A/C works sometimes

Air conditioning electrical troubleshooting 87-93 Mustangs.

Diagrams courtesy of Tmoss & Stang&2birds


Check out TMoss's diagram I posted. Anytime the clutch is energized, the black/yellow clutch power wire should show 12 volts or whatever the battery voltage is measured at the battery.

Make sure the black ground wire is good, and that it connects to a clean bare metal place on the engine. Measure the voltage between the black/yellow wire and the battery ground cable. Good 12 volts, then you have a bad ground. The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges

No 12 volts on the black/yellow wire, check voltage at the pink/blue wire on the WOT relay: it should be 12 volts. The WOT relay is located under the Mass air meter on the passenger side.

Good 12 volts on the pink/blue wire on the WOT relay, ground the orange/lt blue wire on the WOT relay. Recheck for 12 volts on black/yellow wire. No 12 volts on the black/yellow wire, recheck your ground for the orange/lt blue wire. If it is good, replace the WOT relay.

No 12 volts on the pink/blue wire on the WOT relay, check out the low pressure cutout switch. Look for 12 volts on the green/purple wire.

No 12 volts on the green/purple wire, the A/C control switch is bad or the power to the switch is bad.


The next step is valid only if you are sure there is sufficient refrigerant change to make the system operate. The low pressure switch is supposed to shut off the compressor if the refrigerant charge is low. It does this to protect the compressor, since the refrigerant helps to circulate the oil that lubricates the compressor.

If there is good 12 volts at the green/purple wire, measure the voltage drop across the low pressure cutout switch. With the switch connected, probe the back of the switch connector. Use a pair of paper clips or safety pins to measure the voltage between the green/purple wire & pink/blue wire. Do not connect the voltmeter to ground, just the two wires. This measures the voltage drop across the low pressure cutout switch. You should see less than .75 volts. Less voltage drop is always good, but more voltage drop is bad. More than .75 volts, replace the low pressure cutout switch. It unscrews without draining the system or causing more than a short PIFF of gas escaping. The local auto parts store should have a replacement switch. Inspect the low pressure cutout switch wiring harness contacts before replacing the switch. A replacement pigtail is also available at the local auto parts store.



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 86-95 5.0 Mustang wiring Mustang FAQ - Engine Information Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 5.0 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 91-93 5.0 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 94-95 5.0 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

another thing is if it's just comin on and off it could possibly use a charge of freon or maybe a new pressure cutoff switch