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clutch cycles with clutch switch bypassed

flannag1 on Tue July 31, 2012 3:27 PM User is offline

Year: 1996
Make: Ford
Model: Ranger
Engine Size: 2.3
Refrigerant Type: 134a
Ambient Temp: 85
Pressure Low: 35

clutch wasn't coming on at all, jumped cycling switch and it turned on...ran for 5-7 seconds and turned it off. With a pressure of 35 at idle, AC cranked, thought it was cycling switch low pressure switch on the accumulator. Replaced it and no AC. Jumped the switch again and now the clutch cycled on and off about 1 second intervals.

Why is my clutch cycling with the switch jumped? Wouldn't it run constantly? The pressures with it cycling are 22-35, on/off.

efxengr on Tue July 31, 2012 3:50 PM User is offline

it is likely in series with High pressure switch and relay. Try jumping each of those as well.

GM Tech on Tue July 31, 2012 3:55 PM User is offline

If you jumper high pressure switch (which is most likely a three wire transducer -that is not jumperable) then stand back with saftey glasses on- because something may blow....

If it were me- I'd be sure condenser is getting plenty of air- by means of a good fan clutch.....see if spraying water on condenser helps cycling stop.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Tue July 31, 2012 at 3:56 PM by GM Tech

flannag1 on Tue July 31, 2012 4:42 PM User is offline

efxengr....Yep, it's in series......
BM Tech...I am testing this from a cold engine...think it could still be the condenser?

Power is from fuse in instrument panel.
From fuse it goes to on/off switch.
From o/o switch, it goes to the low pressure switch.
From the low pressure switch it goes to the high pressure switch.
From h/p switch it goes to the relay in the engine compartment fuse box. This relay is controlled by the powertrain control module for wide open throttle control.
From the relay it goes to the compressor coil.

Looking ahead, if jumping the high pressure lets the coil run's overcharged? Because the static pressure on the low side is only 35, doesn't that scream low charge? Because I am bypassing low charge.

Edited: Tue July 31, 2012 at 5:08 PM by flannag1

efxengr on Tue July 31, 2012 6:29 PM User is offline

What GM says about high pressure is true, but you only want to jumper the connector long enough to see if it stabilizes. I have two ford trucks; 05 F150 and 07 F350 with very similar schematic. On one I had a bad relay doing something very similar. If after checking the charge and the other switches with no success you jumper the relay (3 and 4) and it works the relay is likely bad. You can also get a voltage reading on 1 and 2 on the relay to make sure the PCM is sending power and ground to that relay coil....on mine the PCM switches the negative side. If you jumper the HP switch momentarily and it stabilizes then you likely truely have a high pressure problem and it could be the condenser (but you need to get a guage on that high pressure side).....either clogging or not enough air.

It could be overcharged but you likely wouldnt see 35psi on the low side and you wouldnt get the momentary good cooling since the evaporator and condenser dont work so well when seriously overcharged. We have several fords in our fleet and I have a personal Expedition. On all of them the HP switch is seperate and has a 2-pole connector that can be jumped. You can also put a volt meter on the clutch plug to see if it is getting a steady voltage supply.

You really need a set of gauges to see high and low pressures, and get the normal pressure spec for you vehicle at that ambient. After fixing the relay on the F150 on first run it was doing something similar to what your is doing switching off and on.....before replacing the relay, nothing at all. At compressor cut on it would draw the suction down from 30 to 20psi, shut off, began to rise back to static, clear the low pressure, kick on, drop to 20psi, kick off...and repeat over and over. That likely is what killed the relay. The charge was low. I brought it back to spec for that vehicle which I think was 35/225 and it has been ice cold now for over month.

mk378 on Tue July 31, 2012 8:10 PM User is offline

Put a proper gauge manifold on and see what the high pressure is. If you've been charging with cans and watching only the low side it's likely overcharged. You need plenty of air through the condenser at all times the compressor is running regardless of engine temperature. It takes much more fan air for good A/C performance than it does just to keep the engine cool.

Jumping the high switch is almost certain to blow the hose, that's why there's a switch there.

Edited: Tue July 31, 2012 at 8:11 PM by mk378

bohica2xo on Thu August 02, 2012 1:46 AM User is offline

Wow, a visit from the Bad Advice Fairy.

Do NOT jumper HPCO switches. Attempting to evaluate a system with only a low side gauge & bypassing the safety equipment is the wrong way to go about it.

A failing fan clutch can cause high pressures on a cold engine. Airflow or refrigerant restrictions can cause high pressure cycling on a properly charged system. Start with the basics, both High & Low side system pressures.


"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.

NickD on Thu August 02, 2012 7:17 AM User is offline

Sounds like a death kit was used, damned single gauge is as inaccurate as hell, if a little free-on is good, a lot more is better kind of thing. The fact the the HPCO switch is cycling means its closed contacts are good. Were sealers used rather than repairing the leak? Seen one of those last week, did a good job of partially plugging up the orifice.

Usually when you see a system this old, lost all of its refrigerant, have to find the leak, flush the system, replace the accumulator, recharge with fresh PAG, that turns to mud with air exposure, draw a deep vacuum. and recharge to proper specifications. Or just roll down the window. Willing to bet a whole pot of coffee, none of this was done.

Never ever short out switches, doesn't tell you a damned thing, should be tested. And you will need more than a pair of safety glasses.

iceman2555 on Thu August 02, 2012 2:29 PM User is offlineView users profile

Perform a diagnostic procedure on the HPCO switch. This could be the problem. The high side service port on this vehicle is on the discharge line. Check the high side pressures with a good set of gauges.....not a set of dial a charge $2.00 units. Also insure that the system is properly charged, evac and recharge. Air in the system can do the same thing. Excessive high pressures, although typically the low side would be higher also. If the high side pressure is in the 450+ range, there is a very good chance the condenser is restricted. The inlet side of this OE condenser is the first four tubes. Loose one tube and discharge pressures will be elevated. If the high side pressures remain in the sub 275- range that the compressor is still cycling, it may be the HPCO is bad. Although the contacts are operational, it could be reading an incorrect operational pressure.
First the system should be recovered, evac and recharged utilizing a machine or metering scale. Charging by cans is so ineffective...well...lets just say ineffective.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

efxengr on Fri August 03, 2012 11:01 PM User is offline

So how would you test the HP switch? If you're watching the gauges and the low and high pressure is in range but the compressor is would you eliminate the high pressure switch as a potential source?

NickD on Sun August 05, 2012 8:15 AM User is offline

For a contact switch, a voltmeter across the contact, will open before showing a voltage higher than 0.1 volts below the threshold trip point. You have the specs on your drawing. Off the vehicle, have to use liquid nitrogen to get those kinds of pressures with a precise regulator and gauge. Use a four amp current source to emulate the load and a voltmeter for a voltage drop of less than 0.1 volts when the contacts are suppose to be good. Most when in doubt, just replace it.

Can also kill your radiator fan and throw a rag over you condenser, risky with an old system, a rubber hose can blow.

Your system is about as basic as it can be, climate control always outputs 12 Volts in the AC mode, in series with the cycling and high pressure switch, both should be closed between 25 and around 430 psi, but need both low and high pressure side gauges to read this. See on yours, only the relay solenoid current flows through these switches, around 100 ma, relay should be closed, and the contacts should be applying a full 12 V to the compressor coil. Actually the battery voltage with the alternator running, will be more like 14.5 volts.

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