Before you condemn a Tecumseh compressor, do the six checks in this order. The compressor is rarely the first thing that fails. I know because I learned the hard way: in 2017, I approved a compressor replacement that was actually a $45 run capacitor. That mistake cost $1,150 in parts and labor—and a very awkward phone call to the customer.
Why I'm the One Writing This Checklist
I've been handling commercial refrigeration and rooftop HVAC compressor diagnostics for about ten years. In that time, I've personally made—and documented—four significant misdiagnoses, totaling roughly $6,700 in wasted budget. I do not say that proudly. But I started keeping our team's pre-replacement checklist because I got tired of buying new compressors that turned out not to be the problem.
If I remember correctly, the first big miss was in March 2017. I had a reach-in freezer that wasn't cooling. The Tecumseh compressor was hot to the touch, the overload was open, and the condenser fan was spinning. I checked the obvious things and decided the compressor was shot. Or rather, I convinced myself the compressor was shot. Ordered a replacement, waited three days, installed it, and the system did exactly the same thing. (Should mention: the capacitor was sitting right there under the cover. I didn't test it. I just looked at it.) Cost: $680 for the compressor, $470 in labor, zero cooling.
The 2022 roof leak call is where the checklist finally took shape. I'll get to that in a minute.
The Six-Check Pre-Replacement Test
This isn't rocket science. You need a multimeter, a capacitor tester, a screwdriver, and about twenty minutes. The goal is to prove the compressor is bad, not just to check the box. Do these in order. If any one of them fixes the unit, stop.
- Check the airflow side first. Why airflow first? Because a compressor can't stay cool if it can't breathe. Dirty air filter, blocked return or supply, greasy condenser coil, bad condenser fan. On many rooftop units the air filter housing is a plain metal box behind a return grille—open it before you open any service valves. A clogged filter can make a compressor overheat, cycle on internal overload, and look like a bad mechanical unit.
- Test the capacitor(s). A weak run capacitor is the most common misdiagnosis in this industry. Remove it, discharge it, and read the microfarad rating. If it's more than 6% low, replace it. While you're in there, check the start relay, potential relay, and contactor for pitted contacts.
- Check the windings for an open or short. With power off, remove wires one at a time. Measure C to R, C to S, and S to R. A single-phase Tecumseh compressor will usually show 1–5 ohms between C and R, 2–12 ohms between C and S, and roughly the sum of those two numbers between S and R. If any reading is open, or S–R doesn't add up, you've got a real electrical problem. Then check each winding to ground. A reading below 100,000 ohms says the winding is breaking down or shorted.
- Check the internal overload and the external safety controls. The compressor can be locked out by an open internal overload, high-pressure switch, low-pressure switch, or a bad thermostat. On the 2017 unit, the overload was open because the capacitor was weak and the compressor was running hot. It wasn't dead. Let the unit cool, replace the capacitor, and the overload closes like nothing happened.
- Measure the amp draw. A locked rotor amp draw that stays high for more than a few seconds points to a mechanical seizure—but only after you verify the capacitor and start gear are good. Compare the reading to the LRA and RLA stamped on the nameplate. A running compressor pulling RLA while doing zero cooling is a refrigeration problem, not a compressor replacement problem.
- Connect gauges and watch the pressures. Low suction, high head, and a starved evaporator usually mean airflow, metering device, or refrigerant charge. If the compressor is running but not pumping, compare the low side to the high side. A healthy small hermetic compressor should create a substantial pressure difference. If it doesn't, and the windings check out, then—and only then—start talking replacement.
That's how to test AC compressor without jumping straight to a replacement order. It takes longer to read than it does to do, and it's saved me from repeating the 2017 disaster more times than I can count.
The Emergency Roof Leak Call That Wasn't a Roof Leak
In September 2022, I got an emergency commercial roof leak detection call in Tecumseh, Michigan. Water had soaked a ceiling tile and a wall below a rooftop package unit. The building owner was convinced the roof was the problem. The evidence seemed to agree: it had rained overnight, and there was water on the floor.
My gut said otherwise. The water spot was too square, too close to the return duct, and the roof membrane around the unit looked fine. I took the ladder up before anyone could sell the customer a new roof.
What I found: The Tecumseh condensing unit was running—sort of. The compressor was hot enough to fry an egg, the condenser coil was caked with construction dust, and the air filter housing was blocked with a filter that should have been changed months earlier. The evaporator had frozen into a solid block, the condensate pan had overflowed, and the "leak" was melted frost draining through the ceiling.
The compressor wasn't dead. It was cycling on internal overload. I cleaned the condenser coil with my Milwaukee air compressor and a blowgun, replaced both the air filter and the run capacitor, and let the unit defrost before restarting. Forty-five minutes of prevention, maybe $60 in parts. The roof needed no roof work. The compressor is still running on that unit as far as I know.
I'll admit, I kept second-guessing myself until the unit pulled down to setpoint. The compressor had been through enough. But it did. That call turned my personal habit into a written procedure. Our team has used the same checklist ever since, and in the 18 months after I printed it, we caught 47 cases where a compressor was about to be replaced—but the failure was airflow, capacitor, refrigerant, or control voltage. Not every one of those was a Tecumseh, but plenty were. I'd estimate the checklist has saved us somewhere north of $8,000 in avoided warranty returns and rework.
When the Checklist Won't Save You
Let me be honest about the limits. My experience is based on maybe 200 mid-size commercial compressor calls, mostly rooftop package units and walk-in boxes with hermetic and semi-hermetic compressors. If you're working on a large screw compressor, a variable-speed inverter system, or a sealed unit you can't get literature for, this checklist is not enough. Those systems need their own test procedures.
The checklist also won't help if the compressor has already let the smoke out. If the windings are open, the body is cracked, or you measure a dead short to ground on every terminal, stop testing and order the part.
It doesn't cover what to do once the compressor is confirmed bad, either. Replacing a compressor without checking the condenser coil, filter-drier, and metering device is how you kill a new one just as fast.
I should add one more thing: this is not a substitute for the manufacturer's service literature. Tecumseh publishes model-specific data for a reason. Use it.
