SAFETY: This is a 12VDC low-voltage control circuit, but the compressor and fans it switches run on 115VAC. Before touching any wire: turn the air conditioner’s breaker off, unplug external shore power cords, switch any onboard generator and inverter off, and disconnect the cable from the positive terminal on every battery in the coach. Confirm the thermostat display is dark before removing it.
Coleman Part of our complete Coleman guide — wiring, setup & troubleshooting.
Coleman Mach thermostats don’t use the R/C/W/Y/G terminals from a house furnace. They’re wired for a 12VDC control circuit supplied by the RV’s own electrical system, not the 24V AC transformer a residential thermostat expects. If you wire one like a house thermostat, the terminal letters will look almost right and the actual voltage will be wrong. Match terminal letters, confirm 12VDC (not 24V AC), and you’re set. For the general concept of terminal-letter matching and multimeter checks, see the thermostat wiring guide; this page covers what’s specific to Coleman Mach.
TL;DR
| Your situation | What to do |
|---|---|
| Replacing a mechanical, electronic, or digital-display Coleman Mach thermostat | All three older types are interchangeable; match R, B, Y, GH, GL, W by letter |
| New digital thermostat (9420381 or 9420382) | Same core letters, plus WHP (electric heat/heat pump strip heat) and SENS (remote sensor) |
| No fan speed selection on the old thermostat | You have GH (high) and GL (low) as two separate outputs, not a single G; pick or wire both |
| Fan, furnace, or compressor stopped working after a rewire | Check the 2-amp fuse on the back of the thermostat before assuming a relay or board failure |
| Wire colors at the ceiling plenum don’t match the thermostat’s colors | Normal. Some plenum series relabel colors partway through the run, so go by terminal letter |
| Your rooftop unit has a heat pump | The WHP terminal is the digital thermostat’s path for this; confirm against your unit’s specific manual before wiring |
The terminal set: R, B, Y, GH, GL, W (plus WHP, SENS on digital models)
| Terminal | Typical wire color | What it does |
|---|---|---|
| R | Red | +12VDC input from the coach’s battery/converter system |
| B | Blue | Ground (12VDC return) |
| Y | Yellow | Compressor output; energizes the compressor relay |
| GH | Green | Fan, high speed |
| GL | Gray | Fan, low speed |
| W | White | Gas heat output (furnace call) |
| WHP | White/black | Electric heat output for heat pump strip heat or an electric-heat accessory (digital thermostat only) |
| SENS | — | External remote temperature sensor input (digital thermostat only) |
There’s no C terminal in the residential sense. B is the DC ground reference, not an AC common wire, and it’s not interchangeable with a house thermostat’s C. If you’re moving a Coleman Mach thermostat’s wiring onto anything that expects 24V AC (or vice versa), stop. That’s a mismatch that can damage both devices, not a wiring puzzle to solve with a jumper.
The digital thermostat’s connectors take 20 AWG solid wire, strip to about 5/16 inch, and press in until the resistance releases; a screwdriver pressed against the connector’s tab releases the wire.
Which Coleman Mach thermostat do you have
Three older styles are treated as fully interchangeable by the manufacturer, all sending the same 12VDC(+) signal:
- Mechanical (by-metal) thermostat: a bimetallic coil switch, heat/cool/fan dial, no digital display.
- Electronic thermostat: solid-state switching, same R/B/Y/GH/GL/W terminals, a small LED bar for temperature.
- Electronic digital-display thermostat: adds a numeric temperature readout, still the same six-terminal wiring.
The current digital thermostat (model 9420381, or 9420382 on units with plug connectors instead of screw terminals) adds WHP and SENS and uses dip switches to tell the thermostat what equipment it’s driving: gas heat only (factory default), electric heat, or both with auto changeover, plus a cool-only mode that disables all heating outputs. Set the dip switches to match your equipment before wiring. A Cool Only setting with a furnace wired to W will simply never call for heat.
Ceiling plenum wiring: the colors can change partway through
The thermostat’s own labeling is consistent, but the wire that shows up at the ceiling plenum (the control box above the rooftop unit) doesn’t always keep the same color. Two documented examples from the same manufacturer’s service manual:
6799-720 & -726, 7330-720 & -726 series plenums
| Thermostat | Plenum wire | Function |
|---|---|---|
| Yellow (Y) | Yellow | Compressor relay |
| Green (GH) | Green | High fan relay |
| Gray (GL) | Gray | Low fan relay |
| Blue (B) | Blue | Ground |
| White (W) | White | Furnace/heat element relay |
6799-730 series plenum
| Thermostat | Plenum wire | Function |
|---|---|---|
| Yellow (Y) | Black | Compressor relay |
| Green (GH) | Green | High fan relay |
| Gray (GL) | Gray | Low fan relay |
| Blue (B) | Blue | Ground |
| White (W) | White | Furnace/heat element relay |
Notice the compressor circuit: yellow at the thermostat becomes a black wire at the plenum on the -730 series. The manufacturer’s own documentation warns these field connections “may not all use color coded wire as noted.” Go by the terminal letter and trace the wire, not by color, especially on a used coach or one that’s had control-box work done.
