Eric Mele investigates a miswiring issue on an older commercial McQuay system that has had a compressor replacement in the past. The surface-level issue is that the compressor runs and runs into a vacuum and doesn't shut off.
Eric's first step is to investigate the controls, including the board, relays, pump-down switches, and more. Old McQuay units have separate compartments for fuses as well as controls and contactors for the compressor and condenser fan. In the case of this unit, Compressor 1 has been deactivated.
After reading the schematics, Eric realizes that the safeties were tied into relay R5, and there could have been an issue with the low-pressure switch. However, that is not the case. After the system pumps down, Eric reads infinite ohms on the wires extending from the low-pressure switch. The next potential issue is that the incorrect wiring was used; the circuit should be incomplete, so Eric turns the deactivated compressor back on to see if the unit doesn't run (as it should) or if it starts and runs. When he turns the deactivated compressor back on, the unit starts and runs.
Further wiring investigation leads Eric to an issue on wire 129; it goes to terminal 2 on the Centronic, but it should actually be going to terminal 2 on the motor protector module. Wire 129 was swapped with 120. After Eric replaces the wires, he starts up the unit after a time delay, and the compressor pumps down and shuts off as it should.
Overall, the wiring was backfeeding power to the compressor all the time. Once the wiring was reconfigured correctly, the compressor stopped running and running into a vacuum.
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Eric's first step is to investigate the controls, including the board, relays, pump-down switches, and more. Old McQuay units have separate compartments for fuses as well as controls and contactors for the compressor and condenser fan. In the case of this unit, Compressor 1 has been deactivated.
After reading the schematics, Eric realizes that the safeties were tied into relay R5, and there could have been an issue with the low-pressure switch. However, that is not the case. After the system pumps down, Eric reads infinite ohms on the wires extending from the low-pressure switch. The next potential issue is that the incorrect wiring was used; the circuit should be incomplete, so Eric turns the deactivated compressor back on to see if the unit doesn't run (as it should) or if it starts and runs. When he turns the deactivated compressor back on, the unit starts and runs.
Further wiring investigation leads Eric to an issue on wire 129; it goes to terminal 2 on the Centronic, but it should actually be going to terminal 2 on the motor protector module. Wire 129 was swapped with 120. After Eric replaces the wires, he starts up the unit after a time delay, and the compressor pumps down and shuts off as it should.
Overall, the wiring was backfeeding power to the compressor all the time. Once the wiring was reconfigured correctly, the compressor stopped running and running into a vacuum.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
So today we're going to look at this old mcquay unit. It's had a compressor replaced on it. Supposedly this compressor just runs and runs into a vacuum and doesn't shut off. So i'm here following up, i got a dig into what's exactly going on with this compressor here now, if you've never messed with one of these before you kind of have controls everywhere, that's fun when the logbook falls out, but here's our control board.
We have some stuff going on here: blower contactors, it's a two-speed blower behind here is just the disconnect itself and you have your control and your pump down switches here. Here's all your relays for various stuff on the unit, your blower, your! I don't think this has heat, but compressor you get the idea. I think they got rid of this compartment on the newer units, but in here in this one it's just fuses. The newer units got a similar footprint, but they did away with this compartment.
Not exactly sure where they stuck the fuses over here we have our controls and contactors for the compressor. Let's see if i can get these doors propped open one-handed, so your compressor contactors there, your condenser fan contactors there, some fuses for the condenser fans. You've got pressure, controls, probably low pressure and you've got your schematic and all of your relays here compressor one has been here. Obviously there's something going on with it.
So we'll go ahead and see what the deal is. So i'm going to dig into the wiring study. My schematic my relays and figure out where everything is and maybe hopefully find a fault without turning it on so i'll, dig in and i'll restart the video. When i have an update all right, so i've been studying the diagram.
Now we have cs1, which is control. Stop one for our compressor one. Now, if we follow that schematic the relevant part we're looking at is, it goes to 102, then to 40, and then it goes down and then it's coming down through our time delay here, pd1 on wire, 112 and out 114 or 113, so 113 goes to 44.. Goes through the low pressure switch, it goes through.
The r5 relay eventually makes its way to 46 on the terminal strip and then over to compressor contactor m1. Now the r6 or the r5 relay is energized by the motor protection, the oil protection um. I forget what r1 relay is doing, but that's just our safeties tied in to that relay. So it looks like if what the previous tech said is running into a vacuum is true.
We could have a problem with our low pressure switch. It seems to be the most likely thing. Let's see what that looks like check that out here. So initially i mean a lot of these.
Labels are worn off the wires, that's not helpful, but from what i can gather the low pressure switch seems to be fine. I initially thought these were going to be them, but that's actually this sub cooling circuit. So we don't got to worry about that. They're hooked to this terminal board they're, not in my circuit.
It ran it pumped down. It should have cut out on low pressure. So you can see here a low pressure switch tied back into these two wires open line. If i hit the control switch, even with them out of the circuit and just to in case it wasn't clear, it was wired with these wires properly in series. Now. Are these wires? The right wires is my next question because they're totally disconnected, they should be an incomplete circuit manually, eliminating the low pressure switch and stopping my compressor from running. But if you see here it starts and runs so after some tracing and relabeling. This wire 129 is going to two on the centronic.
It's supposed to be going to two on this motor protector module. So it's swapped with 120., which is supposed to be going to two on the centronic, so we're gon na have to rearrange some wiring to get that correct. So once the wires were swapped in here, we kind of saw what was going on in here and everything fell in place as far as the wires that were switched. It was a pain in the butt to trace what needed to be done.
Control switches on it's not running now should be in time delay we're about to start it up so now it should pump down and shut off and there it goes all right. Well that looks like it's it thanks for watching. I know it was pretty hard to see from the spaghetti. That's in all these control compartments, but basically, what was wrong was wires.
120 on terminal 2 here and 129 on terminal 2. Here were swapped, so it was just back feeding power to the compressor contactor, all the time simple, once you figure it out once you trace it all down, thanks for watching our video, if you enjoyed it and got something out of it, if you wouldn't mind hitting The thumbs up button to like the video subscribe to the channel and click the notifications bell to be notified when new videos come out, hvac school is far more than a youtube channel. You can find out more by going to hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more. You can also subscribe to the podcast on any podcast app of your choosing.
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