A/C relay not working?

2003 LINCOLN NAVIGATOR
134,000 MILES • 5.8L • V8 • 4WD • AUTOMATIC
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TONYBERTRAM
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A friend asked if I would help him find why his A/C is not working so being familiar with freon and cars and agreed to help. My bad! Navigator's are a pain to work on. Anyway, we started by doing a complete pump down, vacuum to 30hg and waited over night to make really sure the vacuum would hold. It did so got 4 12-ounce cans in before it would take anymore. Started the vehicle and turned on the A/C. Nothing, clutch not pulling in. Checked fuse 11 it was good. Hunted for half a day for the clutch relay, located on the driver's side behind and below the headlight. Had to pull the air breather completely out to get to it. Pull the relay and tested good on the coil no short. Did some reading on here and jumped out I think pins 87 and 30 on the relay plug in and had my friend start the truck. Looked and yes, the clutch had engaged so finished putting in the freon, 54 ounces total and watched the pressures for a bit. High side running a little over 200 at idle and some what higher when revved up some.
Turned the truck off and removed the jumper from the battery and tested the voltage pins for the coil. 5.33 volts. ran a jumper to battery negative and tested again between battery jumper and hot pin, 13.25 with the truck running or not running. Left the jumper to battery neg on and plugged in the relay and heard the clutch pull in. Started the truck and ac working. Had my friend turn the ac off on the controls with the truck running but the clutch stays engaged. Turned truck off, removed the jumper from the battery and back to original problem of clutch not pulling in. My question is does the PCM control the relay through the negative wire?
May 9, 2025 at 6:58 PM
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STEVE W.
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Attached are the wiring diagrams for the HVAC. Yes, the PCM turns the clutch on and off, but the relay should have battery voltage on pins 30 (constant bat) and 86 (key on). There are a lot of sensors and switches that can tell it not to turn on the A/C as well. It sounds like the clutch and its power circuits are okay as you did have it working. So, the next step would be to see if the switches and sensors that control the PCM are the issue (likely are).
For testing you will want a test light and a jumper wire or two. First go to the clutch relay and check the socket for battery voltage. Now stick a light jumper into the terminal 86 and put the relay in. Now with the key on touch the jumper to ground, the clutch should engage. If it does, then the issue is on the control side. If it doesn't then check the ground and the wiring to the clutch. As you had it working directly, we'll assume the clutch circuit is good and that it engages with the jumper test. That means you really should have a scan tool with bi-directional control and the ability to read live data from the HVAC and PCM modules. In particular you want to see the two temperature sensors, one behind the grille and the other behind the dash. Say it is 60 degrees outside, but the interior sensor has failed, and it is telling the PCM that it is 40 below inside the car, the heat will work, defrost will work (and it will run the AC on defrost) because it thinks it's very cold. But you won't see any codes without a scan tool. Then there are the low- and high-pressure switches on the system.
The high pressure should be closed if the system is at the proper pressure, it opens if the pressure goes high. A quick test is to simply jump it in the connector. AC now works? Replace the high-pressure switch. The low pressure can be tested the same way.
For any other testing you do need a scan tool. With it you can command the HVAC and see if it's operational or if there is an issue.
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May 10, 2025 at 2:28 AM
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TONYBERTRAM
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Thanks Steve W. looks like my afternoon will be busy tracing this down.
May 10, 2025 at 9:50 AM
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TONYBERTRAM
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Hey Steve W, man you were spot on. Turned out to be the ambient temperature sensor. Cleaned the connection, cranked her up and turned on the AC and what do you, the clutch pulled in. However, it was the last one we checked but at least every connection is now clean. Thanks, Brother. And thanks for the schematics.
May 10, 2025 at 12:22 PM
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STEVE W.
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Great to hear you got it repaired. Come back anytime.
May 10, 2025 at 12:55 PM