Converting Coleman Mach to thermostat controls

[davescounry]

I did a search for this issue, and see some that are close but not quite the same. Just curious if anyone here has had this specific situation.

I am in the process of installing one of the Airxel Circuit Boards that will allow me to connect my A/C to be thermostat controlled. My particular thermostat is a digital LUX household thermostat. Currently, my t-stat only controls my heater, and my A/C controls are located on the ceiling unit.

Current wiring to the thermostat consists of a single red and single white wire coming off of my heater. So, that is what is giving me my 12VDC (+) and my heater control.

This thermostat wire goes up the wall, past my A/C and over the the thermostat, so I am going to tap right into that wire. Here is where I am a bit confused. The new wiring says I need to connect the control board to the thermostat as follows :
A/C -> green wire-> t-stat for , I believe, high A/C fan
A/C -> gray wire to t-state, I think this is low A/C fan
A/C -> yellow wire to t-stat, this should be A/C relay
A/C -> blue wire to t-stat - this is labeled as 12 VDC(-), and then the diagram shows that there should be a 12VDC(-) connection to the t-stat.

For the most part, I am OK with the green, gray and yellow. Although, probably at first I am simply going to wire the green and only have high fan.

My question is on the blue VDC(-) wire. Since I am using a household t-stat, I don't have a VDC(-) wire running to it. However, at my heater, there is a straight connection to a 12VDC(-). I am wondering how important it is for the control panel on the A/C to connect to this at the t-stat? I could easily connect the new control panel straight to the VDC(-) at the same connection point at the heater and bypass the t-stat.

I saw one thread where someone actually installed the exact same thermostat from an analog t-stat that already had A/C control. The analog t-stat had a blue wire connection. In that case they simply capped off the blue wire and made no connection at the t-stat, but I have no idea if there was any VDC(-) supplied to the A/C elsewhere, or even if it is needed.

See the image for what I am describing, and the blue line is what I am proposing.

 

[cookie]

I have some thoughts on the blue wire.
These are just my thoughts and not necessarily correct but based on my experience installing a couple.
If your thermostat requires batteries to power it you don't need the blue wire.
If your new thermostats requires 12 volts but does not use batteries then you will need the blue (-) wire.
You can always try without the blue wire and if it doesn't work add the blue (-).
Also you can add a high/low fan switch. Check out our manuals for the switch wiring.

Peace
Dave

 

[wdk450]

My post #2 on this thread has links to an older thread with instructions for the Dometic / Hunter thermostat upgrade: https://heartlandowners.org/showthread.php/76462-Dometic-thermostat-fix?

The big thing I found is that the dometic hvac system has -12 volts for the thermostat controls supplied through the AC/Heater control board. The relays etc. in the control board have +12 vdc connected to them and the thermostat closes contacts to -12 vdc to make things work. These AAA battery operated thermostats don't care if they are switching -12 vdc, +12 vdc or 24 vac (typical house hvac). They just close contacts to make the external stuff happen.

 

[jbeletti]

I removed my Coleman thermostats and replaced them with Honeywell thermostats that I could expose to and control by my home automation system. Here's some pics of original trailer wiring to the thermostat before I removed them. Unsure if they'll be helpful at all:
https://beletti.smugmug.com/RV/2020-Sundance-262-RB/HVAC/HVAC-Controls/OEM-HVAC-Control-Wiring/

And here's a link to some pics of how I interfaced the OEM wiring to the new Honeywell thermostat. Again, unsure if they'll be helpful - but maybe
https://beletti.smugmug.com/RV/2020-Sundance-262-RB/HVAC/HVAC-Controls/New-HVAC-Controls/

Finally - attached is a manual for the Coleman Mach thermostats. Perhaps they'll shed some light on your needs.

 

[davescounry]

All - first, thanks for taking the time to reply. A few more questions and comments from me.

First, COOKIE, the link you provided is the exact t-stat I have. In that example, it appears as if the blue wire going to the original t-stat is simply left disconnected. My t-stat is battery powered. So, as I am learning more, I think what I am pondering is whether or not the blue has any purpose in the functionality of the A/C, or is it just there to close the loop on the t-stat. From your comment, it would seem you think it is not relevant to the operation of the A/C.

