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high low side, low high side pressure troubleshooting

1puttpro on Sun August 10, 2008 1:28 PM User is offline

Year: 1990
Make: Ford
Model: Thunderbird SC
Engine Size: 3.8
Refrigerant Type: 134a
Ambient Temp: 70
Pressure Low: 70
Pressure High: 70
Country of Origin: United States

Static pressure is around 70ls and 70hs, with ambient temperature close to 70 degrees. This system was converted from R12 to 134a around 2001. A/C worked fine until the car sat for about a year and a half. Now, no A/C. Compressor turns fine. No noise. The clutch switch will not engage, but if I jump the connector, the compressor runs fine.

When I start the engine with the max A/C and blower... I get 75psi on the low side (slowly climbing) to about 75psi on the high side. If I jump the clutch pressure switch, this engages the compressor and I get suction on the low side... about 5mmHg and the high side goes to 100psi. The compressor continues to run, but the low side seems to cycle between 0 and 5 mmHg in suction.

Any thoughts to problem? I'm leaning to a bad compressor... but if I try to charge the system, the liquid line starts to frost. Could the orifice screen be blocked?

bohica2xo on Sun August 10, 2008 1:51 PM User is offline

You have 2 seperate issues here.

1) The cycling switch is bad. It should short cycle with 70 psi static. Replace the cycling switch.

2) You are low on refrigerant. You have a leak, and it will only get worse. Best to locate the leak & fix it before adding refrigerant.

Your compressor is probably just fine, as long as you don't continue to run it with a low charge. If you have lots of oily spots at the springlock connectors, you should add back an ounce of oil as well when you fix the leak(s) & recharge.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

1puttpro on Sun August 10, 2008 6:24 PM User is offline

I did replace the cycling switch... I added some UV dye, though I haven't spotted any leaks. I also inspected and found no oily spots. I'm removing the bumper to completely inspect the condensor. The only spot I found that may have a leak is the low side coupling, though I suspect that when I shot the dye into the spigot, that is spilled dye. What do you think?

Edited: Sun August 10, 2008 at 6:27 PM by 1puttpro

1puttpro on Sun August 10, 2008 9:24 PM User is offline

I've looked over the condensor and see no signs of leaking. It is holding pressure, so the leak can't be too large... could it? Should I pull a vac?

webbch on Sun August 10, 2008 11:13 PM User is offlineView users profile

Leak or no leak, you should ALWAYS pull a vacuum on the system before recharging. HOLDING vacuum suggests that there are no leaks, but sometime you run into a case where the system holds vacuum, but not pressure.

1puttpro on Sun August 10, 2008 11:47 PM User is offline

So, if the system does hold a vacuum... but not pressure... what would be the cause/fix? I'll pull a vacuum tomorrow morning.

webbch on Mon August 11, 2008 11:43 AM User is offlineView users profile

I've had it happen to me with a bad oring. Logically, o-rings are a likely source in such a case. To be frank, if a system holds a vacuum, the only reason I'd pressurize with nitrogren is if I KNEW the refrigerant had leaked out recently and I hadn't found a smoking gun as to the cause (or possibly if it was an R-12 system where the cost of refrigerant is quite high).

1puttpro on Tue August 12, 2008 9:56 PM User is offline

If I just replace o-rings... do I need to flush the system? Is it necessary to replace the acc/drier if I open the system to replace o-rings?

TRB on Tue August 12, 2008 10:40 PM User is offlineView users profile

No need for flushing. 18 year old accumulator, I would replace it. Might add an ounce of oil to the system while its open.


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