2010 Mountaineer A/C Compressor will not engage. | Ford Explorer Forums - Serious Explorations

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2010 Mountaineer A/C Compressor will not engage.


January 20, 2024
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City, State
Modesto, CA
Year, Model & Trim Level
2010 Mountaineer Base V6
I have a 2010 Mercury Mountaineer Base V6 4.0, A/C is blowing out warm air. I took it to the shop to have the refrigerant pressures checked. They said the pressures were good but the compressor wasn’t engaging and would probably have to be replaced. I took it home to do more troubleshooting. I found the compressor is not engaging, but if I jump the relay it will engage and the air seemed to be cold for the few seconds I let it run. Next I checked the low pressure switch and it tested open with no continuity between the two leads. I tried to bypass the switch with a jumper wire and the compressor still does not engage. I do have 13.7 vdc on the two terminals on the low pressure switch harness. It seems either the pressure is low or the switch is bad because the switch is open, however the compressor still doesn’t engage when I bypass the switch. Am I missing something? I am happy and able to any more troubleshooting. Any help is greatly appreciated, thanks in advance Joel.

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I did swap the relay with the other identical one in the fuse box and it had no effect.

Interior fuses 20 , 10a and 28, 10amp protect deatc

Power box under hood fuses 35, 10amp protects ac clutch, fuse 40, 15 amp protect ac clutch.

You might check those.

Thanks for the help. I checked all the fuses that seemed relevant previously, however I’m not 100% sure exactly which fuses I checked so I will go back a double check the ones mentioned above. I’m thinking I have at least two problems. The low pressure switch is open so either the pressure is low or the switch is bad. Since the compressor doesn’t engage when bypassing the switch something else must be bad right? Where can I find an A/C wire diagram? Any other suggestions or thought?

All fuses double checked and good. I also swapped the A/C relay with fog lamp relay. The fog lamps worked and the ac clutch did not with either relay. The last thing I did was bypass the low pressure and high pressure switch. They low pressure switch only has two leads with 13.7vdc. The high pressure switch had 5vdc across two of the three leads. I ran a jumper between those leads. The compressor still did not engage with both switches bypassed. Is it possible I have a PCM failure or am I missing something else? Thanks again for all the help.

Just to make sure, you jumped the connector correct?

Make sure automatic temp is turned off, and try different setting , defrost.

I do not have a schematic but we need to know if the switching is done at EATC unit, or pcm. I'm not sure about that one

Just to make sure, you jumped the connector correct?

Make sure automatic temp is turned off, and try different setting , defrost.

I do not have a schematic but we need to know if the switching is done at EATC unit, or pcm. I'm not sure about that one
Thanks for the fast response Turdle. I did jump both connectors at the harness connector, not at the switch. Auto was off, I had the fan on high and temperature all the way down. I did try defrost and the other setting and let it run for at least a minute with both switches bypassed. Compressor never engaged.

Do you know where I can find the schematic? I think that would help me narrow down the problem. I would also like to know if I change the low pressure switch will I need to drain and refill the system or is there a check valve to prevent the refrigerant from leaking?

So assuming I have more then one problem I will at least try to fix what I know is bad and then go from there. I know the low pressure switch is open and has no continuity across the leads on the switch. That means either the pressure is low and the switch is working correctly or the pressure is good and the switch is bad. I had a shop check the pressure last summer and supposedly it was good then but to be sure I plan on buying a cheap set of gauges so I can check myself. Does the compressor need to be running to get a reading? If so can I just bypass the relay for a moment without any major concerns? Obviously It would only be a moment just to get a reading on the gauges. If the pressures are good then I will replace the switch. Is there a check valve so the switch can just be removed and replaced, or does the system need to be drained before and refilled after?

So I bought a set of gauges, I also got a low pressure and high pressure switch from a 2007 Explorer in the wrecking yard. I plan to do some more troubleshooting this weekend. I should be able to rule out any pressure, pressure switch failures. If the compressor still does not engage how would I diagnose the PCM or the EATC unit? The Explorer at the yard does have both those parts on it. I know the PCM would need to be reprogrammed to work in my 2010 Mountaineer besides that any other reason why it wouldn’t work?

did the JY pressure switches work? I don't think you can jump those connectors like the older cars. These models have some computer logic that infers the info from pressure switches. I'll see if I can find something in my wiring book to copy

Hey Ruddy, thanks for the response. I would appreciate it if you could find anything on the wiring. As far as if the pressure switches from the junk yard work, I am not sure. If you know the correct way to test the newer computer controlled switches I would love to hear it. Thanks again for the help, Joel.

I couldn't find the article about bypassing jumping on newer cars.
Attached is wiring, parts involved, on sequence and pinpoint test for Pressure Transducer

Climate control on sequence.jpg

Climate Control parts flow.jpg

Climate Control transducer test1.jpg

Climate Control transducer test2.jpg

Climate Control transducer test3.jpg

Climate Control transducer test4.jpg

Climate Control transducer test5.jpg

Climate Control wiring.jpg

Ruddy, thanks again. This will help a lot when I do more troubleshooting. Unfortunately I do not have a scan tool so I won’t be able to do all of these tests myself. Looks like the low pressure switch is the AC Cycling Switch and it should be able to be bypassed with a jumper according to this schematic. The high pressure switch is the A/C Pressure Transducer, that one doesn’t look like it could be easily bypassed with a jumper. Is this model EATC unit capable of running a self diagnosis? If so what is the process to initiate it. I found some other threads for older units but the combination didn’t seem to trigger any testing.

with Forscan and a OBD reader like this
Amazon product ASIN B07MQ8GHG3you should be able to do some of those test or at least see the values it refers to things like ACP_PRESS.
Forscan also has its's own A/C test that goes through the components

I did some more troubleshooting yesterday and I was able to determine my first problem. I bypassed the compressor relay and hooked the gauges up to check my pressures to determine if I have low pressure or a bad low pressure switch. Turns out I do have low pressure. I actually have negative pressure. How does that happen? I would think if all the refrigerant leaked out it would have no pressure but also no vacuum. The only thing I can think, is the shop I took it to drained the refrigerant to measure the amount and “forgot” to refill it. They said it had the correct amount they refilled it and added dye to check for a leak, drive it for a while and bring it back. They also said the compressor was probably bad. Before I took it to the shop my ac was not very cold on the driver side but I bit colder on the passenger side. I added a can of refrigerant and it got colder for a few days then it seemed to stop working. I thought maybe I overfilled it. Am I okay just buying cans and filling it myself.



Those gauges don't show negative. You can get a basic reading on the low side without starting engine. Just need to know the temp inside the engine area and read a table you can find on the Internet.
I believe shops add nitrogen and dye to pressure test. If good they evacuate and add freon.
Too much freon will strain compressor and it is less efecient. That and allowing some air might be how i destroyed mine

Am I ready the gauges wrong? Red is the high side and blue is the low side. On both gauges the reading for r134a is the outer most blue numbers. Red shows below zero, around -6 and the blue shows -8. These were taken with the car running and ac on max, the relay was bypassed and the compressor was engaged.

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Still trying to get this issue resolved. I took it to a shop. He hooked his scan tool to it. He said the high pressure sensor and low pressure switch tested good. He said he should be able to turn the compressor on with his scan tool, but it’s not even giving him the option. He said he can see head unit respond when he turns the ac on but it shows wide open throttle. The car runs and drives fine with no check engine light or P codes. He thinks the PCM is preventing the compressor from engaging. Anybody have experience with a bad PCM with no codes not allowing the ac compressor to engage?