(VDOT) Refrigeration System

Important: Before any work is performed on the air conditioning system, review precautions on handling Refrigerant 134a (R-134a) along with the discharging, recovery and recycling procedures.

The Variable Displacement Orifice Tube (VDOT) Refrigerant system employs the constant run V-5 compressor. The V-5 is a variable displacement compressor that can match the automotive air conditioning demand under all conditions without cycling. The basic compressor mechanism is a variable angle wobble-plate with 5 axially oriented cylinders. The center of control of the compressor displacement is a bellows actuated control valve located in the rear head of the compressor that senses compressor suction pressure. The wobble-plate angle and compressor displacement are controlled by the crankcase-suction pressure differential. When the A/C capacity demand is high, the suction pressure will be above the control point: the valve will maintain a bleed from the crankcase to suction; no crankcase-suction pressure differential; and the compressor will have maximum displacement. When the A/C capacity demand is lower and the suction pressure reaches the control point, the valve will bleed discharge gas into the crankcase and close off a passage from the crankcase to the suction plenum. The angle of the wobble-plate is controlled by a force balance on the 5 pistons. A slight elevation of the crankcase-suction pressure differential creates a total force on the pistons resulting in a movement about the wobble-plate pivot pin that reduces the plate angle.

When the engine is turned OFF with the A/C system operating, the refrigerant in the system will flow from the high pressure side of the expansion tube (orifice) to the low pressure side until the pressure is equalized. This may be detected as a faint sound of liquid flowing (hissing) for 30 to 60 seconds and is a normal condition.

Refrigerant R-134a

Caution: Avoid breathing the A/C Refrigerant 134a (R-134a) and the lubricant vapor or the mist. Exposure may irritate the eyes, nose, and throat. Work in a well ventilated area. In order to remove R-134a from the A/C system, use service equipment that is certified to meet the requirements of SAE J 2210 (A-134a recycling equipment). If an accidental system discharge occurs, ventilate the work area before continuing service. Additional health and safety information may be obtained from the refrigerant and lubricant manufacturers.

Refrigerant performs the following functions in the air conditioning system:

^ Absorbs heat
^ Carries heat
^ Releases heat

These vehicles use Refrigerant-134a (R-134a). Refrigerant-134a is a nontoxic, nonflammable, clear, colorless liquefied gas.

Expansion (Orifice) Tube Description





The expansion (orifice) tube is a plastic assembly containing a fixed diameter tube with a mesh filter screen at either end. The fixed diameter tube creates a restriction to the high pressure liquid refrigerant in the liquid line, metering the flow of the refrigerant to the evaporator as a low-pressure liquid. When the engine is turned OFF with the A/C system operating, the refrigerant in the system will flow from the high-pressure side of the expansion tube (orifice) to the low-pressure side until the pressure is equalized. This may be detected as a faint sound of liquid flowing (hissing) for 30 to 60 seconds and is a normal condition.

When system diagnostics indicate a restricted expansion tube, it may not be necessary to replace it. Metal chips, flakes or slivers found on the screen may be removed with compressed air and the expansion tube may be reused if:

^ The plastic frame is not broken.
^ The brass expansion tube is not damaged or plugged.
^ The screen material is not form.
^ The screen is not plugged with fine gritty material.

Evaporator Description

The evaporator cools and dehumidifies air before the air enters the passenger compartment. The following events occur in the evaporator:

1. Low-pressure, low temperature liquid/vapor refrigerant enters the evaporator.
2. The refrigerant flows through the evaporator's tubing.
3. The refrigerant evaporates.
4. The refrigerant exits the evaporator as low-pressure, low temperature, mostly vapor refrigerant.
5. As the refrigerant evaporates, the refrigerant absorbs heat from the air flowing over the evaporator.

As the process of heat loss from the air to the evaporator core is taking place, any moisture (humidity) in the air condenses on the outside surface of the evaporator core and the moisture drains off as water.

Condenser Description

The condenser receives high pressure, high temperature refrigerant vapor from the A/C compressor. The condenser is made up of aluminum tubing and cooling fins which allow rapid heat transfer away from the high-pressure, high temperature refrigerant vapor. The cooling fins cause the high pressure, high temperature refrigerant vapor to condense into a high-pressure, medium temperature liquid.

