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2010 Mustang GT - A/C Clutch Not Engaging

Dak101 on Mon April 06, 2015 12:50 PM User is offline

Year: 2010
Make: Ford
Model: Mustang GT
Engine Size: 4.6L V8
Country of Origin: United States

Hey everyone! I'm new to the forums, so you'll have to excuse me for any forum rules until I've explored, as I know a lot of them are really specific with what you can post and where and what not.

I'm at a complete loss with my A/C on my 2010 GT, and am not willing to hand over $100 to Brake Masters for "further electrical diagnosis" after they gave me the run around.

In a nutshell, last summer my A/C was evapped/recharged. Everything worked flawlessly. However in Arizona the Summer comes around a lot sooner than for most and a few days ago it was one of those 95 degree days already and needed to kick on the A/C for the season for the first time. Blew fine, but did not get cold at all.

The only thing I changed on the car this Winter that would've had an effect on the A/C compressor possibly is a WOT box install (No-Lift Shift, 2-Step, which if you're considering I highly recommend as it was the best $200 I've ever spent). I followed the directions to a T, making sure every wire went to it's directed source. The WOT box basically takes control of the Throttle Position Sensor, Crankshaft Position Sensor ,Cruise Control Clutch Switch (Clutch Deactivation Switch) and Ignition Coil Power wires that are connected to the PCM's and ECU. The thing works flawlessly so I know I don't have any loose ends or undesired wire connections.

So I figured I would recharge the whole system as I know there's a pressure sensor in the A/C compressor that only engages if it's to a certain level. So Brake Masters took care of that for me, and didn't charge me as it was done less than a year ago, but they said it was an electrical issue I was facing as the compressor is not kicking on. They claimed they checked fuses, but I wanted to be sure, so I checked the specific fuses for the A/C Compressor which are 25 10A** A/C compressor relay and 43 G8VA relay A/C compressor relay in the owner's manual. The fuse was fine and the relay is questionable. I came to the conclusion that if I run a jumper wire from one end of the relay to the other, the compressor's clutch engages, and BOOM! The A/C is as cold as could be. So I said it's the relay! Which are like $35 from the dealership, and I found one on Ebay for $10, so I ordered, waited a few days and tried the new relay today, no dice. So I swapped my other relay for the Fuel pump (41) thinking I just got a cheap relay, but that didn't work either. So either my relay for my Fuel pump and this brand new relay is faulty, or I have some other sort of electrical issue. I cannot figure it out and really need some help before temps hit 115 out here.

Hit Autozone to get a new Low Pressure Sensor Switch to see if that was the culprit, before installing however I took off the electrical plug and jumped it every which way (There's 3 Wires) and still no dice. Just does not want that clutch to engage for some reason.

AC_Doc on Mon April 06, 2015 1:46 PM User is offline

I'm not specifically familiar with the 2010 GT wiring so there may be some differences.
The WOT (Wide Open Throttle) box you refer to (CCRM?) likely contains the compressor power relay (soldered in) that directly supplies the compressor clutch coil. This relay is controlled by a ground signal from the car computer.
I've replaced a lot of melted compressor relays on earlier models.
You may have disturbed something in there.

A quick check of this circuit is to measure across the compressor coil connector for +12V power when its supposed to be engaged. If +12v is there, this is NOT the problem.
Tap on the CCRM box with the end of a broom handle and see if the clutch engages (bad contacts on the power relay)...

Other things that can cause no compressor power are:
Undercharged system.
Bad low/high pressure switches. (Do not apply +12V to ANY sensor pin). Those are powered by the computer's +5V circuits and you can blow out the computer!
Bad connections or open wires (obviously)
Good luck.

Do you have a schematic wiring diagram of the WOT circuitry to refer to?


Sometimes you must accept things at faith value!

AC_Doc on Mon April 06, 2015 1:56 PM User is offline

I missed the part where the compressor engaged with the jumper on the relay.
Disregard the above comments about the CCRM.

The relay you are jumping is likely controlled by a ground signal from the car computer.
The computer checks all the pressure sensors, temperatures, before it enables that relay.

Refer to the A/C wiring schematic and follow the coil connection (the one NOT connected to a power source through a fuse) for that relay back towards the computer.
While running the AC, monitor that relay signal wire with the other voltmeter lead to ground. Should be +12V when relay OFF, and 0V when relay ON). Again, to be safe, don't apply any direct ground or +12V to any AC control circuitry.

Hope this helps.

Sometimes you must accept things at faith value!

mk378 on Mon April 06, 2015 4:54 PM User is offline

You can't jump three wire "switches", these are actually sensors that deliver a continuous reading to the PCM. You would need to use an avanced scan tool to see if the PCM is getting a reading that is within the acceptable range to turn the compressor on.

Very likely the root this problem is that the PCM is not happy about something and it is refusing to pull in the relay.

