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'97 Ford Ranger Compressor Cycling Every 15 Seconds

Bob K on Tue October 02, 2007 10:58 PM User is offline

Year: 1997
Make: Ford
Model: Ranger
Engine Size: 4.0L
Refrigerant Type: R-134a
Ambient Temp: 60F
Pressure Low: 26psi
Pressure High: 160psi
Country of Origin: United States

Good evening. I've been lurking for some time as I get smart on air conditioning systems and troubleshoot my problem. My Ranger has the stock A/C system and has never been altered or serviced. The compressor cycles on for 15 seconds then off for 15 seconds. The vent temp is a steady 40 deg F output. Here are the steps I've taken so far.

Static Reading
Low=79psi
High=85psi
Ambient Temp=60 deg F and falling
Engine slightly warm from use several hours earlier

Dynamic Reading
Low: Compressor kicks on, pressure drops to 26psi and holds for several seconds (or might be falling very slowly). Then the compressor kicks off and the pressure climbs to 47psi and holds.
High: Compressor kicks on, pressure climbs to 160psi and holds for several seconds (perhaps climbing very slowly still). Then the compressor kicks off and the pressure drops to 95 psi and holds.
Ambient Temp=60 deg F and falling
Vent Output Temp=steady 40 deg F

Additional Info:
Doors shut, windows down, hood up. A/C on Max cold. Fan on highest setting.
I tried to rotate the compressor clutch while the vehicle was off to check for bad reed valves. The clutch rotates about 1/4 turn and then meets compressor resistance.
The suction accumulator/drier are plenty cold to the touch and have condensation.
No greasy leaks detected. Some green oil noted at high pressure valve after removing manifold.
Radiator fan is operating and condenser is clean.

While it seems to be functioning fine now, except for the excessive compressor cycling, it did fail me once about a year ago. In stop and go traffic in Las Vegas heat (probably 100 deg F), the compressor cycled every 15 seconds as described above, except this time the vent output went warm and stayed warm until I could get the truck back up to freeway speed.

My first thought is a slow leak and my second thought is a clogged orifice. There is some rust under the accumulator insulation blanket that deserves a closer look, too.

"Recover and recharge" seems to be the standard advice for most A/C issues. Before I pay for that service, is there anything else I can do to narrow down the problem?

Edited: Tue October 02, 2007 at 11:15 PM by Bob K

iceman2555 on Tue October 02, 2007 11:09 PM User is offlineView users profile

Wait for the temp to get about 75° and retest....seriously, there is insufficient ambient temp to make the system truly function.
On a different note....on at 47 psi...off at 26 psi....vent temps in the 40's.....sounds like the darn thing is working.....low pressure cut out...the evap is nearly freezing.....pressure stabilize....47 psi.....LPCO closes and the compressor re engages......over and over....
Want to see if this changes....put the vehicle in a garage....close the doors.....heat up the garage....engine running for a while will do fine.....be sure to vent the exhaust...hate to read about your demise......open the doors...max cool...high blower....engine at idle.....probably see the system operate for longer periods before clutch disengagement.....drop the fan speed and the system will cycle more often.
Wait till it gets a bit warmer....according to Al Gore this will happen at almost any moment....and retest the system.
The best approach....if a problem is encountered.....is to recover and recharge to Joe specs.....takes away a very important variable...at this point it is known how much refrigerant is in the system.....number one 'need to know' for a true diagnosis.
Good luck!!!!

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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

Bob K on Tue October 02, 2007 11:19 PM User is offline

Thanks, Iceman. The compressor cycling symptom occurs regardless of ambient temps. On the hottest days here on the central coast of CA (about 85 deg F), the compressor still cycles at regular 15 second intervals regardless of temps, fan settings, engine speed, etc.

iceman2555 on Tue October 02, 2007 11:27 PM User is offlineView users profile

Service the system...and then retest.....
Good luck!!!!

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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

2POINTautO on Wed October 03, 2007 3:43 PM User is offlineView users profile

Just a couple of notes, I believe your HI pressure fitting is before the OT so if the OT was clogged you should see higher pressures since the clogged OT would hold things back a bit so I think that is OK. How cold is the bottom of the accumulator. Yes ambient needs to really be above 70F and overall I think you are underserviced and it is kicking off due to the low pressure cut off switch, doesnt it cut off at the same LO pressure no matter what your cabin test procedures are, windows open for different fan speeds. HI pressure should be about 2.5 to 2.7 times ambient but at least 70F for better testing. Maybe you could take a quick trip to Death Valley for us with your pressure gauges.

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Give all the dirty details
and dont forget the LO & HI pressures
Year, Make & Model would be nice too

Bob K on Thu October 04, 2007 9:22 PM User is offline

Let me approach this from another angle.

Will a properly serviced R-134a air conditioning system cycle on and off as the ambient temperature dips into the 40s or 50s? If so, what are the physics behind that? Does the low pressure side psi stay too low because it doesn't have a chance to expand appropriately in the cold weather?

mk378 on Thu October 04, 2007 10:52 PM User is offline

Yes. A CCOT system will, by design, cycle a lot. The only time they don't is when the weather is very hot and the system must operate at full capacity.

