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Few years ago I converted my car over to 134. It’s been working fine up until last summer. The car started cycling more off more than it’s on. I evacuated the system and Drew vacuum for an hour. Added 40 ounces of 134To eliminate weather was under or over filled. Started doing some troubleshooting and realized that if I jumped the thermostatic switch that is attached to the pipe just passed the orifice tube the car cycling stops. Thinking the switch was bad I replaced it and it still has the same symptoms of cycling. Any input as to what direction I should be heading? By indication the gauges appear the pressures are normal.
Record pressure when cycling . High and low with ambient temperature and humidity referenced with good property working fan, seals, clutch etc.and hot water shut off valve operation
you already replaced switch correct? And is it adjustable?
did you add any oil after evacuating?
was the orifice changed from original?
did you add refrigerant by cans? Did you allow for loss in hoses and purging? Your charge is the 80% guess compared to r12…use gauges to reference.
is the blower fan on high? With good airflow?
what is temperature at center vent when cycling? It should cycle with fan on low between 34-46f. Typical 70f ambient
for the short time that the system running my pressures are 30/150,, dual spal fans running, seal good, heater bypass is working.
factory pressure switch, compressor runs constantly when temp switch is bypassed, leads me to believe pressure switch isn't failing.
no oil was seen when I evacuated the system, I assumed oil was ok
orifice was changed when system was converted to a blue one
The charts I used showed I should put 40 oz in, I put 41 to combinate for loss.
blower was fine, vent temps when temp switch is bypassed is around 40*
Those pressures (30/150) sound like they were taken when the compressor was off because they are too low for an active healthy working compressor. Here is a page taken from the GM shop manual for the 1979 Corvette. It should help with diagnosis. If the pressures don't reach the appropriate amount then pressure switches won't help.
ive done dozens of a/c replacements and fills. Never do I determine the proper fill by weight of refrigerant only. It’s a pressure sensitive system, that’s how I determine proper fill. The number of oz’s of refrigerant may or may not coincide with the proper fill level and subsequent pressures.
Total refrigerant for 79 is 3.0lbs.. My calculation was 48oz plus 1oz loss per can and 2oz lost in the charging hose. I used 54oz; lost 4oz from 4cans; lost 2oz from hose; equals 48oz.. Then double check with operating pressures and vent temp..
I decide to dive in deeper after adding more 134,,, found metal residue on the orifice screen. Decided to replace the orifice, accumulator, compressor and flush the system. Now on a 70* day I am able to get 32 low and 160 on the high with 52 oz not compensating for loss, not perfect but the system is operating well . Compressor isn't cycling and I'm getting 39* air our of the vents.
I decide to dive in deeper after adding more 134,,, found metal residue on the orifice screen. Decided to replace the orifice, accumulator, compressor and flush the system. Now on a 70* day I am able to get 32 low and 160 on the high with 52 oz not compensating for loss, not perfect but the system is operating well . Compressor isn't cycling and I'm getting 39* air our of the vents.
Metal residue? sounds like the compressor may have been on it's way out. Glad you got it working. 52 oz sounds more like a reasonable amount.
Your pressure is still lowish, but it's harder to tell on a 70* day vs an 85*+ day on the proper fill pressures. If it works then good enough.
i believe the correct r134 charge is about 80% of what the spec called for with R12. i assume another R4 compressor. adjustable cycling switch is also a plus. 134 likes lower low side than 12. you have to buy a 12 switch. the 134 switch is for larger newer fitting size. https://carcody.com/r134a-low-side-pressure-chart/
Last edited by derekderek; Sep 9, 2021 at 08:09 AM.
79 A/C Compressor cycling
