A/C low pressure switch questions

[sunliner]

I've had this problem for a while. AC compressor makes a loud rattle when turned on and also cycles on/off about every 25 seconds. Blows cold air, almost like new. I haven't checked pressure yet...but someone at work suggested I check the low pressure switch and connector so I did. What I got so far.... Testing the connector, I have 4.74 volts with key on.
With the switch plugged in like normal, the system cycles about every 25 seconds, even parked in full sun (it's about 85 deg outside today). If I unplug the connector and jump the terminals, the compressor stays on until I remove the jumper wire, though the noise remains. Ya think I'd be on the right track by replacing the low pressure switch? I'm definitely not an AC expert...

 

[Tiggerr]

Sounds plausible yes, low pressure switch isn't that uncommon of a failure point...
If your compressor is noisy, it sounds like the clutch could be going...which is not available by itself...could just be the clutch bearing, which can be replaced....

 

[Mooseman]

Sounds more like you're low on refrigerant and low pressure switch is doing its job.

 

[mrrsm]

+2 ... Concur that the Low Pressure Switch might be cracked around the plastic fascia at the Gasket and leaking refrigerant as a result. This switch has an in dwelling Schrader Valve that will allow for its replacement with an identical OEM part and not accidentally evacuate the entire system of refrigerant ... and a brief visit to Amazon will let you obtain Red Nylog Visco-Elastic A/C Sealant for around $10.00 to $15.00 or so and then string the sticky stuff around the Switch threads prior to installing it:

After connecting the Switch plug... and as soon as you attach the R-134A can from Autozone with the hose and gauge attached to the Low Side Blue fitting and introduce more refrigerant... when the pressure comes up to around 40 to 50 PSI... the New Low Pressure Switch will make a closed circuit and allow the Compressor clutch to engage. Use the least amount of R-134A necessary as possible to get the system started and use a Digital Thermometer inside one of the AC Vents to watch the inside temps drop down to around 50 Degrees Farenheit or so... and avoid overfilling the AC System with refrigerant as it will damage the Compressor.

 

[sunliner]

something I noticed last night not sure if normal or not. drove with the AC on for about 10 miles. Popped the hood, and that pipe that passes across in front of the engine right in front of the oil fill cap, was frosted over. I've seen that on the accumulator of course but I don't recall seeing it on that pipe. It melts away quickly, so maybe it's normal and I've just never caught it before (?)

 

[budwich]

have you check your pressures yet? It is more likely that you have low "freon" than a bad switch... or possibly a pulled system.

 

[sunliner]

According to the Autozone gauge, I'm on the low end of normal (can't remember the number) I added two cans of 134 and the needle barely moved. so either I have a leak-though it still blows ice cold in the cabin- or that gauge isn't right. What does "pulled system" mean?

 

[DAlastDON]

FWIW...In 2014 a few months after purchase of the envoy the AC quit working. It was just over 100F that day. The compressor would not cycle. Only thing i done was pull the connector off the low pressure switch to jump it out with a paper clip. Compressor cycles and AC blew cold. Been working flawlessly since. Blows cold enough(36F)to keep my water in the cup holder near ice cold with the vent blowing on it. I can only assume it was a faulty connection. Removing the connector from the low pressure switch possibly cleaned it well enought to make contact.

 

[mrrsm]

The Short Answer: The "Frosted Aluminum Pipe" is symptomatic of having a Low Refrigerant Charge.

The Detailed Reasons Answer: Here is the correct sequence of “The Refrigeration Cycle” that should be happening is as follows:

(1) The AC Compressor accepts “Super-Heated” Gas returning to it inside that Large (Frosted Over) Aluminum Tube-Line (Blue Side) and pressurizes the Gas through the Condenser Coils nested alongside the Radiator. AC Compressors work on Gaseous Refrigerant ONLY… Never any Liquids.

(2) This action causes the Heat collected from inside of the Cab areas of the vehicle transferred into the Gas to be expelled from the AC Compressor through the Condenser Coils via the Radiator Cooling Fan.

(3) The Low Pressure “Super Heated” Refrigerant Gas is gradually Condensed back down into a “Sub Cooled” Liquid inside the Condenser Coils using the Radiator Fan to expel this Heat back out into the Environment.

(4) The condensed Liquid then travels out of the Condenser Coils into a smaller diameter, (Red Side) High Pressure Line that guides it through an In-Line Filter and "Pintle" style Expansion Valve routed along the Passenger side Fender Well.

(5) The “Sub-Cooled” Gas & Liquid Mix then passes into the Large Aluminum Accumulator-Drier to draw out moisture and prevent ice from forming up and blocking the tiny tubes inside of the Evaporator Coils and also holds the Liquid temporarily.

(6) The Accumulator routes the Gas-Liquid Refrigerant Mix into it from the "Pintle" metering device called an “Expansion Valve” (sometimes called a TXV if the design uses a Thermostatic Expansion Valve) that streams the liquid through smaller capillary tubes inside of the Evaporator Coils inside the Cab of the Vehicle.

(7) As the Gas-Liquid Refrigerant Mix expands inside the Evaporator Coils… it requires energy to cause a more complete “Phase Change” from a Liquid … back into a Gas. This energy is drawn out of the Hot Air the Cab Blower Motor pushes over the Evaporator Coils that will drop the Air Temperature down to around 45-50 Degrees Fahrenheit.

(8) The Cabin Air is thus made very cool and comfortable because any moisture being carried inside the Hot Air is condensed on the Evaporator Coils and drained outside the vehicle as the Cabin Air becomes De-Humidified.

