1a: True Air thermostat Model 6535-344*

Update: July 2023 – Switched to the MicroAir Smart thermostat, see 1b: MicroAir

See update below: The system thermostat is located in the center of the RV, between the bathroom area and the kitchen/dining room area. It’s mounted on the EMS (Energy Management System) panel. It’s the TrueAir Coleman Mach central heating and air conditioning controller.

This device controls the heat pump and the propane furnace, of which my RV at 36 feet long and two slides only has one. Larger RVs probably have two propane furnaces. There is a programmable device inside the thermostat with a memory that handles all the timing required to operate the system to make it as comfortable for the user as possible. It also controls fan speeds and the compressors individually to reduce energy needs. It’s adaptable to shore power from 20 to 30 to 50 amp. And there are several timers that are triggered or not triggered under certain circumstances. It’s hard to know what state it’s in sometimes because of the complexity.

Here’s a look at the connectors involved. Note that one is 3 terminal…that’s the +12 volt, -12 volt (aka ground), and the ‘low gas heat out’ connection. It’s the power and ground source for the TrueAir thermostat. The other 9 pin connector has 8 wires all going down to the heat pump. I first pulled the 3 terminal connector to shut off power to the thermostat, then disconnected the 9-pin.

It’s a pretty slick thermostat function wise, but I’ve noticed that it can go bonkers in certain situations. Over the 3 years I’ve owned it, I’d notice it would occasionally start doing things incorrectly or not at all. Usually I’d mark it up to my lack of knowledge, shut it off, and walk away. A day or two later, it seemed to function correctly. Leaving it off  a while though powered seemed to do the trick most times. But not always. Not exactly sure why, but it seems that sometimes it needs to be reset somehow…and the exact method to do that is sort of a mystery. According to the Coleman service book though, it can’t be reset. Or maybe they mean some things can’t be reset. I’m unclear about it at present.

Recently, (April ’19) I removed and did quite a bit of maintenance work on the heat pump, new bearings, capacitors, clean up. After that was finished, I noticed that in the cool mornings when I wanted to run the heat pump in the ‘Elec Heat’ mode, it wouldn’t start whereas it had always worked before. In addition, before I had done any work on the heat pump, I’d noticed that the propane furnace wasn’t working where it had before. So I had no heating except from some small electric floor heaters I had. Of course I checked the last thing I worked on involved with it…the heat pump…but I couldn’t find any issues with my work or the wiring. I’d paid particular attention and care tightening and cleaning all the connectors and terminals down there when I worked on it so I felt the problem resided in the TrueAir thermostat device and it was a coincidence that it showed up just after my work on the heat pump’s main unit. Then again, I could have jostled something or removed or applied power in the wrong sequence and it didn’t like that. Who knows?

The thermostat is the device common to both those non-functional items so I concentrated some effort working on it near the end of June ’19. Here in the mountains at 6500 feet where I was staying, it gets down to the 50’s F by morning so I’d like to have some heat when I get up.

After reviewing the documents I had on hand for it, it was clear as mud so I posted a question on WinnieOwners forum and got some help that way. I was reminded that most of those types of problems were caused by the slide switch on the thermostat so I disconnected and removed the board and inspected and cleaned the switches…it looked fine where I could see the contacts and the contact spray didn’t change the symptoms. I checked the resistance measurements of the switch too and they were fine. So that slide switch didn’t seem to be the problem in my case. After that, most of my time was spent concentrating on reading and following the repair charts and schematics shown in the heat pumps service guide.

Everything I tested using the ohmmeter seemed fine so I plugged it back in where it belonged. Still didn’t work. There’s a section of the service guide that suggests you can short (bypass) a section of the circuit in order to test the thermostat but everywhere I found that test in the various documents I found online, there’s confusion about the procedure or the model numbers and/or it doesn’t apply to my setup. It states I should short pin 1 to pin 3 when testing the heat pump function but doesn’t say which connector or PCB pad they’re talking about, my setup doesn’t have a pin 1 installed in the main 9-pin connector shown below in Figure 1 for instance. Assuming I’m looking in the right place, it’s just not clear in my documents.

Even the above document is not right for my RV even though the part number for the thermostat is mostly correct…the lower connector shown above is providing power to the thermo and also has a wire used to activate the gas furnace is not a 4-pin. Mine is 3-pin. I think that 4th pin would be for RVs with 2 propane furnaces but the docs never mention that. And it never explains the difference between ‘Low’ and ‘High’ Gas Heat out either. Though I can pretty much guess what that means.

Another thing: See that “*” after the model number in the above drawing? That’s suppose to be the revision number. My experience with these RV devices is that there’s suppose to be something there on mine, even a ‘0’ if that applies. But on the paper label stuck to the back of my thermostat it just shows 6535-344 and nothing else. Gah!

