Does anyone have any hard data on the refridgerant capacity for a '76 F150 with factory AC? I'm converting to R134A fro mR12 and don't want to over fill, especially since the R134A should be filled to 80% of original capacity.

Thanks in advance!

 

Actually, when changing refrigerants, usually from R12 to R134a, the "correct" charge amount will vary widely from one model of vehicle to the next.
The 80% figure that you see everywhere is a safe baseline starting point and is often considered "good enough" for later model Cycling Clutch Orifice Tube AC systems.
Problem is, 80% usually isn't nearly good enough for the earlier and much better Thermostatic Expansion Valve AC system in your 76 truck. Forget the original R12 spec.
Your system holds "about" 3.5 lbs of refrigerant and you need a manifold gauge set to get it right.

To properly charge your system with R134a:
-Fully evacuate the system to 29+" of vacuum for 30 minutes.
-Add refrigerant with the system OFF until the pressure equalizes.
-Start the engine and set the speed to 1500 rpm.
-Set the AC controls to Max AC, High Blower.
-Open all of the windows
-Add refrigerant until the High Side gauge reads 2.8 times ambient temp (measured near the front of the condenser). Stop adding refrigerant and let the pressures stabilize for 5 minutes.
-Add refrigerant 2 ounces at a time until the High side pressure stabillizes (after 5 minutes with each shot, yes this can take some time) at about 2.8x ambient and the lines from the evaporator (at the firewall) are cold and at the same temperature.
-Once you reach those parameters, you are at optimum refrigerant charge regardless of the original spec. or vent temps.
- Adding refrigerant after this point will NOT improve cooling and can damage the compressor.
-Never let the High Side pressure exceed 350psi! The high side pressure should NEVER reach 3.5 times the ambient temp in normal operation.

Hope this helps.


I know this isn't the answer you were looking for, but...

 

Thanks for the response! We got everything going late last night and have nice cold AC! Was pretty angry with the hose shop who redid the hoses - found a pin hole in the braze joint that caused issues.

 

Good explanation of the steps. Does it hurt if half way of adding 134a just using the vacuum it had; then turning truck on and continuing to charge it. Or are u not supposed to turn truck on till vacuum equals?
93 F150 5.0
I hv had the same problem three times now! High side pressure high and w only 24 ounce of 134a in it blew the purge valve on the suction/discharge manifold.
Everything flushed thoroughly or brand new. Vacuum pulled -30 held for 24 hours. Added 4oz pag oil and new compressor had 3oz in it. System calls for 7 oz.
Is this maybe my problem?: I added the 4 oz of oil through the high side line as it was vacuuming. Let it sit and continue to vacuum for thirty minutes. Added two cans of 134a 24oz total, it needs 32oz; and checked gauges. At this point air was cold inside and doors open. Let truck run for 1 1/2 hours! It stayed cold. I knew I needed to add more 134a to get to the 32 oz requirement. Compressor cycling then staying mostly on. Gauges were good. Low side 29-35 High side 300 no more. Seeing no issues, I drove the truck for 30 minutes. Stayed nice and cool for the dogs. Came home and parked truck. Today turned truck on, no ac. The purge valve on ac manifold of suction/ discharge hose had blown. Third time!
question what is the correct amount of pag oil? And was it wrong to add oil thru the high side gauge line? Per YouTube video …..
It sucked the 4oz right in. Closed valve and let it vacuum again. Vacuumed from low side only. New compressor was turned 10 times by hand.


Seemed like ac works like it should. It was parked an hour and a half and was cold. It’s after I drive it that I ended up w the purge valve blowing from high pressure. Each time, exact symptom. Compressor is F10. Factory ac, now 134a instead of r12 Freon. Advice or suggestions??!! PLEASE. I know there is a solution!
Remember, only 24oz of 32 oz was put in system at this point.

*** I did notice some liquid in the accumulator; which I again replaced this last time. New Red orifice tube, new suction discharge line. Fin tube condenser flushed super good out of the truck. And evaporator too but on the truck. Frontwards backwards etc. liquid was clear, then dried w compressed air.
Second time I noticed liquid in accumulator. You tube said turn can from 12 to 3 o’clock every 4seconds. I watched bubble in the gauge sight glass. When bubbles slowed down I would turn can again to 3 o’clock for just a few seconds… should I not do that bc liquid is getting to the accumulator and probably has moved to the compressor? Not giving up till I fix it!!

 

so ??. What was the outside air temp that day and relitive humidity. Im asking because these pressures you should expect are somewhat relitive to those temps. For instance you may see those temps and pressures because its at the full end of the spectrum it was designed for and its very hot outside. Or it can reach those pressures while still being low pressure, but it could have a filthy evaporator that wont pick ip hear, or a filthy condensor that wont expell it. Or like mine its roasting outside and my outside vent door doesnt want to shut so im drawing max heat but without closeing the vent door even a full 20’ drop ( which im not getting) would struggle to keep up in these old ovens we call trucks.