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What turns the compressor on and off

medium1 on Sun February 01, 2009 12:40 PM User is offline

Year: 1998
Make: Dodge
Model: Dakota
Engine Size: 5.2
Refrigerant Type: 134
Ambient Temp: 75
Pressure Low: 30
Pressure High: 100
Country of Origin: United States

What makes the compressor cycle? How many devices will cause the compressor to cycle and at point, temp, pressure?

TRB on Sun February 01, 2009 5:17 PM User is offlineView users profile

Pressure Switch x 2, CM, Clutch Coil and Clutch Coil relay.

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Chick on Sun February 01, 2009 7:41 PM User is offlineView users profile

It sounds as if you have frequent cycling? if so, your pressures indicate a low charge. I would have it recovered and recharged to factory specs, with some UV dye and find and fix the leak..Hope this helps..

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

medium1 on Sun February 01, 2009 8:15 PM User is offline

What pressure would make it cycle? The pressure doesn't change much and it doesn't get very cold so what makes it cycle? I tried to add freon but the high pressure barley makes 100 and the low doesn't go very low.

mk378 on Mon February 02, 2009 2:40 PM User is offline

This is a CCOT system. Normal cycling is controlled by the low pressure switch on the accumulator. It cuts off at about 20 psi (low side) and back on around 40-50.

When the system is low on charge, the pressure will drop to 20 almost immediately after the compressor starts, leading to rapid cycling. Refrigerant can be added through the low side in this condition, because the pressure there does stay lower than that in the can or tank. Charging should be done by weight whenever possible. This requires starting with the system empty (evacuated). If it's partly charged there's no way to know how much refrigerant is in it to begin with. Add a dose of UV dye if you don't have it already because there's going to be a leak somewhere.

Chick on Mon February 02, 2009 7:04 PM User is offlineView users profile

Just to add, stay away from sealers, they can destroy your system, evacuate, charge the factory amount of virgin R134a with some UV dye into the vacuum, then start your leak detection..hope this helps..

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

medium1 on Mon February 02, 2009 10:34 PM User is offline

Quote
Originally posted by: mk378
This is a CCOT system. Normal cycling is controlled by the low pressure switch on the accumulator. It cuts off at about 20 psi (low side) and back on around 40-50.



When the system is low on charge, the pressure will drop to 20 almost immediately after the compressor starts, leading to rapid cycling. Refrigerant can be added through the low side in this condition, because the pressure there does stay lower than that in the can or tank. Charging should be done by weight whenever possible. This requires starting with the system empty (evacuated). If it's partly charged there's no way to know how much refrigerant is in it to begin with. Add a dose of UV dye if you don't have it already because there's going to be a leak somewhere.

I don't get it, the pressure doesn't drop much lower than 30 or go up much more than 100 so what the heck turns the compressor off. It's not my truck and I'm not trying to fix it I'm just trying to understand how it can cycle at these pressures.

TRB on Mon February 02, 2009 10:46 PM User is offlineView users profile

Clutch cycling pressure switch is mounted on a Schrader-type valve fitting that is part of the accumulator. It is connected in series with the high pressure cut-off switch and control panel, between ground and PCM. Switch contacts open and close due to changes in A/C system pressure. Opening and closing of switches will cause PCM to turn compressor clutch on and off, regulating system pressure which controls temperature of evaporator. Clutch cycling pressure switch contacts are normally open when suction (low side) pressure drops below 20.5 psi (1.4 kg/cm2 ). Extreme cold weather will also open contacts. When contacts are open, PCM will de-energize compressor clutch relay, disengaging compressor clutch. Clutch cycling pressure switch contacts will close when suction (low side) pressure rises above 36 psi (2.5 kg/cm2 ). When contacts are closed, PCM will energize compressor clutch relay, engaging
compressor clutch.



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mk378 on Tue February 03, 2009 10:43 AM User is offline

Are you measuring both system pressures, or just using a single gauge and watching the low side swing between 30 and 100? It's not possible to make a proper diagnosis from a cheap charging hose with a single gauge built in.

medium1 on Tue February 03, 2009 4:04 PM User is offline

Quote
Originally posted by: mk378
Are you measuring both system pressures, or just using a single gauge and watching the low side swing between 30 and 100? It's not possible to make a proper diagnosis from a cheap charging hose with a single gauge built in.

Both, I'm using a good set of gauges. I been fixing my own cars AC for years, mostly leaks.

Chick on Tue February 03, 2009 4:31 PM User is offlineView users profile

It is apparently cycling off when the low pressure hit's 30psi..(probably set for 25-29psi) you have a leak, thats why it's shutting down before the high side goes above 100, it's the low side shutting it off.. Recover and recharge factory amount back into the vacuum and it should work just fine, until more leaks out and it shuts down when the low side hits the cycling pre-set pressure.. Try jumping the low pressure switch on the accumulator, see if the high side goes up, or stays the same and the low side continues to drop.. This will confirm low charge, if the low side keeps dropping and the high side moves very little...Hope this helps.,

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

mk378 on Tue February 03, 2009 4:42 PM User is offline

If the compressor is not coming back on before the low side reaches 100, that's likely a bad cycling (low pressure) switch. It should come on at low side around 40, as long as the high side is within limits (less than 450). It could also be the relay or the compressor clutch is balky and not engaging when it should. But most likely the low side switch is out of spec, since it is turning off too soon as well.

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