Nissan Pick-ups and Pathfinder 1989-1995

Air Conditioning System



To properly discharge and charge the A/C system, a special charging system with quick connectors, as well as complete training in refrigerant recycling and service procedures, and a certification license are absolutely necessary. UNDER PENALTY OF LAW, DO NOT VENT ANY REFRIGERANT (EVEN SMALL AMOUNTS) INTO THE ATMOSPHERE!

The most important aspect of air conditioning service is the maintenance of a pure and adequate charge of refrigerant in the system. A refrigeration system cannot function properly if a significant percentage of the charge is lost. Leaks are common because the severe vibration encountered in an automobile can easily cause a sufficient cracking or loosening of the air conditioning fittings. As a result, the extreme operating pressures of the system force refrigerant out.

The problem can be understood by considering what happens to the system as it is operated with a continuous leak. Because the expansion valve regulates the flow of refrigerant to the evaporator, the level of refrigerant there is fairly constant. The receiver/drier stores any excess refrigerant, so a loss will first appear there as a reduction in the level of liquid. As this level nears the bottom of the vessel, some refrigerant vapor bubbles will begin to appear in the stream of liquid supplied to the expansion valve. This vapor decreases the capacity of the expansion valve very little as the valve opens to compensate for its presence. As the quantity of liquid in the condenser decreases, the operating pressure will drop there and throughout the high side of the system. As the R-12 or R-134a continues to be expelled, the pressure available to force the liquid through the expansion valve will continue to decrease and, eventually, the valve's orifice will prove to be too much of a restriction for adequate flow, even with the needle fully withdrawn.

At this point, low side pressure will start to drop, and severe reduction in cooling capacity, marked by freeze-up of the evaporator coil, will result. Eventually, the operating pressure of the evaporator will be lower than the pressure of the atmosphere surrounding it, and air will be drawn into the system wherever there are leaks in the low side.

Because all atmospheric air contains at least some moisture, water will enter the system and mix with the R-12 or R-134a and the oil. Trace amounts of moisture will cause sludging of the oil and corrosion of the system. Saturation and clogging of the filter/drier, as well as freezing of the expansion valve orifice, will eventually result. As air fills the system to a greater and greater extent, it will interfere more and more with the normal flows of refrigerant and heat.

A list of general precautions that should be observed while servicing the system include:

Keep all tools as clean and dry as possible.
Thoroughly purge the service gauges and hoses of air and moisture before connecting them to the system. Keep them capped when not in use.
Thoroughly clean any refrigerant fitting before disconnecting it, in order to minimize the entrance of dirt into the system.
Plan any operation that requires opening the system beforehand, in order to minimize the length of time it will be exposed to open air. Cap or seal the open ends to minimize the entrance of foreign material.
When adding oil, pour it through an extremely clean and dry tube or funnel. Keep the oil capped whenever possible. Do not use oil that has not been kept tightly sealed.
Use the correct refrigerant only. R-12 and R-134a must never be mixed, even in the smallest amounts.
Completely evacuate any system that has been opened to replace a component (other than when isolating the compressor), or that has leaked sufficiently to draw in moisture and air. This requires evacuating air and moisture with a good vacuum pump for at least one hour.

If a system has been open for a considerable length of time, it may be advisable to evacuate the system for up to 3 hours.

Use a wrench on both halves of a fitting that is to be disconnected, so as to avoid placing torque on any of the refrigerant lines.



In order to prevent heater core freeze-up during A/C operation, it is necessary to maintain permanent-type antifreeze protection of +15°deg;F (-9°deg;C) or lower. A reading of -15°deg;F (-26°deg;C) is ideal since this protection also supplies sufficient corrosion inhibitors for the protection of the engine cooling system.

The same antifreeze should not be used longer than the manufacturer specifies.

Radiator Cap

For efficient operation of an air conditioned truck's cooling system, the radiator cap should have a holding pressure which meets manufacturer's specifications. A cap which fails to hold these pressure should be replaced.


Any obstruction of, or damage to, the condenser configuration will restrict air flow which is essential to its efficient operation. It is, therefore, a good rule to keep this unit clean and in proper physical shape.

Bug screens are regarded as obstructions and should, therefore, be avoided.

Condensation Drain Tube

This is normally a single, molded drain tube that expels the condensation which accumulates on the bottom of the evaporator housing into the engine compartment.

If this tube is obstructed, the air conditioning performance can be restricted and condensation buildup can spill over onto the vehicle's floor.


Because of the importance of the safety precautions which must be exercised when working with air conditioning systems and R-12 or R-134a refrigerant, a recap of the safety precautions follows:

Check with your local authorities before attempting to service you vehicle's A/C system. In many areas, it may be illegal to purchase R-12 or R-134a and service the system, unless you are a certified technician.

The compressed refrigerant used in the air conditioning system expands into the atmosphere at a temperature of -21.7°deg;F (-29.8°deg;C) or lower. This will freeze any surface that it contacts, including your eyes. In addition, the refrigerant decomposes into a poisonous gas in the presence of a flame. Do not open or disconnect any part of the air conditioning system near an open flame.

