Mars Condenser Fan Motor Wiring Diagram

Getting the Wiring Right on a Mars Condenser Fan Motor

The Mars condenser fan motors are workhorses in residential and light commercial HVAC. They run for years, then suddenly they don't. Most of the time the issue isn't the motor itself. It's the wiring. Or rather, someone assumed the wiring diagram matched what was already on the unit. I've replaced enough of these motors to know that the diagram inside the junction cover is often faded, handwritten, or just plain wrong from a previous repair. The motor label tells you the specs. The wiring diagram on the motor housing tells you how to connect it. But neither of those always line up with the unit's existing harness.

Mars Condenser Fan Motor Wiring Diagram Reference

A typical Mars PSC (permanent split capacitor) fan motor has five or six terminals on its motor connector. The common ones you'll see are C (common), FAN, HAZ (high speed auxiliary, some models only), and sometimes a dual-speed terminal pair marked FAN-HI and FAN-LO. The capacitor connects between the C terminal and the FAN terminal on single-speed motors. That's the baseline. Anything else requires looking at the actual diagram, not just guessing from memory. Here's the practical breakdown of how most of these motors wire up in the field: Single-speed PSC motor (most common):

Power comes into the C terminal through the contactor. The capacitor runs from C to FAN. The fan motor lead from the unit's control board or thermostat circuit ties to FAN as well. That's it for a standard setup. The motor spin direction is fixed by the motor design and the terminal you feed power into, so if the fan spins backward, swapping two of the three leads at the motor connector usually corrects it. Dual-speed PSC motor: These have an extra terminal, often labeled HAZ or HI. The contactor feeds C. The low-speed control ties to FAN, and the high-speed control (usually from the indoor blower relay or a second stage on the thermostat) ties to HAZ. The capacitor still bridges C to FAN. This is where most people mess up, because the wiring on the unit side doesn't always match the motor's internal winding taps, and the diagram doesn't account for every manufacturer variation.

I ran into this exact situation last fall on a Carrier unit with a Mars replacement motor. The existing harness had three wires: black, red, and white. The motor diagram showed five terminals. The unit's contactor fed power to the black wire, which went to common. The red came from the indoor blower interlock relay for high speed. The white was neutral. I connected the black to C, the red to HAZ, and the white to C again through the capacitor leg. That should have worked. It didn't. The motor hummed, drew about 4.8 amps on the low setting, and barely moved the fan blade. The problem was the capacitor value. The motor label said 7.5 microfarad. The unit had an 8.0 microfarad capacitor on it from a previous upgrade by someone who probably thought bigger was better. On a dual-speed motor like this, the extra capacitance shifts the phase angle enough to kill torque on the low-speed winding. The motor could spin up fine when I temporarily bridged the HAZ terminal directly to the contactor feed. That proved the motor was good and the issue was purely electrical. My workaround was to swap in a 7.5 microfarad capacitor and recheck everything. After that, the low speed drew 2.1 amps and pulled the right airflow. The high speed came up to about 3.4 amps and spun the fan at the rated RPM. I also verified the rotation by holding a piece of paper against the discharge side of the coil and confirming it pulled air through the fins instead of blowing it away from them. Rotation matters more than people give it credit for, especially on condenser fans where the shroud is designed for a specific airflow direction.

When you're pulling a wiring diagram for these motors, the most reliable source is the motor tag itself, not the unit schematic. The unit schematic assumes the original motor's terminal arrangement. Aftermarket replacements sometimes shift around the terminal numbering, even if the electrical specs stay the same. I learned that the hard way on a Trane unit where the OEM diagram called for a 3-terminal connection but the replacement Mars motor had a 5-terminal block with two capacitors wired internally. I followed the unit diagram, ended up with the motor running on one winding only, and burned out the fan winding in about forty minutes. The motor got hot enough that I could barely touch the housing. The diagram I should have used was the one printed on the motor's own decal, which showed the internal capacitor configuration and told me to tie C to FAN with an external capacitor and leave HAZ unused for single-speed operation. Here are a few things that trip people up regularly: Terminal numbering varies by manufacturer and model year. Mars uses C-FAN-HAZ by default, but their older lines and some international variants use a different arrangement. Always cross-reference the diagram on the motor, not just the part number on the old motor. The part number alone won't tell you if the winding configuration changed between revisions.

