A/C and defrost
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Cross-Country
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A/C and defrost
I have a 1992 F150 5 speed 5.0, when I turn my defrost on the AC kicks on and off repeatedly. I know it’s supposed to be on to dry out the air or whatever to help with fog and defrosting but shouldn’t it stay on. It never stays kicked on for longer than 30 seconds or so and it’ll kick off for just about as long before kicking back on. It’ll do that until I shut the defrost off. AC works great and is charged with canned air or 152a. Any insight is appreciated.
Logistics Pro
Joined: Nov 2013
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From: Mojave, Ca
Sounds like your system is working normally. The Ac pump turns on until the high pressure switch is tripped which turns the pump off. Once enough freon bleeds through your fixed orifice to lower the pressure, the high pressure switch makes contact again and your pump turns back on. Usually happens every 30 seconds depending on engine RPM.
Logistics Pro
Joined: Nov 2013
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From: Mojave, Ca
Ahh. So it does. Any idea what the purpose of the high pressure switch is? Safety shut off switch in case the low pressure fails?
Hotshot
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The opposite of the low pressure switch. If there is too much pressure in the system it will disengage the compressor but that does not apply here. You posted a picture of a ‘94 system which is R-134a and the OP’s truck is a ‘92. That system is R-12 so there is no high pressure switch.
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Logistics Pro
Joined: Nov 2013
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From: Mojave, Ca
The opposite of the low pressure switch. If there is too much pressure in the system it will disengage the compressor but that does not apply here. You posted a picture of a ‘94 system which is R-134a and the OP’s truck is a ‘92. That system is R-12 so there is no high pressure switch.
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Hotshot
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I have no experience with 152a. When doing a conversion from R-12 to R-134a you are supposed to use something like 80% of the R-12 amount. How much are you supposed to use with 152a? Are the pressures good?
Lead Driver
Joined: Feb 2002
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From: Greater Austin, Texas
normally in the first or second notch. should I charge it a little bit and see if it fixes it? The pump kicking on and off drives me nuts and I feel it drag the motor way down.
The clutch cycling should be the same on a/c or defrost IF you have the fan setting the same. If you are blowing max fan speed on a/c then that would make it cycle less, and that's about the only thing that can cause this. You should put a set of gauges on it and see what the pressures are.
Mountain Pass
Joined: Nov 2000
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Ah, I wondered what that switch was mounted on the back of the compressor was for...I guess you can think of that as an OVERpressure cutoff switch...learn something new every day....and I concur, it's probably low on refrigerant and the low pressure switch is at play here.
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Cross-Country
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everything I’ve read says 152a acts pretty much the same as r12 except you can’t use mineral oil. So I went off that and charged it with around 25oz. It kicks off occasionally with AC but it’s either a lot less often or a lot less noticeable.
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Cross-Country
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The clutch cycling should be the same on a/c or defrost IF you have the fan setting the same. If you are blowing max fan speed on a/c then that would make it cycle less, and that's about the only thing that can cause this. You should put a set of gauges on it and see what the pressures are.