I am having AC problems on a 1988 F-150 with 302 and auto trans. The compressor coil will not pull the hub in to engage the compressor. I have ~13 volts at the coil connector. I bypassed the pressure switch, the system is charged, I bypassed the switch to rule that out as a problem. I checked the hub clearance, it is .030.
I purchased a new coil and hub assembly and installed it with a .030 air gap. The compressor will still not engage. I checked the resistance of both the old and new coils. It is at 3.0 to 3.5 ohms. I tried bumping the hub plate while the engine is running and the coil energized with no success.
I have noticed the coil harness appears to have a diode wired in. Could the problem be too little amperage to the coil? Any help will be appreciated.
Thank you,
Larry B

 

Check to see if you have 13 volts at BOTH coil connections. If so, the ground is open and the clutch will never energize. I'd suspect this is your actual problem.

The diode is there to prevent damage to the rest of the electrical devices in the vehicle from the back-emf produced when the coil is de-energized. It doesn't directly affect the operation of the coil itself. There's no such thing as "too little amperage" to the coil. The coil will "pull" as many amps as it can based on its resistance and the available supply voltage. In this case, coil current should be around 4 amps DC (supply voltage divided by the DC resistance)

Steve

 

This is most often down to too much clearance in the ac clutch. As you have tested all the electrics Check the clearance in the clutch, you might get away with removing a shim or you might have to replace the clutch.

 

Nope. He already reported the gap measurement (in spec) and the lack of result when hitting the clutch plate. Both are pretty good indications that the clutch gap isn't at issue.

Steve

 

I was checking the ground and harness connections when I noticed the pressure switch wiring was loose inside the plastic connector. The wire had pulled loose from the metal crimp on the connector. When I jumped the switch yesterday, the connection was probably bad. I had voltage, but not enough current to energize the coil. I crimped a new connector for the pressure switch and now the coil pulls the hub in and operates the compressor.
Thank you for your advice.
Larry B