A/C Clutch will not engage, but air will blow.

glowstang93 said:

Joe, I jumped my low pressure switch and clutch is still not engaging. I checked the voltage from wire to wire at the compressor plug and I got 6 volts. What should the reading be at that point?

I have the electrical type clutch, not the stock one, been in car for 7 years, if this makes a difference in diagnosing.

Click to expand...

just checking to make sure you had the system turned on during the test. i know i didn't the first time i tried it.

glowstang93 said:

Joe, I jumped my low pressure switch and clutch is still not engaging. I checked the voltage from wire to wire at the compressor plug and I got 6 volts. What should the reading be at that point?

I have the electrical type clutch, not the stock one, been in car for 7 years, if this makes a difference in diagnosing.

Click to expand...

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.



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