Air conditioner compressor will not cycle off

[herdfan75]

96 Cobra

I replaced the cycle switch last night as the compressor would not kick on (jumped the wire with a paper clip to make sure it works and it's fine). Turned on the AC and it blows nice and cold. Checked the pressure with one of the freon filler hoses and it's dead center in the green. But I notice that the compressor does not cycle on and off. Also, with it running full time, the car runs a lot warmer than usual...the needle almost goes over to the "L" in the word "Normal." I checked the cooling fan, and it's running strong and stays on all the time with the AC on as it should.

Any idea what could be causing the compressor not to cycle off? Maybe I got a bad switch?

 

[ZeroDCX]

System freon pressure is probably too low. You need a manifold gauge set to determine both high and low side pressures. The disposable gauges packaged with the refill canisters only provide meaningful readings when the a/c compressor is actively on and compressing. It should read higher when the compressor is off and the system equalizes pressure between both low and high sides.

 

[herdfan75]

System freon pressure is probably too low. You need a manifold gauge set to determine both high and low side pressures. The disposable gauges packaged with the refill canisters only provide meaningful readings when the a/c compressor is actively on and compressing. It should read higher when the compressor is off and the system equalizes pressure between both low and high sides.

Yea, I know I need to get a set.

I do know that when I cycle the AC compressor manually by connecting and disconnecting the paperclip from the wire, pressure goes pretty high on the generic gauge and then back to normal when the compressor is on.

Also, I just noticed that the idle is hanging around 1200 rpm before finally settling down with the AC on. With the AC off it is normal and does not hang up.
Any further ideas? What to check first?

 

[ZeroDCX]

Okay, I might have misread your question.

Are you stating that the AC compressor is functioning when turning on the system, but you're wondering why its not cycling off and on during operation?

Also, the system will idle up when the A/C is turned on to compensate for the extra load the compressor puts on the engine. I'm not certain of the exact RPM is will idle, but mine also idles up and then comes down.

 

[herdfan75]

Okay, I might have misread your question.

Are you stating that the AC compressor is functioning when turning on the system, but you're wondering why its not cycling off and on during operation?

Also, the system will idle up when the A/C is turned on to compensate for the extra load the compressor puts on the engine. I'm not certain of the exact RPM is will idle, but mine also idles up and then comes down.

Right. It comes on when I turn the air on but it's not cycling on and off. The AC Cools very well but when the air is on the temp gauge stays about three quarter of the way up. Also with the idle, it doesn't idle down normally between shifts. Very slow to idle down. With the air off it runs normal. I know the IAC idles the car up a bit when the air is turned on to compensate for the compressor but this is beyond that. Like when I come up to a stop light it stays around 1200 for 10 seconds or longer. I think if I can figure out the cycling problem it will help the cooling and idle.

 

[ZeroDCX]

The A/C compressor is functioning normally. The only time it cycles is when system pressure is too low. As you saw earlier when the compressor turns on pressure drops to normal range on your low pressure gauge. It's supposed to hover around that range when the A/C is on outside of WOT.

When pressure is too low and the A/C compressor turns on, the low side pressure will drop below a threshold (I don't remember the value). The low side pressure switch notifies the ECM the system pressure is too low and it turns the compressor off to protect it from damage. When the compressor is not turning, the system equalizes pressure between both low and high sides. The low side pressure switch detects adequate pressure, signals the ECM of this and the compressor turns on... starting the cycle all over again.

Cliffnotes: The A/C compressor only cycles when overall system pressure is low, in need of a refill.

Regarding the high idle, that's how my car runs with the A/C on and as far as I have researched this behavior is normal. I can't recall if it idles that high though, but I will double check.

 

[herdfan75]

That's good to know. I just thought the compressor was supposed to cycle on and off. My new truck does as did my 2014 Mustang. Doesn't seem to hurt anything I guess but I have to watch the temp gauge. But I read that cooking with the AC on on the 96 cars was a problem from the factory.

 

[DSG2003Mach1]

Ive seen a few posts about that cooling issue with the AC on, I believe there are even pictures floating around of what the updated AC condenser looks like so you can verify if yours was updated or not.

 

[ZeroDCX]

Yes, 96 cars have a TSB concerning the A/C condenser having too many fins per inch. This restricts air flow to the radiator.
This TSB was only performed in the warmer states. A key way to tell if it has been done to your car is if the honeycomb grill insert is still present, then it has not been done.

 

[herdfan75]

Yes, 96 cars have a TSB concerning the A/C condenser having too many fins per inch. This restricts air flow to the radiator.
This TSB was only performed in the warmer states. A key way to tell if it has been done to your car is if the honeycomb grill insert is still present, then it has not been done.

The honeycomb grill in the bumper? Mine isn't there I don't think, however, I think the bumper has been replaced.

 

[ZeroDCX]

Note the black honeycomb grill insert on image below. If this was present on your car, then it most likely did not receive the TSB. Part of the TSB is to remove this. There are other ways to determine if it has been done such as measuring how many fins per inch the condenser has. Stock is around 21 fins per inch and the updated model has around 16 fins per inch.

Mark has documented this on his website: http://www.classictiger.com/mustang/UnderTheHood/ACCondenserMod/96ACCondenserMod.htm

TSB info: https://www.stangnet.com/images/stories/docs/sn95_TSBs/96-25-12.pdf