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HELP!!! Strange cycling switch problem.

meck1 on Tue June 17, 2008 7:18 AM User is offline

Year: 1985 -91
Make: Ford
Model: F250
Engine Size: 4.9
Refrigerant Type: R-12
Ambient Temp: 85
Pressure Low: 25
Pressure High: 225

I've suddenly developed a problem that has me stumped, and I'm hoping for help from some of you experienced people. This ac system is a Ford fixed orifice, accumulater type with a Sanden 709 compressor that I added several years ago. I'm using R12. It has given no trouble until this Spring

The compressor is cycling rapidly, about four seconds on and four seconds off regardless of engine speed, not staying on long enough to cool down below 60 degrees. It is not low on refrigerant. The switch/compressor is disengaging at around 35 pounds and engaging at 47 pounds. (The Ford manual says the switch is preset to disengage at 27 lbs.) When I disconnect the ac pressure/cycling switch and jumper across the connector making the compressor stay engaged, the pressures stabilize at 25/225 and vent air is in the forties. I replaced the cycling switch, and it made absolutely no difference. I've also evacuated and drawn a vacuum on the sytem before recharging.

Does anyone have a clue what may be going on here? I can sure use some hints on what to try.

bohica2xo on Tue June 17, 2008 12:48 PM User is offline

Well the jumper rules out any electrical issues. I have had bad cycling switches, and it is possible to get a bad one right out of the box - but I would try adjusting the new one to the service manual spec before I condemn it.

Inside the connector end of the switch, down between the pins is the adjustment screw. Make adjustments in the 1/4 to 1/2 turn increments, and push the connector back on to check the low side pressure. Cabin fan on low speed, doors closed & recirculate air. Keep adjusting until you get cut out around 25 to 27 psi.

Once you get the switch adjusted, open the doors, set the cabin fan to highest speed & run the engine @ 1500 rpm. Check the pressures & vent temperatures @ 1500 rpm, and post them.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

meck1 on Tue June 17, 2008 4:54 PM User is offline

I'll adjust it and see what happens. Thanks for the help.

meck1 on Sun July 06, 2008 3:08 PM User is offline

Followup: the adjustment of the pressure switch did the job. Why it suddenly needed adjusting when it has never needed it before I'll probably never know. For now, thankfully, the air is cold. I appreciate the advice.

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