Coleman-Mach 8 Rooftop Air Conditioner / Heat Pump
Service knowledge for the Coleman-Mach 8 low-profile rooftop unit: how the heat-pump cycle and its components work, run-capacitor diagnosis and replacement with kit numbers, the factory cooling-performance test method, and step-by-step electrical troubleshooting for no-compressor, dead-unit, and no-fan/slow-fan conditions.
Overview
The Mach 8 is Airxcel/RV Products' low-profile rooftop unit for coaches, built either as a straight air conditioner or as a heat pump. In heat-pump trim a solenoid-operated reversing valve flips the refrigerant path: energized in heating, de-energized in cooling. Because the coils swap roles between seasons, service literature calls them the indoor and outdoor coils rather than evaporator and condenser. Metering is done by a by-flow restrictor - two different-sized orifices back to back, one seating and one bypassing depending on flow direction.
Electrically the unit carries a hermetic (non-serviceable) compressor with a clamp-on overload switch, a start capacitor with PTCR relay that drops out near 75% run speed, run capacitors for the compressor and both fan motors, and two fan motors: a squirrel-cage blower for coach air and a propeller fan that pulls outdoor air through the condenser coil. Heat-pump versions add two protection sensors: a thermistor in the compressor relay circuit that cuts out at 26 F (+/-3) and resets at 40 F (+/-3) to prevent evaporator freeze-up, and a freeze switch on the condenser coil that opens the compressor circuit at 18 F (+/-6) actual coil temperature. Units with electric heat carry a 6,000 BTU strip element drawing about 15.2 A at 120 V, guarded by a limit switch in the element frame.
Control identification
Free-delivery (non-ducted) ceiling assemblies use a rotary selector switch and mechanical thermostat; check the switch with an ohmmeter for continuity of L-1-3 in HEAT, L-1 in LO FAN, L-2 in HI FAN, L-1-4 in LO COOL and L-2-4 in HI COOL. Wall-thermostat systems run on 12 VDC control wiring with the standard Coleman color code: red R = +12 V, blue B = -12 V, yellow Y = compressor, green GH = high fan, gray GL = low fan, white W = furnace/heat.
Run capacitor diagnosis and replacement
The classic capacitor symptom: the motor hums but will not start in any speed, yet runs if the blade or wheel is spun by hand. That motor's run capacitor has failed - replace it. If the motor still refuses to start with a known-good capacitor, the winding is open and the motor itself must be replaced (replacement motors ship with a matching capacitor).
Replacement procedure (power removed first - capacitors are a shock hazard):
- Cut the old capacitor's wires as close to the capacitor body as possible and strip 1/4 inch (7 mm) of insulation.
- Discard the old capacitor. The bracket may be broken off to avoid pulling the appliance; cylindrical fan capacitors on some models are instead retained by a nut.
- Insert each stripped wire fully into a red crimp splice on the new capacitor (no polarity), crimp, and pull-test the joint.
- Heat-shrink the splices for a watertight seal.
- Peel the release liner and bond the new capacitor's base to a clean, dry spot on the base pan (or secure a cylindrical fan capacitor with its nut). Dress wires clear of sharp edges and moving parts.
- Restore all assemblies, re-apply power, and run-test every mode.
Verify the microfarad and voltage rating against the motor model before ordering - see the parts list below for the factory kit cross-reference.
Cooling performance test
Farber uses the Airxcel checkout method whenever a unit is accused of weak cooling. Start the unit in HIGH COOL and let it run 15 to 20 minutes before taking any reading, and never use a laser (IR) thermometer - use a probe thermometer in the airstream. Record three temperatures: outdoor ambient, air entering the return grille, and air leaving the closest supply register. The difference between return and supply is the temperature split. A healthy rooftop unit at normal indoor humidity typically delivers roughly a 16-22 F split (Farber guideline; the OEM worksheet asks for the raw numbers). Also capture line voltage at the unit (should be near 120 VAC under load), compressor amp draw measured on the black wire at the compressor, whether the PTCR/start kit tests good, and whether the ceiling assembly is ducted or non-ducted. A small split with high humidity and long run times points at airflow problems (dirty filter, crushed ducting, dirty coils) before it points at refrigerant charge - and the sealed system must never be opened by anyone not trained in refrigeration repair.
Compressor motor bench checks
With all leads removed from the compressor terminals, ohm the windings: zero ohms between any two terminals means shorted windings; anything other than infinity from a terminal to the housing means a grounded winding; infinity between two terminals means an open winding. On a good compressor the highest resistance is R-to-S, the lowest C-to-R, and C-to-S falls in between. For a live check with C, R and S connected, a meter across C and R should show about 120 VAC while the thermostat calls for cooling.
