Mercury Repair Questions? Ask a Mechanic for Answers ASAP
Upon receiving an A/C ON request from the control panel, the PCM looks at engine temperature (ECT), and throttle position (TPS). If those are within parameters, it then looks at system pressures (dual pressure switch) and evaporator temperature (cycling switch). If those are also OK, then it grounds the compressor clutch coil relay.
On an intermittent operation such as you describe, I would look at the dual pressure switch or the cycling switch to see if there is a faulty connection (you can test by wiggling the wires/connectors. See attached graphics for location))
Easiest way to diagnose this system would be with the help of a capable scan tool that could show the signals (PID) from the different components. Not having access to one of those would reduce our diagnostic capabilities to just jumpering the cycling switch to see if that makes the compressor run continuously.
Oh, man... you asking an old time mechanic whether you should replace something that ain't broke? ;)
Seriously now; replacing the switches won't fix it if it's a wiring/connection issue. I would wait until it acted up, then I would test it and see what's happening before replacing anything.
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Can you disconnect and by-pass/jumper the cycling switch and see if that makes the compressor work? If it does, then we'll know the system is low on refrigerant. However, if it doesn't, then we need to do some more testing. Can you get access to a scan tool capable of reading A/C data? Also, we will need a set of gauges to read both Hi and Lo pressures and a voltmeter to check out voltage at different points in the circuit.
Ford designed this system to be diagnosed with a scan tool, so it's going to be tough to do so without it...
With the engine running at idle and the A/C on, check that fuse 26 (5 Amps) in the smart junction box (below center of dash) has 12 volts going through it, then check that the circuit from that fuse to the control head (Black/Light Blue wire) also has 12 volts. Then check the REQUEST circuit from the control head (Dark Green/Orange wire) to the pressure switches and on to the PCM to see if there are 12 volts there as well. If not, replace the control head.