So, I guess I would then ponder, does it do any good or harm to connect the blue wire that leads to the A/C to the VDC(-) that is not connected to the t-stat. Kind of like in the example in the image in my first post.

Second, WDK450, in the link you provided to the the "Upgrading analog..." thread, that is exactly the wiring I have right now, which gives me heat only. Based on the rest of your comments, since my t-stat is battery, can I gather you don't think the blue VDC(-) is needed? As with the above, should I hook it up outside the t-stat loop per my comment?

Third, jbeletti, thanks for the pics. I can't quite tell but it appears you are dealing with a couple t-stats? Some of these pics are kind of throwing me. In your second link, pic 11, I don't see the blue wire, but then it appears in pics 17, 18, you have the blue wire going to W, which in my case I have a white wire (Heater control) going to W. It looks like your heater control is going to G, which for me I think would be feeding my A/C. So, I am really just confused. I don't know if this is just a difference in systems or what.

Again, all, really appreciate taking the time to give me some feedback.

 

[jbeletti]

jbeletti, thanks for the pics. I can't quite tell but it appears you are dealing with a couple t-stats? Some of these pics are kind of throwing me. In your second link, pic 11, I don't see the blue wire, but then it appears in pics 17, 18, you have the blue wire going to W, which in my case I have a white wire (Heater control) going to W. It looks like your heater control is going to G, which for me I think would be feeding my A/C. So, I am really just confused. I don't know if this is just a difference in systems or what.

Pic 11 is of the replacement t-stat. I had to do some wire extending for that one and likely had to do some wire color changing based on what wire I could find at the store.

 

[cookie]

All - first, thanks for taking the time to reply. A few more questions and comments from me.

First, COOKIE, the link you provided is the exact t-stat I have. In that example, it appears as if the blue wire going to the original t-stat is simply left disconnected. My t-stat is battery powered. So, as I am learning more, I think what I am pondering is whether or not the blue has any purpose in the functionality of the A/C, or is it just there to close the loop on the t-stat. From your comment, it would seem you think it is not relevant to the operation of the A/C.

So, I guess I would then ponder, does it do any good or harm to connect the blue wire that leads to the A/C to the VDC(-) that is not connected to the t-stat. Kind of like in the example in the image in my first post.
.

I guess there would be no harm to ťry without the blue wire.
If it don't work connect the blue.
When I did some work at home here the thermostat instructions explicitly said the blue wire must be connected. It did not work till I hooked up the blue wire.

Peace
Dave

 

[wdk450]

I believe (at least on Dometic systems, and the thermostat wiring is supposed to be pretty much universal) the blue wire is used in 2 speed fan systems for Hi fan. Low fan is activated by the brown (or gray) wire connected to -12 vdc through the thermostat switching.

I did a search on Google Images for "RV HVAC Thermostat". There are a number of Coleman as well as Dometic diagrams. Unfortunately, most of the images there are in a format this forum doesn't support (jfif), so I couldn't post them here.

 

[davescounry]

Adding some info in case anyone in the future needs this as a reference.

Of course, I picked the perfect 90+ degree day to be working on this in my driveway. High motivation to get this wired up since it was probably another 10 degrees inside.

The wiring schema in my first post was the correct identification. The blue wire, which was most of my questions, does need to be ground to a VDC(-) in order for the control box to work. However, it does not appear to need to go to the t-stat.

I first wired everything without any connection on the blue wire. Testing V at the control box, I was getting no reading on either the Yellow (a/c) or green (high fan) wire. I then connected the blue wire to a 12 VCD(-) connection at my heater, and directly connected that to the control box. This seemed to do the trick, and it has been working for about 5 hours now.

In this iteration, I have not wired in a switch to toggle between hi fan and low fan. I may see if I can find a SPDT rocker tomorrow and mount it next to the t-stat.

Lots of research went into this, and I never seemed to come across my particular application. It seemed that there were two similar, but slightly different applications -

1. one set were people adding the control box to a Coleman-Mach t-stat in which case the t-stat had a connection for the blue wire.
2. The other set seemed to be folks going with an aftermarket t-stat, and just tucking the blue wire in the wall. I guess in those cases either their control box did not require that ground, or that wire was elsewhere grounded and still supplied connection to the control box.

Like I said, at least it is working now. We will see if anything malfunctions.