Heat-pump quick reference faults
Fan runs but no compressor in heat-pump mode adds two suspects beyond the cooling-mode list: an opened freeze switch (cold condenser coil) and the reversing-valve circuit. Compressor and fan both run but the unit cools when it should heat: wiring, selector switch, or reversing-valve solenoid. Insufficient heat in cold weather is often simply the freeze switch doing its job at low ambient - a heat pump loses capacity as outdoor temperature falls, which is why coaches carry gas or electric backup heat.
Safety
All flowchart diagnostics involve live 120 VAC measurements. Only trained technicians should perform them; disconnect power before touching any component, and discharge capacitors before handling.
Maintenance schedule
| Task | Interval | Parts | Notes |
|---|---|---|---|
| Clean or replace the return-air filter | 1 months | MODEL-SPECIFIC Return-air filter, sized to ceiling assembly grille | Monthly during the cooling season, or more often in dusty duty. A loaded filter is the leading cause of poor cooling complaints and evaporator freeze-ups. Farber recommended interval; OEM requires a clean filter before any performance testing. |
| Inspect and clean indoor and outdoor coils, base pan, and condensate drains | 12 months | — | Annually at spring start-up. Remove the shroud, straighten fins, wash coils gently, and confirm drain paths are clear. Farber recommended; airflow restriction appears in the OEM cause list for both insufficient cooling and insufficient heat. |
| Run the cooling performance checkout (HIGH COOL 15-20 min, record outdoor/return/supply temps, voltage, compressor amps) | 12 months | — | Each spring and any time weak cooling is reported. Do not use a laser thermometer. Compare temperature split to the 16-22 F Farber guideline and log results on the vehicle record. |
| Verify start components: PTCR, start relay, start and run capacitors | 24 months | — | Farber recommended every two years or whenever a motor is slow to start, hums, or trips the overload. Confirm capacitance is within tolerance of the rating printed on the case; replace with the matching RVP kit. |
| Inspect rooftop gasket, mounting bolts, and shroud condition | 12 months | — | Farber recommended annually. A loose unit or degraded roof gasket causes leaks and vibration noise; a cracked shroud disturbs condenser airflow. |
| Exercise electric heat element and limit switch before heating season | 12 months | — | Element should draw about 15.2 A at 120 V (element alone, domestic models). Check limit-switch continuity with an ohmmeter. Farber recommended each fall on units with strip heat. |
Troubleshooting
Nothing runs - no compressor, no fan, no heat
- Confirm the thermostat/selector is actually calling for cooling with the fan set to HIGH before any electrical work.
- Check for 120 VAC between incoming hot and ground at the unit line connection.
- Check 120 VAC between hot and neutral: voltage hot-to-ground but not hot-to-neutral means a break in the line neutral; none hot-to-ground means a break in the line hot (check the breaker). If hot-to-neutral reads but hot-to-ground does not, hot and neutral are swapped somewhere upstream.
- With line voltage good, check 120 VAC between black and white at the 9-pin plug (ceiling assembly side). No voltage on a wall-thermostat system points to the control box; on manual controls, the selector switch.
- On wall-thermostat systems, check 12 VDC between B- and GH at the control box, then between blue and red at the thermostat. Voltage at the control box but not the thermostat means broken wiring in the wall; no voltage at the box, check the thermostat fuse (blown fuse: replace and retest; good fuse: bad thermostat).
- Voltage good at the 9-pin plug but components dead: check between white and black at the motor and between C and R at the compressor to isolate the break between plug and components.
Fan runs but compressor will not start (cooling mode)
- Verify the thermostat shows the unit in COOL and the setpoint is below room temperature; adjust and retest before condemning parts.
- Check for 115 VAC between purple and yellow in the 9-pin harness. If absent on a manual-control unit, check 115 VAC purple-to-white; present means replace the thermostat (allowing for its 3-minute delay), absent means replace the selector switch.
- On a standard single-zone wall thermostat: check 12 VDC between B and Y at the control box, then between blue and yellow at the thermostat. Voltage at the stat but not the box means a broken wire in the wall; none at the stat (after 3-minute delay) means replace the thermostat.
- If control voltage is good, bypass the freeze-circuit sensor: compressor starts, replace the freeze sensor. On zone systems also bypass the SHED (load-shed) circuit; if the compressor then starts, the energy-management system is the fault, not the A/C.
- If 115 VAC is present purple-to-yellow: on systems with a pressure switch, check 115 VAC purple-to-white at the pressure-switch relay, then black-to-white at the relay; if the second is absent, pull the white wire off the relay and retest - same result on a second attempt means replace the pressure-switch relay.
- Check 115 VAC between neutral and the incoming side of the overload; absent means a broken wire before the overload.
