Engine AC Help

flanaganjohn

Member
May 29, 2020
6
6
13
25
New York
Good afternoon all, I recently put a whole new AC system in my 1990 Ford mustang 5.0. I did a reseal to my 6p148 and replaced all the seals. I got a new condenser, all lines new, new evap and clutch. I vacuumed it down held its charge and charged it up. It calls for 2lbs and 2 oz of R12 and converting to R134a i times that by .80 and ended up putting in about 2.25 cans. AC vents blow nice and cold. however my low side pressure is very high. At around 40-45 psi.The most they will come down when revving is 30 psi. High side seems fine at 150. In NY today temps were about 70-75. My issue is that i can't even get the low pressure switch to turn off the compressor. Im leaning toward the compressor being worn out but I wanted to ask some others what they thought. I purged all the lines prior, vacuumed for one hour, have the correct 8oz of POE in the system. Thank you all for your input!
 
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90sickfox

I didn't really have an issue with the stink...
SN Certified Technician
Mar 2, 2015
5,376
4,463
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Your readings are right around where they should be for 75 degrees. If the car had been running with heat in the radiator the low pressure reading may raise a little.

Screenshot_20200530-012645_Chrome.jpg


A bad compressor will usually make the pressure needle shake.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
34,002
10,276
224
Massachusetts
Low pressure switch is set for 25psi for r12. With r134a, you actually want it down to around 18-21psi.

Turning the adjustment screw counter clockwise will decrease the pressure. 1/4 turn is approx 1 psi so you want to go counter clockwise 1 to 1 3/4 turns
 

flanaganjohn

Member
May 29, 2020
6
6
13
25
New York
I did screw the low pressure switch prior, I just can't get the low pressure to drop below 30 even when revving it up at like 3500-4000rpm. does that mean the compressor can't keep up and is worn out? Thank you!
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
34,002
10,276
224
Massachusetts
What's the concern though? Low pressure switch is designed to cut out if the pressure drops too low due to low refrigerant in the system. If your system is properly charged, this shouldn't happen.

With RPM, you'll get a larger pressure drop across the orifice, so the low side will drop and the high side will rise slightly, but as long as your charge is ok you won't get down to 20psi and trip the low pressure switch.

I agree with above post that 35-40psi/150-170psi is where you want to be at 75 degrees.
 

flanaganjohn

Member
May 29, 2020
6
6
13
25
New York
I was always under the impression you did not want the compressor to run continuously? Thank you guys for all the help much appreciated!