1995 Buick Lesabre

[JohnHere]

Had a cooling fan issue (fans on high all the time) and the scanner live data showed a AC PRESSURE - HIGH. (Key in, engine off, AC not run). After several days of having the fuse for the fans pulled I took the car to a mechanic with a GM TECH2 scanner. Put the fuse in and fans operated normally. But, the mechanic noticed the AC PRESSURE - NORMAL. So, we both agreed it’s a faulty AC pressure switch, which commands high/low fan speeds based on high side pressure.

I’m going to replace the pressure switch. I think it’s failure mode in sending the high signal is as inconsistent as it never before sending a high signal.

A faulty pressure switch sounds like a reasonable diagnosis to me as well. You have nothing to lose at this point, so go ahead and change it out. Me thinks they're not all that expensive. I believe you'll find a Schrader valve under the switch, so you shouldn't have to recover-and-recharge the refrigerant again.

Also, this car has the dual climate zone. The passenger side actuator was found to be not moving the blend door to the full cold position consistently. This may have been the cause of me chasing inconsistent vent temps. New actuator installed and the blend door moves full range.

Since your car doesn't have a hot-water valve—and with the heater core remaining warm to hot all the time—an intermittently malfunctioning blend-door actuator would allow some amount of heated air to flow into the refrigerated air stream, raising the vent temperature on that side. Glad you were able to recognize that the actuator was sometimes misbehaving.

[Mark86]

Yes. I watched that blend door actuator (both of them) for 10-12 cycles with no anomaly. Then it went from hot to cold and stopped about 3/8” shy of full cold. Then that was its range for several cycles before returning to normal.

I was going to ask if there was a valve under that press switch. Thanks.

[Mark86]

Ok. Went to pull the high side fan pressure switch. Unscrewed it about 4 turns and it started hissing. Waited a moment and it started hissing heavier and would not stop, so screwed it back down. Sealed up.

Is there a Schrader valve under there? The mounting flange doesn’t look very deep.

[tbirdtbird]

From what you describe unfortunately we have to assume there is no schrader. An unfortunate way to build something. We assume the part got very cold when the port was hissing, indicating refrigerant was flashing.
Are you able to post a pic or 2 so we might get a better idea of the construction

[JohnHere]

Ok. Went to pull the high side fan pressure switch. Unscrewed it about 4 turns and it started hissing. Waited a moment and it started hissing heavier and would not stop, so screwed it back down. Sealed up. Is there a Schrader valve under there? The mounting flange doesn’t look very deep.

I would be surprised if there wasn't a Schrader valve under the switch. From what I can tell, replacement switches appear to have a pin designed to depress a Schrader valve stem, opening the valve on installing the switch and closing the valve as the switch is removed.

However, based on your judicious attempt at switch removal, it appears that a Schrader valve isn't there for whatever reason as tbirdtbird noted. The only way to know for sure is to have the shop recover the refrigerant, remove the switch, and visually inspect the nipple onto which the switch attaches. In that case, you might as well have the shop handle the entire job for you.

[Mark86]

I took a pic, but is too large to upload here. I’m struggling to find a part number for that switch, so I was going to remove, inspect and or take to the parts house in hand to compare to the replacement,

As mentioned I got 4-5 turns on it before it began leaking, which is ample to have a leak. It also kept leaking until tightened with wrench.

Looks like maybe there is a valve, and I was unscrewing that, vs, just the switch. May need to grab that with a needle nose to hold in place,

[Mark86]

Pic. Had to resize

[tbirdtbird]

Agree it does not appear to have enough depth to the fitting on the pipe to include a schrader fitting.
Once someone recovers/recharges, before they recharge, there are adapters available that could be installed to provide a schrader base. Tim, the site owner, may have something available, doubt it would be in his web catalogue, why not call him and ask, he is a most helpful person. Otherwise try online.
I suspect that may be a 1/8th NPT fitting, but it could be machine thread

[tbirdtbird]

On second thought, try to get a shot that is more perpendicular to the pipe, ie not at an angle. There is a possibility that if you were wrenching on the hex, perhaps THAT is the schrader adapter, and the barrel shaped part unscrews separately off that.

Am I seeing a PN on the switch body? Go around it with a mirror...

Also, have you tried removing and installing the electrical connector several times to wipe the contacts ? Sometimes they oxidize.
Also, I would remove the connector and using a VOM measure the resistance across the switch under varying running conditions, such as Off-low cool-high cool before you condemn it

[Mark86]

Red line is pointing to a mating surface. Yellow line is pointing to the boss which the switch mounts. The blue is pointing to the base / female fitting of the switch.

The switch was sending a high pressure signal (verified with scan tool) which was commanding both fans to max in every “key on” situation. After some time (couple weeks) with the main fan fuse pulled, I out the fan fuse back in and the fans did not come on. Hooked up scan tool and verified a low signal / normal from switch (all with engine off). We were able to command fans on/off via scan tool.