Condenser Fan Description

The condenser fan is crucial to the proper operation of the air conditioning system. The fan ensures a sufficient amount of airflow across the condenser throughout the vehicle's operating range. The fan should be checked during all air-conditioning diagnosis procedures. Special attention should be given to the fan whenever excessive high-side pressure is encountered.

When the A/C is off and the engine coolant temperature is below 83°C (181°F), neither the main radiator fan motor, nor the auxiliary radiator fan motor runs. When the A/C is off and the engine coolant temperature reaches 93°C (199°F), each fan motor runs at full speed.

When the engine coolant temperature is below 83°C (181°F) and the A/C is requested, both fan motors are in series and run at half speed.

When the A/C is on and the engine coolant temperature reaches 93°C (199°F) the fans run in parallel at full speed.

Both fans will cycle on and off at full speed as the engine coolant temperature increases and decreases. Since the variable displacement compressor runs constantly, the fans will not cycle on and off with A/C compressor operation.

When terminal 8 of the data link connector (underhood) is grounded, either for engine diagnostic purposes or accidently, the A/C System will be inoperative below 1200 RPM.

Accumulator Description





The sealed accumulator is connected to the evaporator outlet pipe. The accumulator stores the refrigerant (vapor and liquid) and the oil from the evaporator. A desiccant at the accumulator bottom dries any moisture in the system. An oil bleed hole at the accumulator outlet pipe end provides the oil return path to the compressor. The accumulator is not serviceable and should only be replaced when leaking due to the following conditions:

^ Perforation
^ A damaged sealing area
^ Damaged fastener threads
^ Outside air has entered the system for extended periods of time

Heater Core Description

The heater core is the main component of the heater system. The heater core is located inside of the heater and evaporator module. Engine coolant is pumped into the heater core from the engine whenever the engine is operating. The heater core fins transfers the heat from the engine coolant to the air passing over the heater core. The heater core has specific inlet and outlet tubes. The placement of the heater hoses should be noted prior to servicing the heater core or the heater hoses.

The temperature control is linked to the temperature valve by a flexible control cable. When you rotate temperature control counterclockwise to the full COLD position, the temperature valve is held snugly against the air entrance to the heater core. The following actions occur:

^ All of the airflow from the evaporator bypasses the heater core.
^ No heat transfer occurs.

When you turn the temperature control away from the full COLD position, the temperature valve begins to direct air to the heater core. This action allows air to flow through the heater core. The farther the temperature control is rotated clockwise, the more the temperature valve directs air through the heater core. The air discharge is warmer when most of the airflow is heated in this manner. The air discharge is warmer because the heated and unheated air flows join and mix together thoroughly beyond the heater core.

When you rotate the temperature lever clockwise to the full HOT position, the temperature valve blocks off the passage that allows air to bypass the heater core. This action causes all of the airflow to pass through the heater core.

Compressor Description





The compressor (10) is driven by a drive belt from the engine crankshaft through the compressor clutch pulley. The compressor pulley rotates freely, without turning the compressor shaft, until an electromagnetic clutch coil is energized. When voltage is applied to energize the clutch coil, a clutch plate and hub assembly is drawn rearward toward the pulley. The magnetic field locks the clutch plate and pulley together as one unit to drive the compressor shaft.

As the compressor shaft turns, the compressor performs two main functions: it compresses the low-pressure refrigerant vapor (4) from the evaporator (1) into a high-pressure, high-temperature vapor (11). The compressor pumps refrigerant (and refrigerant oil) throughout the air conditioning system.

A/C Compressor Control Module Description

The A/C compressor control module controls the operation of the compressor clutch in response to signals received from the A/C refrigerant pressure switch and the Powertrain Control Module (PCM). With the air conditioning system in operation, the A/C compressor control module performs the following functions:

^ Energizes the compressor clutch in response to air conditioning system request and upon determination of favorable system conditions for air conditioning operation.
^ De-energizes the compressor clutch in response to a high or a low refrigerant pressure signal (from the A/C refrigerant pressure switch), or a high engine demand signal (from the PCM).

Pressure Relief Valve

The compressor is equipped with a pressure relief valve (9) which is placed in the system as a safety factor. Under certain conditions, the refrigerant on the discharge side may exceed the designed operating pressure. To prevent system damage, the valve is designed to open automatically when the high side pressure exceeds a specified amount. Conditions that might cause this valve to open (defective A/C pressure sensor, inoperative cooling fan, etc.) should be corrected, and the refrigerant oil and refrigerant should be replaced as necessary.