At least the above drawing gave me a clear guide to the heat pump control wiring for my RV. With everything plugged in, and the slide switch set to ‘Elec Heat’, I jumpered from the +12 volt at pin 1 of the 3-pin connector to pin 8 of the 9-pin connector. Waited for a bit and viola’! The fan kicked on! Success!

Following the test chart in the service manual, if the heat worked with the jumper that indicated that the thermostat is bad. Oh, bummer. That’s disappointing. Anyway, I let it run like that for a few minutes, and then removed the jumper. And….it kept working. After a couple hours and a couple tests involving resetting the temp later, it’s still working. So I must have accidentally reset the brain somehow and that fixed it or reset it or something. Or at least that’s how it seems right at this point. I won’t know if it’s fixed until a few days of proper operation has past. I’ll leave it set for 68 F at night from now on because the RV does get colder than that near sunrise here. If it’s at 68 F in the morning, I’ll know it’s working. And of course, I’ll set it for 72 F or so after I’m up too. [Edit: Next morning the house was at 67 F, I set it for 74 F and a couple minutes later the heat came on. So it’s working fine. Next job is the propane furnace.]

So that’s it for now, but I gotta say that the documentation for this device, coupled with the documentation for the heat pump just don’t jive. Somehow my system must have slipped through the cracks and no amount of internet searching brought me to the correct documents for my setup. Weird. It likely has to do with the various options marketing was always asking engineering to come up with but still…it’s good to have accurate documentation and Winnie should have provided that eventually.

What I think is going on with the thermostat brain is that it uses it’s memory in a stack setup. And as it goes though it’s routine checks of hardware or customer settings or ambient temp vs actual, it takes a reference value from the stack which drops the rest down. If something happens out of sync, that means the next time the brain pulls the next item from the stack it’s out of sequence, with the brain expecting to see one thing but the stack giving another, and that throws the brain off track and out of wack. And it sort of ends up trying to march into the wall at the end of an alley while still trying to play a band instrument. Only way to get around it is to totally remove power, or press the reset…if there is one (this board doesn’t have one). I’ve seen this sort of firmware mistake many times over during my electronics career so it’s an educated guess. I wonder how many 10’s of thousands of these TrueAir thermostats, @$168 new, have been tossed when all they needed was a re-indexing at the very least and a re-programming with corrected firmware as a permanent fix. Assuming that’s what is wrong with mine. It’s working now, and I’m hoping it will continue to work correctly. I’ll try to be careful with it too. Next time it goes crazy though, I’ll try removing the +12 volt at the thermostat and see if that resets it.

Update: Dec. ’21 – Removed bad thermostat and installed nearly new one

So I’ve had to resort to disconnecting power behind the panel for the Thermo several times this winter AND had to flip both CB’s (circuit breakers) for the basement heat pump as well…and then when I got down here to Tucson AZ and it gets hot in the afternoon, I’d need the AC and it wouldn’t start unless I pushed real hard on the slide switch. Even after I’d used contact cleaner on it. So I used an Exacto knife and cut away a tiny sliver of plastic to give relief for the edge of the switch handle to allow it to travel further (to perhaps a better area of switch contact) but that didn’t help much.

The result is that, finally, it’s time to replace the TrueAir thermostat. I got a lot of years out of it, nursing it along (see the stories above/below) and as a full timer, it’s had a lot of use. I have been following a company that makes a smart Thermostat for RVs, EasyTouch RV Thermostat, but they still don’t have one available to replace this TrueAir. I just checked today and still get the message that they are working on it. So, time to check the online markets. And I’ve found several that carry this old TrueAir and list it between $150 to $300. Gah! With no guarantee I’d get the updated or higher Rev model. In other words, I could get a new one with the same problems (except for the slide switch issue).

So I was complaining about this to my friend here in Tucson and he offered me his brand new, but removed TrueAir thermostat. I don’t remember why it didn’t work for him but he’d already bought a replacement that worked and this new one was a spare. Offered $50 for it but he refused and just gave it to me. Very nice of him. He thought it was blown but it doesn’t seem to be. I was just going to remove the slide switch to put into mine but tested it first and it’s working so far.

This is what they look like side by side. I’m a bit disappointed that the paper label on the back of mine suggests that the one I just got (and that John had bought NEW off the internet last year? or so) is actually older than mine. Produced by Coleman before mine it appears, AM7868-E20 vs AM7868-G15. My old one is the G15. Which would mean that it still has the old firmware in it so it’ll likely require the occasional unplugging and plugging back in that my old one on the right in this picture did…

They look exactly alike. Damn. Remember mine was likely installed in 2002, and John got that ‘new’ one in 2019. Sigh. Coleman, if you just refused to recall these defective thermos, screw you.