The low side pressure is related to evaporator temperature. The CCOT turns the compressor off when the pressure gets low to keep the evaporator from getting too cold and frosting up on the air side.

Bob K on Fri October 05, 2007 12:38 AM User is offline

I'm gobbling up A/C physics info as quickly as I can in other threads, but some things still don't make sense to me. Seven years ago, when this truck was almost brand new, I don't recall the compressor hardly ever cycling, regardless of ambient temp or engine/road speed. In fact, I don't recall seeing many automotive A/C systems cycle on and off. Maybe that's ignorance on my part. I'll have to pay better attention.

Help me through a couple scenarios and see if my understanding of the system physics is correct.

Scenario 1
The system is functioning normally and is properly charged with 25oz of R-134a. Ambient temp is 60 deg F. A/C is on max recirc. Low side pressure slowly drops to 26 psi over the course of 45 seconds and then the LPCO switch turns the compressor off. It does so to keep the evaporator from freezing and so that the compressor will receive adequate flow and lubrication. The reason the pressure dropped and the LPCO activated is because the relatively cool recirculated air ( let's say 45 deg F) flowing over the evaporator was not sufficient to heat the refrigerant enough to cause a corresponding increase in pressure. Without an adequate expansion of the refrigerant at the evaporator, the suction side of the pump pulls the pressure down faster than the evaporator can keep the pressure up. Is that a correct assessment?

Scenario 2
The system is low on refrigerant but otherwise not damaged. Ambient temp is 100 deg F. A/C is on max recirc. The compressor cycles on and off every 10 seconds and vents blow warm air. This occurs because there isn't adequate liquid refrigerant supply to the orifice. As a result, an inadequate amount of refrigerant is delivered to the evaporator. A low quantity of refrigerant in the evaporator means the refrigerant can't pick up enough heat to keep the pressure above the LPCO point. Hence, the compressor continues to cycle. Is that a correct assessment?

Edited: Fri October 05, 2007 at 12:40 AM by Bob K

2POINTautO on Fri October 05, 2007 2:00 AM User is offlineView users profile

I like scenario 2, I would also concede that with a properly functioning system in cooler weather and a temp gauge in the vents, that the compressor would kick on and off at a predetermined vent temp due to a properly or improperly calibrated thermistor in the evap case.

My old Odyssey would shut off at 17C every time, hot or cooler day, the thermistor was out of spec and I could not get ice cold temps from the vent and on cooler days in September, the compressor would cycle frequently and I could predict within one second of its shut off and turn on by watching my digital thermometer in the vent.

So I challenge you to monitor vent temp and your pressure gauges to determine what is causing your cycle, I am guessing LO pressure is cutting off your compressor.

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Give all the dirty details
and dont forget the LO & HI pressures
Year, Make & Model would be nice too

Bob K on Sat October 06, 2007 3:31 PM User is offline

I am confident that my A/C is cycling because the low side pressure is regularly reaching the LPCO. The compressor consistently turns off at 26psi.

I'm just trying to understand the physics behind why compressors in properly functioning and serviced systems cycle on and off. I don't believe the A/C compressors in my '93 Saturn and my '95 Saturn ever cycle off, regardless of ambient temp. So...why does cycling happen in some properly functioning systems but not in others?

Chick on Sat October 06, 2007 3:43 PM User is offlineView users profile

Saturns use expansion valves rather than O tubes, and also use variable displacement compressors. They will seldom if ever cycle..Just duifferent systems... Like the GM systems that use the V5 and V7 compressors, they seldom cycle either... Toi much cyclimng can be a bad thing, but a lot more info is needed, like both pressures along with ambient temp, fan speed etc..The hotter it is, the less a CCOT system will cycle, (working to remove heat) the colder it is out, the more it will cycle..(Less work needed) Variable displacement compressors de-stroke as needed to keep pressures from reaching cycling pressure..

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

Bob K on Sat October 06, 2007 4:07 PM User is offline

Thanks, Chick. That helps me understand more. It just seems odd to me that any vehicle manufacturer (Ford) would make a newer A/C system where the compressor cycles regularly in cooler ambient temps. That can't be good for longevity.

One more A/C theory question. Assume we're dealing with a properly serviced '97 Ford Ranger (thus, high/low pressures within acceptable range), ambient temp at 60 deg F, max recirc., vent temp at 40 deg F. How often would I expect the compressor to cycle? Once per minute? More often? Less often? In other words, if I know how often a properly serviced system will cycle in those conditions, I'll have a better idea of what to expect from my system after I get it serviced.

Chick on Sat October 06, 2007 4:11 PM User is offlineView users profile

At 60 degrees you can expect a lot of cycling. (will still vary car to car as to how long between cycles) I would suggest leaving it on fresh mode to introduce as much heat as you can, (not much at 60 degrees) or you're just cooling the cold air in the cabin, and thus, lot's of cycling... Proper clutch Gap adjustment will make it less noticable though..A wide gap and you'll hear it everytime...

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

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