A/C relay switch or sensor

1999 FORD MUSTANG
18,000 MILES • 4.6L • V8 • 2WD • MANUAL
Avatar
GR VERNER
  • MEMBER
  • 2 POSTS
My car is the GT model. A/C does not work and I took it to a shop and they tested the whole system and said the system is fine it is holding Freon. The compressor and clutch are good.

They told me the next step was to take the inner fender off to check a relay or a circuit. I did not have the special lug nut tool so never did get that checked.

Now I moved to Texas and it is hot!

My question is: What relay or circuit could it be? If the system is good and it is an electrical problem? what could it be? what is the name of the sensors or relays involved with the A/C system?

The car has low mileage and garage kept. I did not even run the air much up north in Pennsylvania. Any direction here would be helpful. :)
Nov 22, 2017 at 6:14 AM
Advertisement
Avatar
STEVE W.
  • CERTIFIED EXPERT
  • 15,113 POSTS
They were probably talking about the compressor relay. It is part of the "constant control relay module" (CCRM) The module also contains the relays for the fuel pump, cooling fans, A/C clutch and the PCM. It is behind the coolant reservoir.
There are a couple other items that should be tested as well, the high and low pressure switches can fail and either one will prevent the system from working. They can both be tested or replaced without needing to discharge the system.
The low pressure switch(aka cycling switch) opens if the pressure in the system drops below 22-27 psi and closes at around 35-40 psi. So if it is tested while it is installed it should test for continuity if the system has R134A in it.
The high pressure switch is tested the same way, it should be closed until the system sees a high pressure. Setting still and the clutch not working it will be closed.

So if there is correct R134A amount in the system both switches should be closed.
Easy way to test, unplug them one at a time and jump the two wires, turn on the AC, if it works with either one jumped then the switch is bad.

Those switches are either on the accumulator or in the lines, Ford used a couple locations depending on actual build date.
Image (Click to enlarge)
Attached Media
Nov 22, 2017 at 11:57 AM
Avatar
GR VERNER
  • MEMBER
  • 2 POSTS
Thank you so much, that is good advice I googled the CCRM and that is what they were talking about as it is inside the inner fender well. The shop did all the tests above and said all was good so it has to be-be the relay that is inside the CCRM. Looks like radio shack makes a replacement relay that will work. I will be looking to do that.

Thank you Again!
Nov 22, 2017 at 1:52 PM
Advertisement
Avatar
STEVE W.
  • CERTIFIED EXPERT
  • 15,113 POSTS
Be careful if you are replacing the relay, the board has two crimped ground connections so you have to work between the backing plate and the board. For that a very narrow tip on the soldering iron is useful.

Good luck with it and thank you for using 2CarPros.com.
Nov 22, 2017 at 3:08 PM