Automotive Air Conditioning Information Forum

I am working on a 1984 corvette with 37k miles. It has the original R12. On the low side with a manifold gauge it showed about 7 lbs. My gauge set high side doesn't have the right adapter so i am blind on the high side. I am lucky to have an R12 tank. When i add freon upt to 30 /40 lbs and the clutch kicks in - it drops to around 10 psi and the compressor frequently cycles. When the clutch releases the pressure spikes up. Is that normal? Should i jumper the low pressure switch or keep adding so that when clutch is engaged the pressure is at 30 PSI?

With this frequent cycling there is no noticeable cooling.

The pressure holds overnight so i hope its a tolerable slow leak to get it into this condition.

Is the right procedure to jumper low pressure sensor and fill to 30 lbs with compressor running?

My first thought is that you should get hold of a quality Manifold Gauge Set that will indicate the pressures on both the low and high sides. My second thought is that you want to conserve that precious R-12 by checking the system for leaks using a good "sniffer" before adding any more refrigerant.

It sounds to me like the system is indeed low on charge--especially since the car hasn't run for 15 years. But other issues might also be in play on a 37-year-old vehicle, such as the R4 compressor and/or the viscous-drive fan clutch. In the meantime, don't run the system with only 7 PSI on the low side (in fact, I'm surprised it runs at all), and definitely don't jumper anything. Otherwise, you risk damaging the compressor.

If there's no under-hood decal that indicates the correct charge amount, my sources specify a charge weight of 48 ounces of R-12. And don't try to charge it by the low-side pressure, either. You'll need to weigh-in the charge using a precise refrigerant scale.

Yes, it's normal for the pressures to rise when the clutch cycles off.

Do you happen to know the history of this car and whether anything was ever done to the A/C system?

I say to keep it R-12 for sure, and NEVER add any sealant !!!

R4's do ok in the shaft seal, ignore the kid talking out his backside.

Never jumper the cycling switch - it is there for a reason.

No, you can't just charge to 30 psi on the low side. You need to charge by weight.

I see no information about corrosion in the condenser, so again see my first statement.

Cusser is absolutely right. Keep this vehicle R12. Converting older Corvettes to 134a never seems to provide good performance.