I've had no a/c in my 97 F150 truck since last sept. It quit on me out of the blue - one night i came out to the car after a long drive with a/c on the whole time (i had parked it maybe 4 hours when i got home) and there was a BIG puddle of water under the car. (it was at least 3 feet around..not typical drips) I figured it was frozen in the late night 32degree weather and engine temps slowly melted the big block
o ice when it froze.

Anyway, the compressor will now only come on when i short it. I bought a low side gauge and filler kit to see what the deal was. Turns out, my low side showed 125psi with the car off, on and running, and even with the compressor manually kicked on via a jumper wire.
SO, i figured i'd try letting some pressure out down to about 43psi where it should be to see if the compressor comes on by itself.

Keep in mind....wHen the compressor is jumpered and forced on, the air blows cold - regardless of pressure in the system.

ANyway, no matter how long i let pressure out of the low side, it always went to equilibrium regardless of what the compressor was doing. I'd drop low side pressure to 40 or so, then the gauge would slowly start increasing back up. It just seemed like i was dropping the entire system pressure down. then i filled it back up, and it went to 125 again and held with compressor on or off. Let air out and it would drop to about 70psi on or off. I could only get the pressure down to where it should be temporarily by bleeding pressure out, but it would just even out again by itself. I dont have a high side gauge, so i dont know what thats doing. I'll find out for sure after borrowing one.

Id get cold air as long as the wire was jumpered, but it would always shut off and blow warm air when i took the short out. There seems to be no way to get the low side pressure down to the 40's where it should be. It always raises back up with the compressor off or on.

WHats going on? Do i have a leak somewhere thats letting high-side air get into the low side? It's like the system is acting like it does regardless of the compressor state. car off, gauge on the low side shows 125psi. car running, compressor on, it even raised to 140 for a few min before slowly dropping back to 125psi. Then i let pressure out of the low side and it acted the same way, but at 70psi.

if anyone is an a/c guru - help me out please. im pulling my hair out.
thanks in advance for any help you can give

 

The standing pressure should be the same high side and low side. When the compressor comes on it should pull the low side down and increase the high side. The exact pressures will depend on the temp and humidity of the ambient air. If you have to jumper the compressor then that is your first problem. Could have low pressure and cutoff switch will not let it come on. Could of had the over heat switch cut off the compressor. It goes on and on. If you don't see any pressure change with the comp. running then I would say that your comp. is bad.

 

Quit screwing around letting your R134a out. The static pressure is supposed to be that high depending on ambient temperature.

When the low side pressure RISES above (approx) 45 psi, it closes the cycling switch and enables the compressor clutch. If you have high enough pressure in the system and the clutch only engages when the switch is jumpered, YOU HAVE A BAD SWITCH.

If the compressor is running while the switch is jumpered, you have cold air, and the low side pressure reading is not dropping, YOU HAVE A BAD GUAGE. The low side pressure HAS to drop when the compressor runs and there is cold air coming from the vents.

Steve

 

wow - bad gauge definitely! I just bought this thing yesterday and it's bad. SO much for a Napa gauge.

When my buddy hooked up his professional gauges, it showed 1psi low and 102psi high. I had a tad bit left in my r134 can and we put it in and raised the high to 120 and the low to 3psi.

Im gonna go back to napa and take this gauge back..it was also leaking r134 out of its low-side fitting...real nice. I couldnt see it last night, but in the sunlight outside today it was just jetting right out to atmosphere. When i unplugged the gauge from the low side and the can from the other end, the gauge just sat there with nothing connected to it reading 45psi.

I wouldnt have let anything out if the gauge was accurate and not showing me 125psi low side with the compressor running. Now, we verified it operates as normal so next step is proper pressure and see if the switch works.

Based on my Haynes manual - it says 43.5psi is optimal. Is there a minimum/max low-side pressure before the switch will activate so i can be CERTAIN that it's filled properly?

Thanks for all the help guys - im almost home free

 

Typically, the switch closes around 45 psi ascending. It then opens up again around 20-25 psi, then cycles if needed.

These numbers are approximate.

The actual pressures in the system will depend on the ambient temperature and humidity.

Steve

 

According ot the Temp/Pressure chart 45 PSI is about 50F which is the temp that the owners manual says teh A/c doesn't operate below.
So I guess there is not separate "switch" to lock out the A/C at low temps when using the defrost mode ...

°F R717 R22 R134a R404a R507
Temp psig psig psig psig psig
40 58.4 68.6 35.0 87.02 90.3
41 59.2 70.0 36.0 88.72 91.3
42 61.4 71.5 37.0 90.50 93.3
43 62.9 73.0 38.0 92.98 95.3
44 64.5 74.5 39.0 94.08 97.3
45 66.1 76.1 40.0 95.90 99.3
46 67.6 77.0 41.1 97.75 101.3
47 69.3 79.2 42.2 99.61 103.3
48 70.9 80.8 43.2 101.5 105.3
49 72.6 82.4 44.3 103.42 106.3
50 74.3 84.1 45.4 105.36 108.3
55 83.2 92.6 51.2 115.41 119.3
60 92.6 101.6 57.4 126.08 130.3
65 102.8 111.3 64.0 137.39 141.3
70 113.8 121.5 71.1 149.38 154.3
75 125.5 132.2 78.7 162.05 166.3
80 138.0 143.7 86.7 175.45 180.3
85 151.4 155.7 95.2 189.54 195.3
90 165.5 168.4 104.3 204.51 210.3
95 180.6 181.9 113.9 220.22 226.3

 

ok guys..pressure's dead-nuts. Jump the switch and it works great. Take the switch off and it wont kick on. It's gotta be the switch.

So - does the switch on my 97 F150 come with a schraeder valve so i wont loose all the r134 i just put back on it, or will i have to let everything out to replace it?

I was thinking about machining a connector to go on the low-side fill port to take advantage of it's schraeder valve if not..i could connect a switch in there and easily replace it without losing anything next time around. If i need to add r134, just pop it off and plug the can in. I'd use a modified filler-hose to lockon to the filler port and on the other end just modify it to add a switch in there.

So - which way do i have to go? Please tell me i wont lose R134 when replacing the switch!

 

yes, you can remove the switch from the accumlator without loosing pressure.

 

woohoo!!! man thats GREAT news at this point!

thanks a TON for all your help guys - i think i got this prob taken care of then. I'll pickup a switch and have a go. THis forum is great!

 

pressure cycling switch was the fix! it works great now...thanks everyone!!