Nissan Pick-ups and Pathfinder 1970-1988

Air Conditioning System



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

The most important aspect of air conditioning service is the maintenance of 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 of refrigerant, and 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 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eventually, the valve's orifice will prove to be too much of a restriction for adequate flow even with the needle fully withdrawn.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 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, and freezing of the expansion valve orifice will eventually result. As air fills the system to a greater and greater extend, 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:

  1. Keep all tools as clean and dry as possible.
  3. Thoroughly purge the service gauges and hoses of air and moisture before connecting them to the system. Keep them capped when not in use.
  5. Thoroughly clean any refrigerant fitting before disconnecting it, in order to minimize the entrance of dirt into the system.
  7. 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.
  9. 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.
  11. Use only refrigerant-12. Purchase refrigerant intended for use in only automotive air conditioning system. Avoid the use of refrigerant-12 that may be packaged for another use, such as cleaning, or powering a horn, as it is impure.
  13. 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 12 hours (overnight).

  1. 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°F (-9°C) or lower. A reading of -15°F (-26°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 specified.

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 the 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.

Condensation Drain Tube

This is normally a single molded drain tube which expels the condensation, that 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 necessary safety precautions that must be exercised when working with air conditioning systems and R-12 refrigerant, a recap of the safety precautions are outlined.

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 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°F (-29.8°C) or lower. This will freeze any surface, including your eyes, that it contacts. 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 open flame.

  1. 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.
  3. 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 tube or line wrenches when tightening these flare nut fittings.)
  5. Never start a system without first verifying that both service valves are backseated, if equipped, and that all fittings are throughout the system are snugly connected.
  7. Avoid applying heat to any refrigerant line or storage vessel. Charging may be aided by using water heated to less than +125°F (+51°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.
  9. Always wear goggles when working on a system to protect the eyes. If refrigerant contacts the eye, it is advisable in all cases to see a physician as soon as possible.
  11. Frostbite from liquid refrigerant should be treated by first gradually warming the area with cool water, and then gently applying petroleum jelly. A physician should be consulted.
  13. 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.
  15. 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.


Most of the service work performed in air conditioning 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 and the pressure graduations read from 0 to no less than 60 psi.

The high side gauge measures pressure from 0 to at last 600 psi.

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, flow of refrigerant through the manifold. During all testing and hook-up operations, the valves are kept in a close position to avoid disturbing the refrigeration system.


Sight Glass Check

See Figure 1

You can safely make a few simple checks to determine if your air conditioning system needs service. The tests work best if the temperature is warm (about +70°F/21°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 systems 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 for a few seconds (about 1500 rpm) either with the help of a friend 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 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.

Exercise the air conditioner for a few minutes, every two weeks 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. 1: Arrow points to the sight glass on top of the air conditioner's receiver dehydrator


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


See Figure 2

It is not necessary to discharge the system for compressor removal. The compressor can be isolated from the rest of the system, eliminating the need for recharging.

  1. Connect a manifold gauge set.
  3. Close both gauge hand valves and mid-position (crack) both compressor service valves.
  5. Start the engine and turn on the air conditioning.
  7. Turn the compressor suction valve slowly clockwise towards the front-seated position. When the suction pressure drops to zero, stop the engine and turn off the air conditioning. Quickly front-seat the valve completely.
  9. Front-seat the discharge service valve.
  11. Loosen the oil level check plug to remove any internal pressure.

The compressor is now isolated and the service valves can now be removed.

Click image to see an enlarged view

Fig. Fig. 2: Manifold gauge set


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


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!