Ford Repair: A/C clutch doesnt engaged, jumper wire, compressor clutch

Question
I just bought a Winstar 1998 3.0L and found that its A/C clutch didn't engage when I switched on A/C. Since it uses R134a, I could but a pressure gauge to measure the pressure and it was up to 68 psi at lower port. I guessed that it was an electrical control problem other than lack of refrigerant but I didn't know how to check and fix it. Could you tell me? Thank you.  

Answer
Your reasoning sounds correct - I would check for power at the cycling switch (mounted on the accumulator or in the low-pressure line near it).  You should have power available on one side of the switch if the A/C is turned on, and the switch should close with pressure above 26psi and allow the current to flow to the clutch coil.  That switch is easy to replace - it just screws onto a threaded fitting.  An easy test is to unplug the switch and use a jumper wire between the two terminals - if the A/C comes on, the switch was open circuited and should be replaced.  This was a common problem on the mid-90's Taurus and Windstar.
If that doesn't turn on the compressor, check for power at the compressor clutch coil.  You may also need to check for ground on the other side of the clutch, which is supplied by the PCM when conditions are OK for A/C.  (for example, if the PCM sees an engine overheat condition, it will not ground the A/C request)  This may require a specialist with schematics and diagnostic equipment to find the cause.
Hope this helps...
Clay