(9) This action becomes an endless cycle of withdrawing Heat Energy into the Large Expansion Return Line exiting the Evaporator Coils that brings the Low Pressure, “Super Heated” Gas right back to the AC Compressor.

(10) However… If there is an Insufficient Amount of R-134A inside this Closed System… The “Sub Cooled” Liquid Refrigerant will suddenly “Flash Over” through the Expansion Valve… and drop the temperature of the Evaporator Coils down from 45-50 Degrees F to below Freezing at 32 Degrees F.

(11) As soon as this occurs… any moisture laden air that passes over the Evaporator Coils under the Dashboard will gradually collect on the coils and eventually Freeze Over into a Solid Block of Ice.

(12) After this happens, the Evaporator Coils will be unable to allow the Cabin Air to pass through the Frozen up coils...and the Large Aluminum Line will “Frost Over”. The only solution to this problem is to find and repair any Refrigerant Leaks and then Weigh in the proper amount of R-134A Charge into the system. This action always takes place on the Low Pressure (Blue) Side of the system.

 

[budwich]

According to the Autozone gauge, I'm on the low end of normal (can't remember the number) I added two cans of 134 and the needle barely moved. so either I have a leak-though it still blows ice cold in the cabin- or that gauge isn't right. What does "pulled system" mean?

If you are using one of those "can gages" to get your measurements, then you are not likely to understand what is happening to your system. Those only provide a "very limited view". You need to look at both pressures (ie. high and low). You might be able to get by reading the high side (compressor side) via the torque app.

As suggested and with the excellent info provided, it is likely that you have a low pressure issue. This could be related to low levels of r134 BUT it isn't necessarily the only possibility. You may have a "gummed up" system which can cause similar symptoms. The "gumming" could come from a degenerating compressor, someone adding "sealer", etc. such that the expansion valve is not allowing adequate gas thru.

 

[mrrsm]

@budwich ...That is an excellent observation… the Pintle/Filter unit is referred to as an “Orifice Filter Tube” in the automotive world as it sits inside of a hidden High Pressure Line along the upper inner fender well of the Trailblazer vehicles… and in other models is mounted at the Compressor Coils take-off pipe down by the front of the Radiator.

It is a difficult thing to access because it remains stuffed deep inside of a bolted up and sealed flange and requires a pair of long Needle Nose Pliers or a sturdy, long Hemostat to reach down inside and grab the thin rectangular plastic end of the unit and pull it out. It will appear to look like a toilet inside of a turn-of-the-century abandoned Bordello… and it will not respond well enough to cleaning. So replacing it with a brand new OEM Quality Orifice Tube will be necessary. This is the kind of Crap that when flushed off of the screens of the Orifice Tube will show the destruction of the AC Compressor inside:



Of course, the AC system will need to be evacuated to do this work and the entire system High and Low Pressure lines will need to be blown clean with a special AC Line Flush that is available at Autozone. Never flush the AC system with the AC Compressor installed… remove the Hi-Lo line manifold from it first and capture whatever blows through that two lines into a bucket. Avoid breathing the extremely vaporous stuff.

After installing a Brand New Orifice Tube… Replace the Accumulator with a Brand New one and do not remove the pre-sale protection plugs from it until you are ready to lubricate new Green “O” Rings with PAG 150 (Poly-Alkyl-Glycol) and tighten the fittings down carefully enough not to over torque them. They are Aluminum and very delicate… it is best to remember that its the Green “O” Rings that do the sealing… NOT the strength of your arm when putting any of this AC Hardware together.

The Rule of thumb is to first… quickly pour in 2 Ounces of PAG Oil into the Accumulator… 2 Ounces into the Compressor if being replaced with a Brand New OEM unit...and 2 Ounces of PAG Oil into the Condenser Coil opening at the top before sealing up all the lines and fasteners. This all must be done fairly quickly as the PAG Oil will soak up moisture right out of the the air and within 20 minutes after being “uncorked” the 6-8 Oz. Bottle of this stuff will become useless and saturated with too much water. So ‘Keep it Kapped’ and note that there is an Ounce Measurement Scale listed as you pour the stuff out marked clearly for the ease of use clearly visible right on the edges of the PAG container.

When all of the components are installed… use a Harbor Freight 2.5 CFM (Cubic Feet per Minute) or better Vacuum Pump and use the BEST Vacuum Oil you can buy...to evacuate the system for about an hour to remove any Air, Moisture and/or Non-Condensible Gases and bring the vacuum down as close to 25 Microns as read on the indication scale of your AC Manifold Gauge Set as possible in order to make everything inside a Bone Dry Vacuum like Deep Outer Space. Then the system can be re-charged with R-134A… using a Gram Scale to weigh in the charge as per the Owner’s Shop manual.

I have penned a “How To Repair The AC System” under the Articles Section for more specific details on how to do this job. In order to search and find AC Refrigerant leaks… You can purchase an inexpensive Digital Leak Detector tha is worth its weight in Platinum for making this job a lot easier to run down the leaks. This chart will show you where to use the device:

 

[sunliner]

All you guys, I REALLY appreciate the help. Most AC work appears to beyond my level of expertise though. Given the racket that the compressor makes and the indications that there's a leak developing, I probably just need to accept the fact that it's time to take it in to the shop. The current AC was put in three years ago and I hoping it would last longer...but maybe it's time. My local mechanic is a great guy though...time to give him a few bucks. Again, thanks..this forum is awesome.