- Check 115 VAC between C and R at the compressor: absent means a bad overload (replace it); present with the compressor not running means the compressor itself is bad - replace the unit.
- Compressor hums but starts if the fan blade side is assisted / motor spins manually: replace the start components (PTCR/start capacitor) or run capacitor before condemning the compressor.
No high fan, or fan running slow
- Put the fan in HIGH speed - the whole test assumes high fan.
- If the motor does not run at all: spin it by hand with power applied (carefully). Runs when spun: replace the fan capacitor and retest.
- Motor still dead: check voltage black-to-white at the motor. Good voltage: bad motor. No voltage: check black-to-white at the ceiling-assembly plug - good there means a bad connection to the motor; none there, trace to selector switch (manual controls) or control box.
- Wall-thermostat systems with no voltage at the plug: check DC volts between B- and GH at the control box. Good voltage: bad control box PC board. None: check 12 VDC between blue and green at the thermostat - good means a broken wire in the wall, none means a bad thermostat.
- If the motor runs but slowly: determine the correct amp draw for the motor and measure amps on the motor's white wire.
- High amp draw: check whether the fan-capacitor wires are on opposite posts. If not, rewire the capacitor correctly and retest; if wired right, the capacitor is bad.
- Low amp draw: measure black-to-white voltage at the motor. Good voltage: bad motor. Low voltage: check the same pair at the ceiling-assembly plug - good there means a bad connection; low there, check 120 VAC hot-to-neutral at the incoming line (low incoming voltage: find and repair the supply problem, e.g. undersized cord or park power). Good incoming with low at the plug: bad selector switch (manual) or bad control box PC board (wall stat).
Heat or compressor runs but no fan
- Check wiring at the wire-nut junctions: motor black to selector/control black, motor red to red, motor white to thermostat white, brown pair to the fan capacitor.
- Test the fan run capacitor (motor runs when spun by hand = capacitor).
- Check selector-switch continuity (L-1 low fan, L-2 high fan).
- If all pass, replace the fan motor - and check rotation direction on the new motor before final assembly (condenser fan pulls air through the coil).
Fan and heat pump run but no electric heat element
- Check limit-switch continuity with an ohmmeter (opens on overtemperature from restricted airflow - find the airflow cause too).
- Check the thermostat/selector heat circuit and wiring.
- Check freeze-switch state.
- Check the element for continuity; confirm about 15.2 A draw at 120 V when energized.
Unit cools when it should heat (heat-pump mode)
- Check wiring to the reversing-valve solenoid (valve energized = heating).
- Check the selector switch or thermostat heat call output.
- If the solenoid has voltage but the valve does not shift, replace the reversing-valve solenoid coil / valve.
Compressor and fan run, insufficient cooling
- Check for airflow restrictions: dirty filter, blocked return, crushed or leaking ducting, dirty coils, damaged shroud.
- Run the cooling performance test (HIGH COOL 15-20 minutes, probe thermometer) and evaluate the return-to-supply split.
- Consider load: extreme ambient heat or an undersized unit for the space gains more heat than the unit can remove.
- Verify line voltage under load and compressor amp draw against nameplate.
- Only after the above: suspect refrigerant charge or compressor. The sealed system must not be opened except by a trained refrigeration technician.
Service parts reference
| Part number | Description | Type |
|---|---|---|
| 1499-6441 | Run capacitor kit, 10 uF / 370 V - blower motor 1468*338 (115 V/60 Hz) | repair |
| 1499-6451 | Run capacitor kit, 12.5 uF / 370 V - fan motor 1468*339 (115 V/60 Hz) | repair |
| 1499-6431 | Run capacitor kit, 5 uF / 450 V - blower/fan motors 1468*340, 1468*341, 1468*344 (240 V/50 Hz) | repair |
| 1499-6411 | Run capacitor kit, 2 uF / 450 V - fan motor 47103*301 (240 V/50 Hz) | repair |
| 1499-6461 | Run capacitor kit, 2.5 uF / 450 V - fan motor 47301*301 (240 V/50 Hz) | repair |
| MODEL-SPECIFIC | Return-air filter for ceiling assembly | consumable |
| MODEL-SPECIFIC | Compressor start kit (PTCR + start capacitor) | wear |
| MODEL-SPECIFIC | Freeze sensor / freeze switch (opens 18 F +/-6 coil temperature) | repair |
Based on: Airxcel RV Products: Mach 8 Service Manual (1976-665), Run Capacitor Replacement Instructions (1976-695), Cooling Performance Worksheet, flowcharts T-002/T-003 and No Compressor Flow Chart. This guide is an original work; always confirm procedures against your unit's build record and the manufacturer's current documentation.