A/C not working

Tiny
DIVIDES
  • MEMBER
  • 2007 FORD MUSTANG
  • V8
  • 2WD
  • MANUAL
  • 177,000 MILES
Bought the car like it is with the A/C not kicking on, clutch not kicking on, does not overheat. Where is the A/C compressor fuses for starters? I see someone probably tried to test the wires to the compressor already, but it could be the compressor, Freon level, fans, etc.
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Tuesday, October 5th, 2021 AT 9:51 AM

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Tiny
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With engine off.
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Tuesday, October 5th, 2021 AT 10:00 AM
Tiny
DIVIDES
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Engine no same thing. Compressor not kicking on fan not coming on.
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Tuesday, October 5th, 2021 AT 10:03 AM
Tiny
KASEKENNY1
  • EXPERT
Hi again,

This could be a couple of things since the static pressure looks fine. I would suggest starting with removing the relay and let's check the entire control system from there.

Let's start with checking for 12 volts at pin 86 and ground, then 30 and ground with the key engine on and A/C on.

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

If you have 12 volts on both, then both fuses are fine. Then jump one of those wires to terminal 87 of the relay and see if the compressor kicks on.

Lastly, we need to check the PCM to see if it is commanding it on. You do this by checking pin 85 for ground so put your meter on battery positive and then the black lead on pin 85. The with the engine and AC on the meter should read 12 volts. This shows that the PCM is commanding the relay closed and the compressor should be on. This means you have a relay issue, or the compressor clutch has failed.

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

However, if you don't have 12 volts then the PCM is not commanding it on. This could be a pressure switch or a PCM issue.

So, let's get to that once we know all this info. Please let us know what you find. Thanks
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Wednesday, October 6th, 2021 AT 3:23 PM
Tiny
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So, there are two relays to the A/C are we talking about testing the one on passenger side? When you say check the ECU readings with A/C on meaning car running?
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Wednesday, October 6th, 2021 AT 4:34 PM
Tiny
KASEKENNY1
  • EXPERT
There may be another relay that is tied to the A/C but not one that directly feeds the clutch.

Take a look at the wiring diagram that I attached last time and the one in the engine compartment directly feeds the clutch and gets turned on and off by the ECM so we need to start with this one.

Correct. The engine on and A/C on this way the clutch will come on. You can do it with the engine off, but you will have to listen for a single clunk from the clutch and that can be missed.

Just make sure you hook the meter to battery positive so you can check for ground from the ECM.

I also attached the info on the fuse box layout and relay so you can see the terminal layout of that as well.
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Wednesday, October 6th, 2021 AT 5:28 PM
Tiny
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Pictures say f47 and f49 are both A/C relays, but these are actually fuses. Both are good. I am having hard time finding which is the A/C relay. Also, I am using a power probe to check fuses, will it be okay to do so to check for 12v when I find the relay?
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Saturday, October 9th, 2021 AT 7:45 AM
Tiny
KASEKENNY1
  • EXPERT
Yes. Sorry. Maybe I caused that confusion. Fuses 47 and 49 are the two fuses that feed the power circuits for the relay on pin 86 and 30.

So, if those fuses are good then we need to remove the relay on the same fuse block to jump the terminals of the relay.

See the attached for the relay information. It includes the terminal layout of the pins.

So, we need to jump 30 to 87 with the engine on and A/C on to see if the compressor starts running.

If it does then we need to check 85 for ground with the A/C on. Put your power probe clip on battery positive and then touch the probe to terminal 85 of the relay and your probe should show 12 volts.

If both of these check out, then the relay would have to be the issue.