Anyway, just now installed it, it’s cooling fine. I’ll be able to test heating tonight when it gets coolish. I did add a furnace switch several months ago so this picture shows both the new thermo and the gas furnace switch below it. I added that because I like to be able to decide myself when to use gas, not have the thermo decide for me.

To use it, I slide the thermo’s switch to Gas Heat or Elec Heat, than flip that toggle switch below it to start up the gas furnace. One extra operational step but like I said, I like to decide when to use gas, not have the thermo do it for me. It’s now Jun ’22 and it’s working fine, haven’t needed to remove the thermostat and disconnect the connector at all so far. Hoping that is a thing of the past.

Update: Late August ’19

Just wanted to mention that the thermostat had been working fine for 2 months, at least in regard to the cooling function, which was basically all I was using it for at the time. And occasionally using the heat pump in the mornings setting for heat had been working fine too. Up until late August that is.

This time the heat wouldn’t come on when called for when it was in the 50’s outside, and not much higher than that inside the RV. Tried it over several cool mornings, later in the day, the A/C would still work though. I would move the slide switch to electric heat and several minutes later, it still hadn’t come on. So today, moved the slide switch to off, unplugged the thermo’s 3 terminal power connector and left it off for 3-5 minutes. Plugged it back in, a few seconds later slid the switch to electric heat, and a couple minutes after that, the heat pump starts up. So whatever is causing the occasional failure of the thermo’s heat pump sequence can be reset by removing then restoring power. Sort of like a power cycle sequence used with computers. I still have to work on the gas furnace but I’ll get to that in October of this year.

What’s interesting about this heat pump problem is that it’s so weirdly intermittent. It’ll work providing heat in the mornings for months, and then just stop working. Than if power is removed from the thermostat for a few minutes, than restored, it starts working again. Really seems that it has bad firmware. I’m not ignoring the possibility of that slide switch being bad but when I measured it with an ohmmeter there wasn’t any reading that would suggest it’s going bad or is presently intermittent. All the contacts are bright and clean, solder joints all good, nothing to indicate a bad or potentially bad switch. And I’ve tested enough switches over the years that I have an eye for a bad switch. This sort of problem may be fixed by replacing the thermostat…or so I’ve read in the forums…and it’s a shame that if it is just a firmware problem that Coleman didn’t recall them and replace them to save the owners that expense. I’ll just kee​_p using my power sequence fix for now. It doesn’t happen often, and it’s simple to restore so no need for a new thermo just yet.

If it happens again, I’ll just try adding my own circuit to automate resetting it. Push a button, a relay drops power to the thermo, times out, restores power. That should do it. And it’ll save $168.

Update: Sept. 2019

Well, as far as I’m concerned it’s official. There is nothing wrong with the True Air  thermostat that resetting won’t fix. As I said above I thought it was bad firmware and I’m now convinced that’s the case.

What I have happen is that the ‘Heat’ on cool mornings just won’t come on even though the inside/outside temps are within range of operation such that it should. So what I do to fix it is to disconnect the power from the thermostat, wait 5 minutes, then plug it back in. Two minutes after that, the compressors start running and delivering heat shortly afterwards.

What annoys me the most is that Coleman management likely knew that’s all that was wrong with the thousands of thermostats they probably shipped out at $168 each retail and they just liked that money more than helping their own customers. What would have been the right thing to do is to recall them and rework them. Instead they continue on pretending that the thermos with the symptoms I had are bad. Nowhere I’ve looked on-line about this issue ever mentioned just removing power and restoring 5 minutes later resets the thermostat. In fact, Coleman’s documentation specifically says that it can’t be reset. And for an issue like mine they show in the trouble tree that the thermostat needs to be replaced. That has not been the case here.

Many RV’ers say that the switch goes bad if they have symptoms like I did, but as an electronics professional, I know some things about switches and mine looks and measures fine. So I wonder how often people mistake this problem I’ve had for a bad switch and end up replacing a perfectly good thermostat.

If I had a one finger salute emojii, I would send it to Coleman for their corporate ineptitude at best and malfeasance at worst regarding this thermostat.

What I’m doing as a permanent fix eventually if I ever get around to it is to add a switch to the EMS panel near the thermostat. So when the thermostat won’t start the heat pump feature, I’ll just flip it off for 5 minutes and then back on.

Update June ’20: It’s been several months and the ‘Warm’ feature of the heat pump has worked fine each time I’ve tried it. Haven’t needed to reset it in a very long time now. So I haven’t installed any ‘Reset’ switch yet.