The capacitor is part of the circuit, not just an accessory. Running a PSC motor without a capacitor will destroy it. I've seen units where the capacitor was bypassed with a jumper during testing and left that way because it "worked for a while." It doesn't work. The phase shift the capacitor provides is what creates the rotating magnetic field in the secondary winding. Without it, you get a vibrating hum and a motor that pulls locked-rotor current until the windings fail. Wire gauge and voltage drop matter on long runs. If the motor is more than fifteen feet from the contactor, the voltage drop on the switched leg can be significant with undersized wire. A twenty-foot run with 18-gauge control wire and a 3.5-amp motor can drop nearly a volt under load. That's not enough to shut the motor down, but it's enough to reduce airflow by maybe ten percent and make the motor run warmer than it should. Use at least 16-gauge for runs over ten feet. Eighteen gauge is fine for short jumps between the terminal block and the motor connector. Capacitor tolerance and temperature derating. A capacitor rated at 7.5 microfarad might actually read 6.8 or 8.2 depending on the manufacturer and the ambient temperature. Most HVAC capacitors are rated for 45°C ambient. If the condenser sits in direct sun or the fan compartment is poorly ventilated, the effective capacitance drops. I tested a capacitor labeled 7.5 that measured 6.1 microfarad after the unit had been sitting for an hour in ninety-degree weather. Not dead, but significantly off spec. That kind of degradation is why motors sometimes start sluggishly on hot days even though they work fine in the morning.

One thing the wiring diagram won't tell you and you need to account for: the motor's rotation must match the fan blade's pitch direction. Mars fan motors are typically rated for clockwise rotation when viewed from the motor shaft end (the hub side). If the blade is mounted backward or the motor is reversed, you'll move far less air and the motor will run hotter. Check the decal on the motor for the rotation arrow. If there isn't one, look at the fan blade. The curved side of the blade should face the direction of rotation. Concave side leads, flat side follows. That's the standard for most condenser fan blades. When you order a replacement motor, make sure the frame size, shaft diameter, shaft length, and mounting bolt pattern match the original. Mars uses a 5/8-inch shaft on most of their residential condenser fan motors, but some commercial units use 3/8 or 1/2 inch. The frame dimensions matter too. A TEFC (totally enclosed fan cooled) motor in a 56-frame will bolt right in where an ODP (open drip proof) 48-frame won't, even if the electrical specs are identical. Check the mounting hole spacing on the old motor before you order. It's usually something like 3-15/16 inches center-to-center on the elongated slots, but it varies. If you need the actual wiring diagram for download, most of the information is on the motor itself. Mars doesn't publish individual wiring diagrams as standalone documents the way they used to. The closest you'll get is the installation and service manual for the specific motor series, which is usually available through the distributor or the manufacturer's website under the service literature section. For a C-FAN-HAZ configuration, the standard diagram shows power entering the contactor, feeding common. The contactor closes, power goes to the common terminal on the motor. The capacitor connects from common to fan. The control circuit for the fan speed (usually the indoor blower relay) connects to HAZ for the secondary winding tap. That's the configuration for the vast majority of residential condenser fan motor replacements. Anything outside of that is a special case and you should verify it against the motor nameplate before making connections.

The most useful thing you can do before disconnecting anything is photograph the existing wiring. Not the whole unit. Just the motor connector and the wires going into it. Five seconds with your phone saves twenty minutes of guesswork later. You'll be surprised how often someone has already modified the wiring and the existing connections don't match what the diagram says should be there.