Let me know what you find with this, and we can go from there depending on what you don't have as that will determine the direction.
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Saturday, October 9th, 2021 AT 8:29 AM
Tiny
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Another thing I noticed is that the relays can all be installed one way or 180 degrees. So, how can I tell which is 87 as it can be in 2 different places?
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Saturday, October 9th, 2021 AT 9:23 AM
Tiny
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Oh, I see. Those numbers correspond to the clips in the fuse box not the relay.
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Saturday, October 9th, 2021 AT 9:25 AM
Tiny
DIVIDES
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Jumping 87 to 30 turns the compressor on. Touching positive probe to battery positive and touching the probe to 85 does nothing. Connecting positive and negative ends of the probe to battery the touching the probe to 85 I get 0 value but the negative indicator makes a sound. Even with multimeter touching red to battery positive and negative to 85 does nothing. Touching red cable to positive battery and the negative to 87 turns on A/C compressor. Also touching a tester light to battery positive terminal and to 87 light comes on, nothing when I touch 85, so all tests are showing the same.
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Saturday, October 9th, 2021 AT 9:53 AM
Tiny
KASEKENNY1
  • EXPERT
Okay. That is showing that the module is not grounding the relay to turn the compressor on, but the compressor and wiring are able to turn on.

So, we need to get into the control side.

Let's start with the pressure switches. I would go to the dual pressure switch and check for voltage on pin 4 to start.

If you have voltage on this wire, then the cycling pressure switch is fine.

Then check for voltage on pin 1. This is the signal that is going to the PCM to tell it that the low pressure is fine. Then check voltage on pin 3. This is telling the PCM that the pressure is not too high.

Looks like these are all on/off switches so you should have 12 volts on all 3 wires with the engine running and A/C on when you back probe the connector with it plugged in.

If you want to bypass this dual pressure switch then you need to remove the connector, jump pin 1 to pin 4 and then apply 12 volts to pin 3.

If the compressor comes on, then this switch is the issue.
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Saturday, October 9th, 2021 AT 10:49 AM
Tiny
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And this is the switch on left front of the engine I assume. So even though the relay is kicking the compressor on, with the A/C on, the switch should still have these values?
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Saturday, October 9th, 2021 AT 11:30 AM
Tiny
DIVIDES
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One on the front left has 4 wires not 3.
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Saturday, October 9th, 2021 AT 11:49 AM
Tiny
KASEKENNY1
  • EXPERT
That is correct. We are now onto the control side, so we need to check the inputs to the PCM to find out if something is telling it to not turn the compressor on.

Sorry. Yes. That is a 4 wire. One is ground so I just called a 3 wire because you have 3 "signal" wires, but it is a 4 wire.

Here are the connector views for each of the switches on this vehicle.

The second is the dual pressure switch that you are looking at.
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Saturday, October 9th, 2021 AT 12:02 PM
Tiny
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By passed the switch by jumping 1 and 4, then applied 12v from battery and with the probe. Compressor did not come on. There is ground on pin 2. Nothing on pin 4, power on the other 2.
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Saturday, October 9th, 2021 AT 1:23 PM
Tiny
KASEKENNY1
  • EXPERT
Okay. Let's do one more thing before we condemn the PCM. Jump 1 and 4 again, then 2 and 3.

Looks like the PCM is looking for ground on pin 3 so applying 12 volts may have not be exactly correct.
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Saturday, October 9th, 2021 AT 3:27 PM
Tiny
DIVIDES
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Jump 1 and 4 and at same tome jump 2 and 3 at the same time? And no battery voltage to supply?
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Saturday, October 9th, 2021 AT 4:06 PM
Tiny
KASEKENNY1
  • EXPERT
Correct. Jump them at the same time then turn the key on.

If you look at the wiring diagram pin 3 is a voltage supply from the PCM to the sensor and when the sensor is closed, it supplies ground for that circuit.

So, if the PCM sees an open circuit on this circuit then it knows the pressure is out of spec and doesn't allow the compressor to run. So, if the circuit is closed the PCM will turn the compressor on so jumping it is just telling the PCM that it is good pressure.
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Saturday, October 9th, 2021 AT 4:44 PM
Tiny
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Did that and turned it on the fan cane on but not the compressor. When 1 and 4 are jumped I probe 2 and it shows ground, 3 shows positive and battery voltage.
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Saturday, October 9th, 2021 AT 5:00 PM
Tiny
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But then it is possible it's low on Freon so compressor not coming on.
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Saturday, October 9th, 2021 AT 5:03 PM

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