Wiring/replacement steps
- Kill power completely. Breaker off, shore power cord unplugged, onboard generator and inverter switched off, and the cable disconnected from the positive terminal on every battery. This is the manufacturer’s own warning, not an optional precaution: the compressor and fan relays this thermostat switches carry 115VAC.
- Photograph the old thermostat’s wiring before touching anything.
- Label each wire by terminal letter (R, B, Y, GH, GL, W), not by color; see above.
- Disconnect and remove the old thermostat.
- Land each labeled wire on the matching terminal of the new thermostat. On the digital model, strip 5/16 inch and press each 20 AWG conductor in until it seats; tug gently to confirm it’s held.
- Set the dip switches (digital model) to match your equipment: gas heat, electric heat, or both, and cool-only if there’s no heat source at all.
- Restore power and check fan, cooling, and heat functions in order.
Troubleshooting with a multimeter
With power restored and the thermostat calling for each function, these are the manufacturer’s own diagnostic checks, done at the low-voltage terminal strip in the ceiling assembly:
- Fan only (high): 12VDC between GH and B.
- Cool, low fan: 12VDC between GL and B (fan) and between Y and B (compressor).
- Cool, high fan: 12VDC between GH and B (fan) and between Y and B (compressor).
- Heat: 12VDC between W and B (furnace). This can’t be checked at the ceiling assembly if you have a gas furnace with its own control board; check at the furnace instead.
If any of these reads 0V, work backward toward the thermostat: check for 12VDC at the thermostat’s own R and B wires first (confirms the thermostat has power), then the specific output wire. A digital thermostat with a blown internal fuse won’t produce voltage on any output even though R and B read correctly. Replacing the fuse (or the thermostat, since the fuse isn’t user-serviceable on the PTC models) is the fix. A freeze thermistor on the evaporator coil can also open the compressor relay if the coil drops below 32°F; it’s a working safety feature, not a fault, and it recloses once the coil warms back to 55°F.
Skipping the wiring diagram
The wire-count diagrams elsewhere on this site (2-wire through 8-wire, heat pump O/B) are drawn for residential 24V AC systems with R/C/W/Y/G-family terminals. None of them represent a 12VDC RV circuit accurately, so none are included here: using one would show the wrong voltage and the wrong terminal set. If you want a visual, the manufacturer’s own wiring diagrams (linked in the FAQ below) are the accurate source for your specific ceiling plenum series.
⚠️ Know when to stop
Thermostat wiring is 24 V AC low voltage — safe to handle once you cut power at the breaker (do it anyway; a short can kill the transformer or blow the board fuse). But stop and call an HVAC pro or electrician if you hit any of these:
- Line voltage (120/240 V): thick wires with wire nuts, often on baseboard or wall-heater thermostats. Miswiring these can injure you or start a fire.
- Gas components: anything past the furnace control board — gas valves, ignitors, pilot assemblies.
- Refrigerant lines or sealed-system parts on heat pumps and AC units.
- No idea which system you have: if the wiring doesn’t match any diagram here, photograph it and get a pro’s eyes on it before guessing.
FAQ
Is a Coleman Mach thermostat 12V or 24V? 12VDC. It’s supplied by the RV’s own electrical system (battery and converter), not a 24V AC transformer like a house furnace. Never assume residential wiring rules apply.
What do the GH and GL terminals mean? High fan and low fan. Unlike a house thermostat’s single G terminal, Coleman Mach splits fan speed into two separate outputs so the thermostat (or the installer, on older ceiling-plenum wiring) can choose one speed.
Where is the fuse on a Coleman Mach thermostat? On the back of the thermostat itself — not at the ceiling plenum or a separate fuse box. Electronic and digital models use a small fuse: 2 amps on the current digital thermostat, a self-resetting PTC type. If a function stops working after a rewire and the voltage checks out at the terminal strip, check this fuse before replacing anything else.
Are all Coleman Mach thermostats interchangeable? The manufacturer states the mechanical, electronic, and electronic-digital-display styles are completely interchangeable, with the same six core terminals. The newer digital thermostat (9420*381/382) adds WHP and SENS but keeps the same R, B, Y, GH, GL, W core, so it’s a compatible upgrade path from any of the three older types.
My wire colors don’t match the chart. What do I do? Trust the terminal letter, not the color. The manufacturer’s own service manual documents cases where the same function (compressor) uses a different color at the plenum end than at the thermostat end. Photograph the old wiring, label by letter, and trace with a multimeter in continuity mode if you’re unsure.
Do I need a licensed tech for this? The thermostat wiring itself is low-voltage DC and reasonable for a comfortable DIYer who follows the full power-disconnection steps above. Anything involving the compressor’s 115VAC side, refrigerant, or a control board replacement is past the DIY line; call an RV-certified tech.
Manufacturer references: Coleman-Mach digital wall thermostat installation guide (9420*381/382), RV Products/Airxcel 12VDC wall thermostat service manual.
Last reviewed: July 2026
