Skip to main content ↑ ↓
Topic: A/C pressure switch question. (Read 1500 times) previous topic - next topic

A/C pressure switch question.

Okay i'm still trying to get A/C in my car.  I bought a new dryer however it doesn't have a pressure switch like the original unit had.  If i bypass this feature will the compressor still operate normally?  I realize the risk of ruining the compressor if the lines were empty but I plan on verifying that the system isn't leaking before continuous use.  Any input would be great.  Thanks.
84 TurboCoupe with 302 .010 over - 9.8 : 1 Forged Pistons - Edlebrock Goodies:  Aluminum Heads #6037, Intake #3821, 65mm TBI/EGR #3824/3827, Camshaft #3722 -  Interactive System & Technologies Mass Air with 24# Injectors - A9L - 3g Alternator - BBK shorties, Cat Converter H-pipe, Magniflow lers - World Class T5 1352-169 (1986, V8, WC, 3.15 3.35 1.93 1.29 1.00 0.68) OEM Clutch Cable - 7.5 Trac Lock with 3.08 and slapper bars from 82 Mustang.

A/C pressure switch question.

Reply #1 –
My A/C system had to have one. I have two NOS switches if you need one.
'88 Sport--T-5,MGW shifter,Trick Flow R intake,Ed Curtis cam,Trick Flow heads,Scorpion rockers,75mm Accufab t-body,3G,mini starter,Taurus fan,BBK long tube headers,O/R H-Pipe, Flowamaster Super 44's, deep and deeper Cobra R wheels, Mass Air and 24's,8.8 with 3.73's,140 mph speedo,Mach 1 chin spoiler,SN-95 springs,CHE control arms,aluminum drive shaft and a lot more..

A/C pressure switch question.

Reply #2 –
You don't want to bypass that switch.  Since you're likely to ask why not, here we go.  That switch is the A/C clutch cycling switch, based on pressure in the low side (which is always roughly equal to the evaporator temp) this switch turns the clutch on and off.  This is important because if the system continues to run with the low side below approximately 22-25 psi, the evaporator will freeze in to a solid block of ice (more on that later).  The switch opens and shuts off the clutch around 22 psi which allows the system to begin to equalize, once pressure rises above around 44 psi, the switch will close again, turning the clutch back on.  The detectable result, if you had a digital thermometer in the vent, would be a temp fluctuation of around 3 to 5 degrees in the discharge.  Some systems also have a high side safety switch which prevents an over-pressure in the high side, by interrupting the clutch over 350 psi high side for situations like system obstruction or cooling fan failure. 

The frozen evaporator question.  Under normal operation, the evaporator is very cold.  You could take a surface temp reading of a normally operating evaporator of round 33 to 36 degrees F, so when warmer air passes over it, the moisture in the air condenses on the evaporator and drips off into the HVAC case, and then out of the A/C drain onto the ground (you may have noticed a puddle of water under cars running A/C).  If the clutch never switches off, the evaporator temp is unregulated, and that water condensing on it will freeze before it can get out the drain.  A frozen evaporator blocks ALL airflow, so your blower will make lots and lots of normal sounding noise, but benefit you with no airflow until you shut off the A/C and let it thaw.  Too much of this will cause the evaporator to suffer damage.  When water freezes it expands, and I have replaced evaporators that were leaking from the distortion and materials fatigue caused by repeated freezing. 

There's much more to tell on the subject of A/C, but the rest is probably not relevant to your post.