Avoid contact with a charged refrigeration system, even when working on another part of the air conditioning system or vehicle. If a heavy tool comes into contact with a section of copper tubing or a heat exchanger, it can easily cause the relatively soft material to rupture.
When it is necessary to apply force to a fitting which contains refrigerant, as when checking that all system couplings are securely tightened, use a wrench on both parts of the fitting involved, if possible. This will avoid putting torque on refrigerant tubing. (It is advisable, when possible, to use tubing or line wrenches when tightening these flare nut fittings.)
Never start a system without first verifying that both service valves (if equipped) are backseated, and that all fittings throughout the system are snugly connected.
Avoid applying heat to any refrigerant line or storage vessel. Charging may be aided by using water heated to less than 125°deg;F (51°deg;C) to warm the refrigerant container. Never allow a refrigerant storage container to sit out in the sun, or near any other source of heat, such as a radiator.
Always wear safety goggles when working on a system to protect your eyes. If refrigerant contacts the eye, it is advisable in all cases to see a physician as soon as possible.
Frostbite from liquid refrigerant should be treated by first gradually warming the area with cool water, then gently applying petroleum jelly. A physician should be consulted.
Always keep refrigerant can fittings capped when not in use. Avoid sudden shock to the can which might occur from dropping it, or from banging a heavy tool against it. Never carry a can in the passenger compartment of a truck.
Always completely discharge and recover the system before painting the vehicle (if the paint is to be baked on), or before welding anywhere near the refrigerant lines.


See Figure 1

Most air conditioning service work requires the use of a set of two gauges, one for the high (head) pressure side of the system, the other for the low (suction) side.

The low side gauge records both pressure and vacuum. Vacuum readings are calibrated from 0-30 in. Hg (0-101 kPa), and the pressure graduations read from 0 to no less than 60 psi (414 kPa).

The high side gauge measures pressure from 0 to at least 600 psi (4137 kPa).

Both gauges are threaded into a manifold that contains two hand shut-off valves. Proper manipulation of these valves and the use of the attached test hoses allow the user to test high and low side pressures.

The manifold valves are designed so that they have no direct effect on gauge readings, but serve only to provide for (or cut off) the flow of refrigerant through the manifold. During all testing and hook-up operations, the valves are kept in a closed position to avoid disturbing the refrigeration system.

Click image to see an enlarged view

Fig. Fig. 1: Common manifold gauge set


The compressed refrigerant used in the air conditioning system expands into the atmosphere at a temperature of -21.7°deg;F (-29.8°deg;C) or lower. This will freeze any surface that it contacts, including your eyes. In addition, the refrigerant decomposes into a poisonous gas in the presence of a flame. Do not open or disconnect any part of the air conditioning system near an open flame.

Sight Glass Check

See Figure 2

You can safely make a few simple checks to determine if your air conditioning system needs service. The tests work best when the temperature is warm (about 70°deg;F/21°deg;C).

If your vehicle is equipped with an aftermarket air conditioner, the following system check may not apply. You should contact the manufacturer of the unit for instructions on system checks.

  1. Place the automatic transmission in Park or the manual transmission in Neutral. Set the parking brake.
  3. Run the engine at a fast idle (about 1500 rpm) for a few seconds, either with the help of an assistant or by temporarily readjusting the idle speed screw.
  5. Set the controls for maximum cold with the blower on High.
  7. Locate the sight glass, if so equipped, in one of the system lines. Usually, it is on the left alongside the top of the radiator.
  9. If you see bubbles, the system must be recharged. Very likely, there is a leak at some point.
  11. If there are no bubbles, there is either no refrigerant at all or the system is fully charged. Feel the two hoses going to the belt-driven compressor. If they are both at the same temperature, the system is empty and must be recharged.
  13. If one hose (high pressure) is warm and the other (low pressure) is cold, the system may be all right. However, you are probably making these tests because you think there is something wrong, so proceed to the next step.
  15. Have an assistant in the truck turn the fan control or A/C switch on and off to operate the compressor clutch. Watch the sight glass.
  17. If bubbles appear when the clutch is disengaged and disappear when it is engaged, the system is properly charged.
  19. If the refrigerant takes more than 45 seconds to bubble when the clutch is disengaged, the system is overcharged. This usually causes poor cooling at low speeds.

If it is determined that the system has a leak, it should be corrected as soon as possible. Leaks may allow moisture to enter and cause a very expensive rust problem.

Operate the air conditioner for a few minutes, every month or so, during the cold months. This avoids the possibility of the compressor seals drying out from lack of lubrication.

Click image to see an enlarged view

Fig. Fig. 2: Arrow points to the sight glass on top of the air conditioner's receiver/drier


  1. Park the vehicle in the shade.
  3. Connect a gauge set.
  5. Close (turn clockwise) both gauge set valves.
  7. Mid-position both service valves.
  9. Start the engine, set the parking brake, place the transmission in NEUTRAL and establish an idle of 1500 rpm.
  11. Run the air conditioning system.
  13. Insert a thermometer into the center air outlet. Evaluate the temperature and pressure readings of the A/C system.


To properly discharge and charge the A/C system, a special charging system and quick connectors, as well as complete training in refrigerant recycling/special service procedures, and a certification license are absolutely necessary. DO NOT VENT ANY REFRIGERANT (EVEN SMALL AMOUNTS) INTO THE ATMOSPHERE. THIS PRACTICE IS PROHIBITED UNDER PENALTY OF LAW!

If you do not have the certification and equipment necessary to service the A/C system, take your truck to a local repair facility and have the system recovered. Once service is finished, you can return the truck to the facility for proper evacuation and charging.


If you do not have the proper equipment and training, take the truck to a certified technician.


Some leak tests can be performed with a soapy water solution. There must be at least a 1 / 2 lb. charge in the system for a leak to be detected. The most extensive leak tests are performed with either a Halide flame-type leak tester or the more preferable electronic leak tester.

In either case, the equipment is expensive, and the use of a Halide detector can be extremely hazardous!