whats there for the thermostat contact/controls.
 
124 volts is good, usually you want a bit more voltage than less.

But it tells you that it doesnt have a control transformer, andif there is a starter it will be 120 volt input. It should keep things simple.

Now what exactly did we want to do again? (ill re-read the first post and then give ya some more direction)
 
Last edited:
Ok, if the t-stat has 120 volts (124) then you will need a thermostat that will handle line voltage (120) to replace that one. Im not sure off the top of my head if programable line voltage t-stats are hard to find.


If you want to be able to have the compressor and fan kick off, you will need to find the wiring and a way to interupt it. We may be able to interupt them both with the same thermostat, or run a relay in the compressor wiring from the t-stat to shut it off.

Now it would be good to get an idea of what the wiring to the fan looks like and where we could put in a relay, or see if there is a way to get the thermostat to switch both. Sizing of both (amperage or watts) and voltage is important info to have before looking at t-stats so you know what you need.

You will need to know the amperage of the fan and compressor to make sure you get a thermostat that is large enough to handle them, or if you cant find one to get a relay sized apporpiately.

I would suggest looking at leviton, lutron and copper-crouse's websites for simple t-stats, not sure where to start for programable.

It would be nice to find a programable t-stat that has two sets of contacts, one which opens up later so the fan can run for a minute after the compressor kicks off.
 
Last edited:
I will get up on the roof and take off the cover to get some pics of the wiring and start looking for some line voltage thermostats. Thanks for the help so far.:cheers:
 
one option would be to use the supply voltage to the ac and pull a line from there to a plug where a control transformer could be installed and used to power a low voltage stat and relay. run the line voltage trigger circuit for the ac thru the open contacts of the low voltage relay which is operated by the low voltage thermostat. the relay could be installed in some spare space inside the ac, the transformer could be the plug in 120vac to 12 vdc that you can get at radio shack and the just run an lv cable to your new stat. Be sure and use the same circuit that powers the ac to take your control voltage from. If you wanted the fan to be able to run w/o the compressor, then you would need 2 relays- one for the fan, one for the compressor...This is just a basic run-downHTH
 
Lamb has it and if that is to much just use a step down transformer for the ac controls and a dimmer switch seperate for the fan
 
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