Fix Coleman-Mach Multi-Zone AC Control Box Problems

The Quick Answer

You're absolutely right - you've got the wrong control box for your 2003 Fleetwood Excursion's Coleman-Mach system. The 9430A751 control box you installed is designed for a true multi-zone setup with two separate air conditioning units, which is why it has SIG 1 and SIG 2 terminals. Your original system only had one AC unit controlling multiple zones through dampers, so it used a single-zone control box with just one SIG terminal. The thermostat switching from Zone 1 to Zone 2 and becoming unresponsive is happening because the new control box is looking for a second AC unit that doesn't exist in your system.

What's Causing This

Your Coleman-Mach ZC thermostat is designed to work with both single-zone and multi-zone systems, but it needs the correct control box to function properly. When you connect it to the 9430A751 dual-zone control box, the thermostat detects that it's in a multi-zone configuration and automatically switches to Zone 2 mode, waiting for communication from a second AC unit that isn't there.

The three green LED flashes you're seeing on the control box indicate it's receiving power and initializing correctly, but it's stuck waiting for a response from Zone 2. Your original system likely used either a 9430A731 (single-zone heat pump) or 9430A741 (single-zone cool only) control box, depending on whether your Coleman-Mach unit has heat pump capability.

The fact that your original control box had only one SIG terminal confirms this was a single-zone system. Even though your RV may have had multiple vents and zones, it was controlled by dampers and a single AC unit, not multiple AC units like the 9430A751 is designed to handle.

Tools You'll Need

• Correct replacement control box (9430A731 for heat pump or 9430A741 for cool only)
• Wire strippers and electrical tape
• Phillips head screwdriver
• Digital multimeter (for voltage verification)
• Wire nuts (if any connections need to be remade)
• Flashlight or headlamp for working in tight spaces

Step-by-Step Fix

⚠️ SAFETY WARNING: This repair involves working with 120V AC power at the control box, which can cause serious injury or death. Always turn off power at the breaker and verify it's off with a multimeter before beginning work.

Step 1: First, determine whether your Coleman-Mach unit has heat pump capability by checking the model number on the unit itself or looking for a heat pump wire harness. If it has heat pump, order the 9430A731 control box (around $85-110). If it's cool-only, get the 9430A741 (around $75-95).

Step 2: Turn off the main breaker or disconnect power at the RV electrical panel before beginning any work. Then turn off all power to the AC system at both the thermostat and the main electrical panel. Take photos or document all wire connections before disconnecting anything. Remove the incorrect 9430A751 control box by disconnecting the wire harness and unscrewing it from its mounting location.

Step 3: Install the correct single-zone control box in the same location. The wiring should be much simpler - you'll have your purple signal wire connecting to the single SIG terminal, plus the main power connections (verify terminal designations on your specific control box model as these may vary).

Step 4: Set the jumpers on the new control box according to your system configuration. Since you mentioned no heat pump, ensure the heat pump jumper is in the "no heat pump" position. The zone jumper should be set to Zone 1.

Step 5: Restore power and use a multimeter to verify proper 24V AC supply at the control box before testing the system. Your Coleman-Mach ZC thermostat should now stay on Zone 1, display the current temperature instead of dashes, and respond properly to button inputs for cooling, fan speed, and temperature adjustments.

When to Call a Pro

Call an RV technician if you're uncomfortable working with 120V AC wiring or if you discover additional wiring complications when removing the incorrect control box. Also seek professional help if the correct control box doesn't resolve the issue, as this could indicate problems with the thermostat itself or the AC unit's control board.

If your system has any unusual wiring configurations or if you find evidence of previous modifications that don't match standard Coleman-Mach installations, a professional can ensure everything is wired safely and correctly. The cost for professional diagnosis and installation typically runs $150-250, but it's worth it to avoid potential damage to your AC system or electrical hazards.