AC low pressure switch

CMSRock · **Joined:** Wed Mar 14, 2018 10:31 pm **Posts:** 12

Worked on my daughters 06 crd today. The AC would not cool because the compressor clutch would not engage.

Usually this is a low refridgerant issue, if there isn't enough in the system to close the low pressure switch the clutch won't get any power for its electromagnet. With the engine off the AC system will balance and you can read the static pressure with your manifold gage set at either port. At around 80 degrees F it should show around 50-60 psi in the system. If this falls below the switch setpoint (low 30's) due to low refridgerant levels the switch opens and your clutch won't engage. Idealy to add refridgerant the compressor should be running.

The low pressure switch is on the aluminum accumulator can by the firewall under the coolant box. If you pull the switch connector you can jumper it and engage the clutch in order to add refridgerant or you can pull the ac clutch relay in the electrical fuse center near the battery and jumper the two large spade sockets to engage the clutch briefly to add to the system. Take care not to overfill the system if you use the relay jumper, just add enough to overcome the low pressure switch issue.

My problem today was not a low refridgerant issue though. The static pressure was over 50 psi, more than enough to close the low pressure switch. The switch is normally open but held closed by the pressure in the system. You can test it with an ohm meter and it should show almost 0 when close and infinite when open. This one was showing open. Fortunately the switch is mounted on a schrader fitting and just unscrews with little refridgerant loss, you don't have to drain the system. $20 later at the FLAP and I had a new switch, problem solved.

The AC system fired right up and at 1000 rpm in an 83 degree F environment was showing low side at 35psi(25-35 is range), high side at 255 psi (200-300 is range)and on the test drive the center vent output ranged from 41F-47F. At idle after returning parked in the shade fan on 3 and recirculating the air it dropped to 36F which may indicate a charge down by an oz or two but when I drove it back to her this evening, about 26 miles, it got stable at 45F.

Posted this as fyi in case your switch goes bad someday, its not always low refridgerant.

WWDiesel · **Posted:** Tue May 22, 2018 10:00 am

Good writeup!
I always check the LP switch first when an AC system clutch will not engage.
As you stated, low freon level may not always be the culprit.
Chevy's/GM's was notorious for the LP switch going bad for some reason. I always kept a couple new ones in stock for them.... :roll:

stevegsmith · **Joined:** Thu Jan 06, 2011 6:35 pm **Posts:** 11

Just wanted to post an additional comment as a thank you to the author of this thread and to make a contribution to the forum.

Last week, my AC stopped working (2006 CRD). Popped the hood, couldn't hear the clutch engaging...could barely see it since the darn compressor is at the bottom left side of the engine. Wasn't moving. Checked fuse, all good. Checked relay by trading it with another, that was good too.

Took it to my AC/radiator shop who re-charged my freon last year and he confirmed there was no power getting to the compressor. My first knee-jerk reaction or fear was a failing compressor. He said that he thought the compressor could be fine but that something was interrupting power to the compressor..so he said "take it to an electrician before you take it to a shop for a new compressor".

LOSTJEEPS to the rescue. Took it home and logged on before taking it to the shop. Did an "AC no working" search and landed here.

After reading CMSRock's very accurate description, went out to look under the hood to identify parts. Pressure switch is relatively easy to get to. Pulled the plug and did a continuity test...no beep....switch open. This part was a little tough since the two electrical terminals are hard to get to.

Logged onto O'Reilly and searched for the part.
https://www.oreillyauto.com/search?q=MRY+20922
Climate control 2 terminal pressure switch

Picked up part and decided to try removing without tools. Surprise, didn't need any. Unscrewed it from the top of the accumulator (aluminum can on right firewall). Heard a little pressure pop. Removed the old o-ring and installed the new o-ring that came in the box with the new part, then screwed on the new low pressure switch. Re-attached the plug and voila! Cold air again. Heard the clutch re-engaging.

Thank you CMSRock and LostJeeps admin.

steve

AC low pressure switch

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