* Specific A/C system design: Even within the same year and engine, slight variations in the A/C components could affect the charge.
* System leaks: If the system has leaks, the charge will be lower than the factory specification.
* Condenser and evaporator condition: A clogged condenser or evaporator will reduce efficiency and affect the necessary charge.
You cannot determine the correct freon charge without consulting a service manual specific to your 1987 Dodge Ram. These manuals usually list the correct refrigerant type and charge amount.
Warning: R12 is an ozone-depleting substance and is no longer legally manufactured or sold in many places. Any servicing will likely require a retrofit to a compatible refrigerant (like R134a) and the system will need to be properly converted to handle the new refrigerant. A qualified mechanic is crucial for this process.
Instead of trying to guess the charge, take your truck to a qualified automotive A/C technician. They will have the proper tools and knowledge to diagnose any problems, evacuate the existing refrigerant (safely and legally), repair any leaks, and recharge the system with the correct amount of refrigerant (or its appropriate replacement).
