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Discussion Starter · #1 ·
Hello Everyone,

I've spent all today troubleshooting this AC problem. Yesterday I replaced the entire AC system on my '99 Pathfinder; compressor, radiator, evaporator, condenser, and receiver/dryer, AC lines are the only things I didn't replace (but I did evacuate them). Pulled a -30psi vacuum that held for over an hour, then evacuated the system for nearly 2 hours. My sticker called for 6.8 ounces of PAG, my compressor came with 6.1, I drained 2.5 ounces from it and distributed it (and the additional .7 ounces) to the new components and lines.

This morning I went to fill it up and the clutch wouldn't engage. I assumed this was because there was no refrigerant in the system, bypassed the relay and the clutch engaged, allowing me to add refrigerant. I filled the system up with 22 ounces of R134a, as the sticker recommended. Beautiful, cold air was blowing into the cabin, I was so happy. Unfortunately, when I removed the relay bypass and put the fuse back, the clutch again refused to engage. I swapped a confirmed working relay with the existing one, didn't make a difference.

Static pressure in the system measured ~100 degrees for both the Hi & Lo (it was around 88 degrees, at the time). When I bypassed the relay again, my Lo dropped to 45psi but my Hi only rose to 150psi. From what I've read, my Lo is in range, but my Hi is too low. Since my static measurement matched ambient temp, pretty sure I have enough coolant.

I'm stumped, hoping someone with greater knowledge could give me some things to test/try. I'm in Florida and it's at least another 3 months of unbearable heat....I'm feeling desperate/despondent.

Left pic is static, right is with clutch engaged
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If the clutch isn't engaging and it's cold when you engage it manually, then the problem isn't system pressure. The 150 psi is a little low and the 45 psi is a little high, but not a lot. Check the Dual Pressure Switch on top of the Receiver/Drier. It should show near 0 ohms with a normal charge in the system. If it shows open-circuit (infinity) then that's your problem. Since you just changed a bunch of under-hood parts, a damaged switch or connector is a good bet. If it checks out, back-probe the Blue/Black wire at the switch and measure voltage with the A/C and blower on, it should measure near 0V. If it doesn't, there's either a wiring issue between the A/C control head and the switch, or the control head isn't sending a compressor "on" signal for some reason (a bad control head, no blower signal, etc).
 

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Discussion Starter · #3 ·
If the clutch isn't engaging and it's cold when you engage it manually, then the problem isn't system pressure. The 150 psi is a little low and the 45 psi is a little high, but not a lot. Check the Dual Pressure Switch on top of the Receiver/Drier. It should show near 0 ohms with a normal charge in the system. If it shows open-circuit (infinity) then that's your problem. Since you just changed a bunch of under-hood parts, a damaged switch or connector is a good bet. If it checks out, back-probe the Blue/Black wire at the switch and measure voltage with the A/C and blower on, it should measure near 0V. If it doesn't, there's either a wiring issue between the A/C control head and the switch, or the control head isn't sending a compressor "on" signal for some reason (a bad control head, no blower signal, etc).
Wow, thank you! Feels like you just turned on a flashlight in my pitch black darkness. That all makes sense, time to bust out (find) my multimeter :)
 

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Discussion Starter · #5 ·
One other lingering question that's been worrying me. As I stated in the original post, the compressor I have (Welcome to UAC) came with 6.1oz of PAG; since my system calls for 6.8oz of PAG and I'm installing all new components, I drained 2.5oz out (I couldn't get anymore to come out). What's worrying me is where I drained the oil from.

The screw I removed and hole I drained the oil from is in the picture below. After I drained, I noticed there is also a drain screw at the rear of the housing at the bottom. There aren't two separate/exclusive chambers in a/the compressor for oil, are there? I'm worried I drained the oil out of the chamber and it won't be replaced by the oil in the system and the pistons, etc., might be running dry. The oil is shared by the whole system, right??
 

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Yes, there's no segregation of oil in the system. You can't drain or fill in a "wrong place", the oil will still go where it needs to go once the system operates.
 

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Discussion Starter · #8 ·
A little more backstory (that might give crucial insight), I haven't been running the AC since it went out almost a year ago. I tried adding the refrigerant with dye you get from the automotive store back then to find any leak, but it never appeared when I searched over the system with a black light. I tried to run the AC multiple times after that, but gave up after only hot air blew out and I never found any dye. When I was blowing out the hoses on Wednesday, all the dye shot out of the Lo hose, it seems all of it just stayed right where I injected it at (the hose expands to a larger, round cylinder at the low pressure nozzle, pretty sure it accumulated right there).

I'm thinking now that this current clutch issue wasn't introduced by my changeout, it was probably the reason my AC stopped blowing cold last year. I don't know if that changes the order of things I should be testing, but it gives me hope I didn't screw something up during the changeout...
 

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You said jumping the relay gave you cold air, so fretting over anything operational in the system is pointless. You know it works. The question is why the ECM won't activate it, and the triple switch is the most likely culprit.
 

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Discussion Starter · #11 ·
Well, score one for the Easy Button. It was recommended to detach the battery for awhile to let the ECM reset, so I did just that for ~30 minutes. Reattached the battery and I was greeted by slightly cool air coming out of the vents as soon as I turned the engine on. A few seconds later it was colder and it kept going down. Resetting the ECM worked, clutch is working as expected now!! I didn't have to put a multimeter on anything....words can't express how happy I am right now.

Thanks for steering me in the right direction VStar! 24 hours ago I was miserable, now I feel like taking a long drive somewhere in this awful Florida heat :D
 
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