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A/C compressor is not turning on

Tiny
BHAMDOC1973
  • MEMBER
  • 2003 FORD MUSTANG
  • V8
  • 2WD
  • AUTOMATIC
  • 138,000 MILES
I need to find out where the relay/fuses for the compressor, it turns on by hand. Attached gauges while car running, and they read 90 on low and 90 on high side. Checked fuse 24 which is 20 amps and it's good. Checked wires at compressor but both are ground. Also jumped the low-pressure switch and nothing. It has power and ground. There is also this switch, which has two powers and two grounds, tried to jump power to ground and nothing. It may have also caused the fan to stop working after I did that. I think I blew a fuse when I jumped it.
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Friday, June 17th, 2022 AT 8:44 AM

13 Replies

Tiny
BHAMDOC1973
  • MEMBER
  • 1,575 POSTS
Never mind fan is on, but still 2 wires at compressor are showing ground
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Friday, June 17th, 2022 AT 9:58 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,758 POSTS
Hi,

At the compressor, you should have a black wire (which is ground) and a black wire with a yellow tracer (which is power).

If you disconnect the connector from the compressor, with the engine running and the AC turned on, do you get power to the black/yellow wire? That wire comes from the compressor relay and should power the clutch coil, so it engages.

Check that and let me know what you find. As far as the wires, if you have continuity to ground via the black/yellow, pull the compressor relay and recheck. If it still shows ground, the wire must be pinched or broken and is shorted to ground.

Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-check-wiring

Also, I attached the wiring schematic below of the A/C system for your reference.

Let me know what you find

Take care,

Joe

See pics below.
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Friday, June 17th, 2022 AT 8:03 PM
Tiny
BHAMDOC1973
  • MEMBER
  • 1,575 POSTS
I am having a problem recognizing where the A/C clutch relay is. I thought about removing the compressor's wire and checking for power and ground but didn't do it. Will do that first.
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Friday, June 17th, 2022 AT 8:09 PM
Tiny
BHAMDOC1973
  • MEMBER
  • 1,575 POSTS
Removed the wire, there is ground, there is no power but sometimes I get a reading that isn't power cause the probe doesn't light or beep and it reads 9.9.
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Saturday, June 18th, 2022 AT 10:12 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,758 POSTS
Hi,

Is that 9.9v? If it is, the light should turn on. Do you have a different way of testing it other than the test light? You could use a voltmeter or multimeter. Also, you indicated the 9.9v is sporadic. How much of the time is it non-existent?

Let me know.

Joe
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Saturday, June 18th, 2022 AT 10:40 PM
Tiny
BHAMDOC1973
  • MEMBER
  • 1,575 POSTS
Didn't say 9.9v just 9.9 no beep or light and not always there. I am using a probe and if its volts/power it would beep. I have a regular test light as well.
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Sunday, June 19th, 2022 AT 5:41 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Okay, I'm not familiar with the tool used that provided the 9.9. However, if you used a test lamp and it never turned on, it can't be getting power.

By chance, have you checked for voltage at the relay?

Joe
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Sunday, June 19th, 2022 AT 7:30 PM
Tiny
BHAMDOC1973
  • MEMBER
  • 1,575 POSTS
It's a regular probe. I don't know where the relay is or which one in the box and how do I test it or jump the wire to see if the compressor kicks on?
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Sunday, June 19th, 2022 AT 7:32 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,758 POSTS
Hi,

According to the manual, the relay is mounted on a bracket behind the engine. If you want to test it, here are the directions for checking a relay.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Let me know if this helps.

Take care,

Joe

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Sunday, June 19th, 2022 AT 8:08 PM
Tiny
BHAMDOC1973
  • MEMBER
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Do you know where behind the engine? Some videos show it behind the fender liner.
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Monday, June 20th, 2022 AT 8:53 AM
Tiny
JACOBANDNICKOLAS
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Hi,

Check behind the coolant reservoir. It should be located in the constant control relay module. Keep in mind that some of these relays are integral. If that is the case, the module would need to be replaced. And based on the model year, that may be the case. I attached what the CCRM looks like on the newer Mustangs. See if this is what you find. And as far as the relays are concerned, they are integral. I believe you can take it apart, but the relays are usually soldered onto a printed circuit board.

Let me know.

Joe
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Monday, June 20th, 2022 AT 10:06 PM
Tiny
BHAMDOC1973
  • MEMBER
  • 1,575 POSTS
Replaced the relay module and all is good. Thanks
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Saturday, June 25th, 2022 AT 7:57 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,758 POSTS
Hi,

You are very welcome and thanks for the update. I'm glad to hear it's working.

Take care of yourself and please feel free to come back anytime in the future.

Joe
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Saturday, June 25th, 2022 AT 8:50 PM

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