Model: Grand Cherokee
Engine Size: 4.0
Refrigerant Type: r134a
Ambient Temp: 80
Pressure Low: 24
Pressure High: 190
Country of Origin: United States
My father-in-law's '96 Grand Cherokee has a puzzling problem. The compressor clutch cycles on and off every 5 seconds or so even though the system appears to have a full charge. Cooling is marginal as a result (and this can't be any good for the clutch. I determined the pressures after jumpering the cycling switch (it is screwed into the accumulator) so I could get stable pressure readings. Jumpering the switch stopped the cycling and I got the readings that I listed here at a 1500 RPM fast idle with the fan on high on recirc with the door shut. Vent temps dropped to below 40F on a quick test drive, switch still jumpered. Based on what I saw, I figured that the cycling switch was bad.
I replaced the switch with a brand new one and the original behavior is back - cycling compressor every 5 seconds or so even after initial system start-up. Any thoughts on what I am missing here? Is this the way this system should work?
It's low on refrigerant. The proper way to recharge it is to recover whats left in the system. Evacuate and recharge to the proper weight.
I don't see how it can be low enough on refrigerant with those pressures showing (measured when the cycling switch is jumped) to cause the clutch to cycle every five seconds (with the swith hooked up normally.) When I took it for a ride with the switch jumped I got great cooling on the road and at idle. I have charged a dozen cars over the past 8 years using pressures (the 2.2 - 2.5 times ambient temp on the high side rule) and never had this happen before (cycling too fast with what seems like a normal level of refrigerant based on pressures.)
If it is undercharged I guess I am suprised. I just don't want to have him pay to evacuate and recharge by weight at a shop (I don't have a scale) if that isn't the problem. Does the 2.2 x ambient rule not apply all the time?
Listen to Chris.
The proper way to charge is by weight.
The is no general formula or rule of thumb to charge by. If there is some other problem, the rule of thumb or formula goes out the window.
An undercharged system can cool very well... until poor oil returns due to the lack of charge kills the compressor. Either that, or the short cycling is the result of a partial restriction.
Jeep's have been notorious for leaking evaporators. An undercharged system is clearly a possibility here.
OK - an evacuation and recharge by weight is the way we'll go. I definitely learned something from both you and Chris on this one.
Thanks very much,
the pressure switch wont close until the lo side reaches a minimum psi....not sure what the psi is for 134A.. the switch stays closed till the lo side comes up to the peak lo side prssure then it opens again and the compressor unloads..system pressure drops to 20-25? then the switch kicks back on...being low on charge your lo side psi is below 20-25 psi so the switch never closes..you can only guess how much refridgerant is in there so you need to recover, evacuate and charge to the full system capacity..when you evacuate the system make sure the vacuum holds w/o any drop for a half hour...far as a vacuum pump those venturi pumps that run off an air compressor will not boil off moisture in the system..they might put the system under vacuum but not a good enough vacuum to eliminate moisture ...not a bad idea to go ahead and inject some dye before you charge..Tracerline products has an oem approved dye for finding leaks.
Edited: Tue August 05, 2008 at 4:16 AM by neilh
Thanks for the advice as well. A full recovery, vacuum and recharge by weight will be the course of action.
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