Electrical Ac Compressor Clutch Is Acting Weird...

[Konrad Kocon]

Hey guys! So I have my 98' Sn95 Svt Cobra, and I feel my compressor clutch is acting funny. I live in Tucson, AZ so it gets hotter than hades out here. Now, when I start the car, and switch to Normal AC the compressor will kick on after about 30-45 seconds, start blowing a little cool, then shut of, so I switch it to Max AC and after 30 seconds to a minute, it will switch on and start cooling without issue. Any advice?

 

[Konrad Kocon]

So, update, the problem here is intermittent, so far, I've clocked it to when it's mid day and about 105 degrees and over.

 

[robertsa]

The ac clutch is supposed to cycle on and off during normal operation. Are you seeing something abnormal about the normal, intermittent operation or simply unfamiliar with how the system works?

 

[Konrad Kocon]

im still getting familiar with them, but, from the looks of it, it's a consistent cycle. I propped the hood open and sat there for a bit and watched it, the fan just keeps on going because it's hot out here. But other than that, as far as I can tell, it's staying cool in the car when it's running, but I'm not seeing it cycle in and off.

 

[Konrad Kocon]

I know it's normal for R-134 systems to cool less at idle, so when it's idling it actually cools pretty well, it just has such a delay on the initial start up to engage. Also, the intermittent issue is this, sometimes on start up, or even after running for a bit, if I switch it to Normal AC, it will shut off, and blow nothing but hot air, I'll feel it turn off, then I have to turn it off entirely, wait a second, then switch it back to MAX AC.

 

[Konrad Kocon]

I know it's normal for R-134 systems to cool less at idle, so when it's idling it actually cools pretty well, it just has such a delay on the initial start up to engage. Also, the intermittent issue is this, sometimes on start up, or even after running for a bit, if I switch it to Normal AC, it will shut off, and blow nothing but hot air, I'll feel it turn off, then I have to turn it off entirely, wait a second, then switch it back to MAX AC.

 

[robertsa]

As long as the clutch is engaging and disengaging rythmically, it's fine. The fan will stay on because it's supposed to when the ac is on to draw air through the condenser coil.

Stop by a shop and have them check the system pressure. It's normal for a system low on refrigerant to cool uncommonly well and then not at all when the evaporator coil freezes up.

 

[Konrad Kocon]

Okay, so update, I work in a shop, and so far the compressor doesn't cycle on and off, it just remains on. The fan is the least of my worries because it's hot out, and I just replaced the suction line, Evac recharged, and its holding pressure well, no loss of refrigerant.

 

[robertsa]

Did you vacuum test the system? What are your high and low side pressures?

 

[Konrad Kocon]

I can't tell you exactly what it was, but it was around 350-400 on the high side. I recharged a week ago, and it's still blowing strong. The compressor I found to be an aftermarket MAHL unit. I checked the surrounding lines with UV light and there are no signs of leaking anywhere. My guess is the High Pressure cutoff Switch maybe that's causing it to be under a consistent cycle?

 

[robertsa]

Your pressures will tell you if there's something wrong with the system like being over or undercharged, clogged, etc. The vacuum test is for finding leaks.

Unplug the low pressure sensor and see if the clutch disengages.

 

[wmburns]

For your Model year Mustang the high pressure compressor turn off is 425 PSI.

Also your Model year Mustang will engage the high speed fan at 325 PSI head pressure. When the head pressure drops below 275 the high pressure switch will re-open. However the PCM will hold over the high speed fan for 20 additional seconds. This is done to prevent short cycling of the high speed fan.

Your high side pressure seems a little high to me even for the high outside temperatures. If this were my car I would make sure that the outside coil is super clean. Use a garden hose running water from the front and back. Double check that all fan shrouds and air dams are in place. Double check there aren't any dented AC lines.

But if you know that the high side pressure is not exceeding 400 PSI and you think that the high side pressure switch is the cause, that is very easy to test for. Test the high pressure switch DB/V and DG/O wires to confirm the switch is closed. If the high pressure clutch switch is opening too low (below 425 PSI) this could be your problem.

It helps to know the low side readings as well as the temperature of the cool air coming from the vents. Understanding the low pressure side can help determine IF your AC system is over charged.