AC Pressure Fluctuations - High and Low Sides

Jeff M

LVC Member
Nov 24, 2017
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Orlando, FL
The car is a 2003 V8 and the AC is not cooling well at an idle. Bringing the revs up helps and it functions pretty well when the car is moving 30+ mph. Still not normal though. I have a video of the gauges taken at an idle in my garage with an ambient temperature of 85 degrees. This is at max cool with the windows down and recirculation off. There is quite a bit of history to get to this point so I need to go over that but here's the video first. The video starts at the low point in pressures and covers a full cycle through the sweep up and down. You can hear the fan ramping up & down during the cycle. The only self diagnostic codes I'm getting are 1242 (recirc door) and 2796, 2426 which are the sun load sensor ones. Reading previous posts I don't think these codes would be causing this.

The car sat for 16 months without being started and I fired it up again for the first time in early August. It had previously been overheating so I had to deal with that. During the time it sat the DCCV went south so that was replaced with a genuine Motorcraft one first. The overheating was a fan issue and I assumed incorrectly that the fan needed to be replaced. It's an aftermarket unit. In reality it was a control wiring issue and once that was fixed the fan would cycle properly and the overheating issue is gone. AC was not cooling at this point so I added a portion of a can with my cheapie fill hose and was able to get some cool air but not really cold out of the system. The pressure gauge sucked so I invested in a set of real gauges. With the new gauges I added another can of R134a and that helped but the pressures were in the mid 30's and almost 200. It took nearly another can before I started getting reasonable temperatures but the pressures were high in the mid 40's and I think around 225. I left it at that and have been running that way for at least a month. The car needed quite a few other items fixed so this past weekend was the first time I put gauges back on it and I got these readings. I know I put a bunch of refrigerant in the system and I'm prepared to have the system evacuated, replace the drier and recharge but don't won't to do that if something else needs attention first.
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Well, from the description I thought it was going to be the scroll control valve or the evap sensor. Then I watched the video and thought it was going to be the expansion valve, but finally, I listened to the video. I think it's the aftermarket fan. It's more like it's going on and off or high and low instead of being variable like it is supposed to.
The fan stops or goes really low at about 150 PSI and then comes back on at over 300 PSI. It should be varying its speed to hold it at around 200 PSI. Also, it should ramp back down much more slowly than it is doing. I can turn my AC off, and it still takes a pretty good while before the fan starts slowing down.
I could be wrong (and it's your money), but I think it's the fan. Maybe you can get a good stock one from a junkyard at a reasonable price.
Also, when you evacuate and recharge, you need to drain the oil out of the compressor and put back the correct amount. There's no telling how much you lost. You have a leak. You need to find and fix it. One can in five or ten years could just be out gassing through the hoses, but this is too much for that.
Thanks Joe, I still have the stock fan and I know it's good since replacing it didn't fix anything. The new fan only started working after the wiring was fixed. I'm going to put the stock one back in but I'm also going to check out the wiring repairs. I'm no good with electrical gremlins and I paid a shop to fix it so I'm curious exactly what was done. I know they hard wired it and I hated the idea when they told me what they had done. There is an 04 V8 in a yard reasonably close to me so I think I'll go grab as much of the body side fan control harness as I can and properly splice it in with solder and heat shrink. The original fan still has the factory connector in tact so repairing the body side will make it all proper again.

I'll worry about the leak once I get this issue resolved and I'll admit it hadn't even occured to me but I know you're right.
Just wanted to close this out for future reference. It was entirely a fan control issue that was causing the pressure fluctuations. The after market fan is still in the car and after sorting a previously botched control wiring repair the fan is functioning as designed. The pressures immediately went rock solid and after adjusting the refrigerant level a wee bit everything calmed down and the A/C is cooling like it should. Yes Joe, I know I likely still have a leak so I'll cross that bridge hopefully a little later.

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