Dak101 on Mon April 06, 2015 6:04 PM User is offline

Well had it looked at by a local mustang shop with a Matco ECU reader. Guy claimed that the ECU wasn't even seeing inputs from the A/C components and that the section had been completely wiped out. So I've concluded that during the WOT box install, a faulty wire spliced into the PCM had fried the ECU in the A/C area. So he quoted me $300-$500 to get a new ECU in with proper VIN and mileage.

wptski on Tue April 07, 2015 8:12 AM User is offline

Aren't you taking a chance on another second PCM if "possibly" it's the WOT box's fault?

James89dx on Tue April 07, 2015 12:00 PM User is offline

Originally posted by: wptski
Aren't you taking a chance on another second PCM if "possibly" it's the WOT box's fault?

He said a "faulty wire spliced into the PCM" - since they located the "faulty splice" I assume they would have also then removed the splice and wired it correctly. But if not surely it's going to do the exact same thing to the new ECU.

I'm curious how the WOT box operated properly given that it was wired incorrectly

Dak101 on Wed April 08, 2015 1:52 PM User is offline

Well more information on the mystery, I contacted the WOT box company N2MB and they responded with, "What you are describing sounds like a failure that is completely unrelated to the WOT Box. The WOT Box connections at the ECU (YELLOW, GREEN AND BLUE) are high-impedance. That is, they cannot provide a signal to the ECU. They can only read signals from the ECU. So, there is no way to damage the ECU by installing the WOT Box, no matter which wires at the ECU you connect the WOT Box wires to." and then they suggested that my Bama Performance tune could have removed the A/C from the ECU, which seemed really unlikely as I've had tunes from Bama for months that have worked just fine, however thinking about it more I remembered they had sent me an updated tune with some changes, didn't mention anything specifically about A/C components and they typically wouldn't mess with that I would think. So I retuned my ECU back to one of my first tunes, one that I knew had working A/C in hopes that the ECU was just not being fed the right information. Didn't help, so looking through some related forum topics with the Bama tunes and A/C not working properly they suggested that I try resetting the ECU by unplugging ground for 30+ minutes, didn't work, or by turning the key on, not the engine, waiting till the beep notifications stopped, pressing the gas pedal to the floor slowly in 3 seconds, then back up in 3 seconds, and then waiting 6 seconds, still no cold air. So I performed an idle reset. Loaded the initial street tune that came with my tuner (That I know had working A/C) and let the car idle for 5 minutes, A/C on full blast for 2, off for 2, then back on for 2, and then drove the vehicle for 10. Still nothing at all. I just dropped off the car back with the shop as he suggested he takes it to Ford himself to have them run diagnostics on it that may see farther into it then his Matco tool. Just seems like it's a finger pointing game at this point. I emailed Bama about the issues but haven't heard a response yet.

Dak101 on Wed April 08, 2015 1:58 PM User is offline

The WOT box is currently removed, but if I put it back in I'm now sure I know where everything goes. The directions that came with it were for a different model year, and then on top of that a lot of the colored wires were rearranged, so I was putting together a puzzle attempting to install it. Once installed though it makes you run through a checklist, if something isn't right it makes it very clear, I didn't have it right the first few times, but by the end, it was in correctly and working properly. Just with the information I got from that company about it not being able to do any damage, unless you switch your ground and power, which I wouldn't be able to do as the ground wire is like 3 inches long and power reaches the engine compartment and are very clearly labeled, and even then they claim it would blow the WOT box before doing anything to the vehicle. At this point I think I either have a faulty ECU for unknown reasons (Possibly the Bama tune) or Brake Masters lied to me and I have too low of pressure in the A/C system for it to automatically kick on, however that doesn't make sense because the air is freezing cold when the relay is bypassed, and the ECU isn't seeing any of the A/C components at all.

mk378 on Wed April 08, 2015 8:29 PM User is offline

Yes, since it gets cold when bypassed, the refrigerant amount is about OK. Compressor lock-out only happens when the charge is so far off that you wouldn't be able to get any cold air.

Logic to drop the A/C clutch out at WOT is a common performance feature (both stock and aftermarket) to maximize hp to the wheels. Are you sure this WOT modification has nothing to do with the A/C?

wptski on Thu April 09, 2015 10:57 AM User is offline

Originally posted by: James89dx
Originally posted by:

He said a "faulty wire spliced into the PCM" - since they located the "faulty splice" I assume they would have also then removed the splice and wired it correctly. But if not surely it's going to do the exact same thing to the new ECU.

I'm curious how the WOT box operated properly given that it was wired incorrectly

Whoops! I missed that.

Dougflas on Thu April 09, 2015 11:27 AM User is offline

Moral of the whole episode....don't mess with the manufacturer's systems.

Dak101 on Mon May 25, 2015 9:42 PM User is offline

Turned out to be the high pressure sensor switch which I was unable to locate on my own, the harness had melted and was telling the ECU a constant extremely high pressure due to it grounding out. A local mustang swap changed out the switch, harness and evapped and recharged my whole system for around $400 and the issue has been completely resolved since. Which was caused by unrelated issues than the WOT box install.

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