I have a list of my own screwups. It's not a fun list, but it's honest. The first Tecumseh compressor start relay I replaced took twenty minutes. The second one took two days, because I didn't verify the wiring diagram before trusting a universal relay. That mistake made it onto my list as mistake #4: $38 in parts—maybe $42, I'd have to check—plus $180 in wasted labor and a callback I still cringe about.
I've been a field compressor technician handling Tecumseh compressor and condensing unit service for nine years. I've personally made and documented 11 significant mistakes, totaling roughly $8,400 in wasted budget. Now I maintain our team's checklist so the next tech doesn't repeat them.
What we're really comparing
This article is a comparison of two ways to handle a Tecumseh compressor start relay wiring diagram: the careful way and the quick way. I'm not going to give you a fake "both sides have merit" essay. I'll tell you which method wins, where it loses, and when I still make the wrong choice.
I'm comparing three field decisions:
- Which diagram to follow—the faded one on the access panel or the generic one in the relay box.
- Whether to test the compressor windings before you spend money on a relay.
- Whether to choose the OEM Tecumseh relay or a universal replacement.
There's a fourth comparison too: how the repair is priced. That one decides whether you ever call the same tech again.
My experience is based on about 300 single-phase Tecumseh compressors in residential and light commercial AC and refrigeration—maybe 270, I'd have to check the log, not rack systems. If you're working on a three-phase screw compressor, stop reading and call someone who specializes in those.
The diagram problem: OEM drawing vs universal drawing
Back in 2017, I had a Tecumseh AE series compressor that wouldn't kick on. The homeowner had already bought a universal start relay from an online parts store. The box had a clear drawing with relay terminals labeled 1, 2, and 5. The compressor had three terminals labeled C, R, and S. The drawing didn't say which relay terminal goes to C, R, or S. It said "see unit diagram." So I looked at the unit diagram on the access panel. It matched the compressor model. I wired it exactly like that. It worked.
The next week, I did the same thing on a different Tecumseh model. I didn't look at the panel. I used the same universal relay drawing, assuming the terminal positions were identical. The compressor hummed, drew locked-rotor amps for a few seconds, and tripped the overload. I had swapped the start and run leads. The generic diagram was correct for that relay—but I had ignored the compressor diagram that showed a different hookup.
Winner: the access-panel diagram, after you verify the model. The generic diagram is fine for understanding relay pin functions, but it's not a substitute for the wiring diagram specific to your compressor. On Tecumseh HVAC units, the diagram is usually on the access panel or the compressor nameplate. Let me rephrase that: the generic diagram is a map of the relay, not the compressor. If the panel diagram has been painted over or faded, don't guess. Get the model and serial number and use Tecumseh's parts lookup.
Test the compressor before you blame the relay
I know you came for the wiring diagram. But here's the thing: when I started, I replaced relays first and tested later. Later usually meant after I heard the compressor try to start with a bad run capacitor and cook the relay.
If you searched "how to test AC compressor," this is the part you actually need. The question isn't "relay vs no relay." It's "how to test the compressor windings before you spend $40 on a part you might not need."
Lock out power. Wait for the capacitors to drain. Then use a multimeter on the compressor terminals: C to R, C to S, S to R. According to Tecumseh's service literature, winding resistance values change with model and temperature, but on a single-phase motor the reading from C to S plus C to R should roughly equal S to R. Also check from each terminal to ground. If you see a dead short, an open winding, or a reading to ground, a new start relay will not fix it. The relay might work fine and the compressor is still broken. I've made that exact mistake.
In September 2022, I replaced a start relay on a Tecumseh compressor without testing the run winding. The compressor was shorted to ground. The new relay clicked, the breaker tripped, and the customer looked at me like I was the reason the power went out. That was mistake #9 on my list: $62 for the relay, about $120 of my own wasted time, and a 3-day delay while the replacement compressor arrived.
Winner: test first. It takes ten minutes. Replacing a relay blind takes one hour plus a callback. The math is simple.
OEM relay vs universal relay: where I sit now
I have mixed feelings about universal replacement relays. On one hand, they've saved me on a Sunday when the local wholesaler was closed and a customer's freezer was full of food. On the other hand, I've seen too many universal relays wired wrong by people who trusted the box instead of the compressor.
If the Tecumseh model and serial number are available, I try to get the OEM relay part number first. Tecumseh's parts lookup usually shows a specific relay. That part is meant to sit on that compressor and connect the same way as the original. You still need the diagram, but there are fewer variables.
Universal relays are not all the same. Some have 3 terminals, some have 5. Some need a jumper between terminals. If the universal relay's wiring diagram says "see compressor diagram" and you don't have the compressor diagram, stop. You're not ready to wire it.
To be fair, universal relays are generally clear if you've done one before. The issue isn't the part; it's the assumption that all compressor terminals are laid out the same. They aren't. On a Tecumseh, C, R, and S might be arranged in a triangle; on another brand, they might be in a line. The compressor doesn't care about your brand loyalty.
Winner: OEM for certainty, universal for convenience—but only if you verify the compressor hookup. The surprising part: universal relays aren't as cheap as they look. The part might save you $27, but the time you spend figuring out an unfamiliar pinout costs more than that. Don't hold me to exact prices—they change—but the logic doesn't.
The pricing angle nobody talks about
This one has nothing to do with wiring, but it has everything to do with why you're reading an article instead of calling a tech. If a service company advertises a $69 diagnostic fee, then tells you it's $180 to install a relay and another $90 for a capacitor and a $30 hazmat fee because they touched the compressor, that's not an honest price. It's a headline.
I've learned to ask "what's not included?" before asking "what's the price?" The tech who says "the total will be $240, and that includes the relay, capacitor, labor, and tax" is a tech I trust. The total may be higher than the headline price, but it's a real number.
"The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end."
In 2024, I started putting a single line on all my service invoices: "No surprise charges. If I find something else, I call you before fixing it." A few clients were clearly shocked, but then they started sending their friends. That's the transparency lesson.
Granted, this approach takes more discipline. It's easier to quote a low number and add on later. But I do not want to be the reason someone doesn't call a tech next time.
The day the EGO leaf blower almost cost me a compressor
One more thing that doesn't fit neatly into a comparison: your attention. In October last year, I was in a hurry to finish a compressor relay job so I could check a faulty outdoor heater for another customer. My neighbor was clearing leaves with an EGO leaf blower, and the noise was making me rush. I mounted the relay, connected the wires, and was about to power up before I realized I hadn't confirmed the wiring diagram against the model number. I stopped, checked, and found I'd swapped C and R. The compressor would have hummed and tripped overload, again.
So glad I double-checked. Almost re-created my 2017 mistake because of a leaf blower and a heater call. Now our team has a simple pre-power checklist: model verified? Diagram verified? Windings tested? All costs in the estimate? If the answer to any of these is no, stop.
So which should you use?
If you're asking specifically about a Tecumseh compressor start relay wiring diagram, the answer is not "trust the internet image." The answer is:
- Use the diagram on the compressor or access panel if it matches the model. If not, pull the model-specific diagram from Tecumseh.
- Test the compressor windings before you replace the relay.
- Prefer the OEM relay when you can wait; use a universal only if you're comfortable reading its diagram and matching it to C, R, S.
- Quote the total price up front, and expect the same from anyone you hire.
That checklist is not exciting. It came from a pile of my own mistakes, roughly $8,400 in wasted budget over nine years. I don't want you to pay for my education.
If you're working on another brand, most of this still applies. I've only tested Tecumseh and a few Copeland and Danfoss models. For anything else, your experience may be different. And if you find out otherwise, tell me—I'll update the list.
