Automotive Air Conditioning Information Forum

Low Side pressure. How could low side pull a vacuum with the low pressrue switch working. would the pressure switch cause the compressor to shut off ?

the reason i ask is b/c my system imediately pulls to the low side cut off and shuts down the compressor. then 5 seconds later the whole cylce repeats. low side moves down, the high side starts rising. then the cut off triggers and all repeats. the compressor and whole cycle keeps cycling every 10 - 15 seconds or so

i heard of people saying the low side is pulling a vacuum becasue the expaansion valve is stuck or there is a restriction. but i dont understand how could get to a vacum bc the low pressure swithc would shut it all down. like i say my real problem is the 10 - 15 second cycling. as described above. any help would be greatly appreciated to understand what i need to fix ?

Make/model/year always helps.

"my system imediately pulls to the low side cut off and shuts down the compressor. "
From here it sounds low on refrigerant. It sounds like the LPCO is doing its job.

Have an AC shop test for leaks, evacuate, and recharge by weight

BasicUser323 wrote: ↑Fri Mar 08, 2024 10:50 am i heard of people saying the low side is pulling a vacuum becasue the expaansion valve is stuck or there is a restriction. but i dont understand how could get to a vacum bc the low pressure swithc would shut it all down. like i say my real problem is the 10 - 15 second cycling. as described above. any help would be greatly appreciated to understand what i need to fix ?

Not all systems have a low pressure cutout. I have seen a lot of older systems that only have a trinary switch on the reciever dryer if they even have that.

I retrofited a marine cold plate unit that used a Sanden compressor on the Starboard engine of a house boat from R12 to R134a a few years ago. I added a low pressure cutout to protect the new Sanden that activates at 7-8 psi. That system lacked any kind of pressure protection. I also wired up a binary switch in the reciever dryer as well. The codenser was raw water cooled with what looked like a 12V bilt pump supplying it water. The Port engine also had an engine driven compressor for a cabin ac system.

"I have seen a lot of older systems that only have a trinary switch on the reciever dryer"

A trinary switch includes a LPCO:

"The AC trinary switch has three functions: high pressure protection, low-pressure protection, and fan control"

tbirdtbird wrote: ↑Fri Mar 22, 2024 9:54 am "I have seen a lot of older systems that only have a trinary switch on the reciever dryer"

A trinary switch includes a LPCO:

"The AC trinary switch has three functions: high pressure protection, low-pressure protection, and fan control"

While technically that is true, a trinary switch on the high side does not prevent low pressure on the suction side. As long as the system has ~40 psi on the high side, the compressor will run. If the system had a complete loss of refrigerant it would keep the compressor from running, but short of that, the compressor will run even with a very low charge or restriction in the system.

It is also thus possible for a system with the low side running in a vacuum to draw in air and moisture into the system through a leak in the low side of the system.

Without re-reading this thread, I don't think we know the make, model, and year of this vehicle and whether it has a TXV or OT. Regardless, it's possible that the charge is "off" and/or that it has a restriction someplace, as suggested earlier.

If it's an older vehicle and the A/C system has never been serviced (accumulator or R/D replaced), it's possible that the desiccant bag has ruptured, setting the beads loose and clogging the metering device, subsequently causing the system to pull into a vacuum on the low side.

Needless to say, this system won't cool very well under such a condition.

JohnHere wrote: ↑Sat Mar 23, 2024 10:09 am Without re-reading this thread, I don't think we know the make, model, and year of this vehicle and whether it has a TXV or OT. Regardless, it's possible that the charge is "off" and/or that it has a restriction someplace, as suggested earlier.

If it's an older vehicle and the A/C system has never been serviced (accumulator or R/D replaced), it's possible that the desiccant bag has ruptured, setting the beads loose and clogging the metering device, subsequently causing the system to pull into a vacuum on the low side.

Do not assume that a dessicant bag cannot rupture because it is new either especially now. When I worked in a Nissan dealership about 10 years ago their supplier had a batch of bad dessicant bags that were in a lot of trucks and SUVs. Nissan issued a recall on several vehicles with the bad units. I probably changed out 100 of the condenser mounted receiver dryers. I actually found a few that had contaminated the system and plugged the H blocks. I seem to remember it being mostly Titans and Armadas. Unfortunately new does not mean it is good or even of reasonable quality now. The parts manufacturers cannot even make condensers correctly for GM vehicles now, they are shipping one model out without it even having the correct mounting bracket to bolt it into place. I even went so far as to contact Agility about the fitment issue and was met with an argumentative attitude. Even offered to drive the whole vehicle 10 miles away to the guys office so he could look at it in person, never heard back again, ghosting. I have noticed the suppliers do not even care now, so the market is loaded with junk parts.