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Jaguar Air Con – facts, faults and use

The following refers to aircon systems fitted to Jaguar XJS, XJ Series 1,2,3 and will have some relevance to later cars.

See also our R134a conversion mythbuster.


The air conditioning system is often regarded as just the air cooling side but should be regarded as the entire air blend, heating and cooling systems.

Below you will find brief descriptions of the components, how best to operate the system, common faults and how to resolve them. KWE specialises in repairs of these systems, from a simple re-gas to complex fault-finding.

The system used on the XJS and saloon cars is quite sophisticated and embodies true climate control, that is, automatic temperature control and dehumidification. The major components are: airblend box, system controller or computer, air blowers (fans), heater matrix, hot water valve, refrigeration compressor, condenser radiator, expansion valve, filter dryer and cooler matrix (evaporator).

Refrigeration principles

A compressor circulates a special refrigerant gas around a circuit comprising in order: Condenser radiator (in front of engine coolant radiator), filter-dryer (contains a dessicant to remove any water, and a filter to trap debris), expansion valve, evaporator matrix, fuel cooler (on most cars) and back to the compressor.

The gas is chosen to be able to be a gas or a liquid at reasonable temperatures and pressures. In its liquid state the refrigerant reaches the expansion valve where it rapidly expands into its gas state and becomes very cold (like the outlet of aerosol cans). This coldness is held in the evaporator matrix where air on its way to the cabin is blown through it thus becoming cold itself. Much of the water vapour held in normal air condenses out on the evaporator surface and drips down and out through vents.

The refrigerant gas is sucked to the compressor which then compresses the gas making it very hot and at high pressure. This hot gas is passed to the condenser radiator where as a result of being cooled by outside air passing through it, the refrigerant condenses back to a warm liquid. This passes to the filter-dryer (also known as a receiver dryer) where any moisture is removed by a desiccant and any particles shed by the compressor or hoses is trapped (there is another filter in the expansion valve).

Then the refrigerant liquid passes to the expansion valve and the process is repeated.

Airblend Box

This large assembly sits under the dash and in front of the controls and radio. It contains the heater matrix through which hot engine coolant passes, a cooler matrix or evaporator through which refrigerant passes, and an arrangement of flaps or air valves which control the mixture of hot and cold air and thus the temperature of air blown into the cabin. It also controls air direction to dash top (screen demist), dash front (corner face vents), centre vent, and low level vents front and rear. The systems were designed by Delanaire and the early cars (up to ca 1987) used the Mark 2 system, and later cars the Mark 3. The Mark 2 used a motor-driven rotary controller which operated the air blend flaps directly with levers, and the various other flaps and valves by vacuum. It senses the difference between the temperature of the cabin and that which the control knob is set to, and varies the proportion of hot to cold air accordingly. The Mark 3 system carries out the same functions but the controller is a microcomputer and the main air blend flaps are operated by electric motors.

The refrigeration system runs all the time so providing dehumidification of all incoming air. When heat is demanded the water valve is opened which heats the matrix and the cooled incoming air is directed through this heater to provide warm, dry air. When cold is demanded the water valve is closed and the air is directed to bypass the heater matrix and so the cold, dry air reaches the outlets unheated. When the correct cabin temperature has been reached the water valve is opened and the blend flaps are automatically operated to provide a mix of hot and cold air. As the cabin temperature varies the system automatically compensates by changing the proportion of cold to hot air reaching the vents.

There is a thermostat inserted into the evaporator matrix which switches off the refrigeration compressor when the matrix approaches freezing point. This is to prevent icing up which would stop air flowing through it. On early systems this can be unreliable.

The airblend box itself is simple enough to be reliable but can suffer from vacuum leaks (often resulting from careless installation of radios and alarm systems) and seizure of flap operating motors in the Mark 3 system. The vacuum valves on the Mark 2 can also become clogged with dirt. The condensate drains can become clogged. See faults below.

System Controller

On the Mark 2 this is an assembly of a small electric motor with servo feedback potentiometer, and a shaft carrying a number of cams, positioned under the airblend box on the right hand side. The motor can often be heard whirring as it changes position. As the motor turns the shaft these cams operate steel rods which move the airblend flaps, and also vacuum valves which operate the various vacuum ‘motors’ for screen demist, centre vent, recirculation flaps and water valve. The position of the shaft driven by the motor is controlled by an amplifier which measures the difference between the temperature of the cabin sensor (mounted in the dash) and the setting of the temperature control knob. The amplifier also controls a block of relays to set blower speed. The amplifier is notoriously unreliable and cannot be repaired as it is solidly potted. There are aftermarket replacements available.

On the Mark 3, the controller is a small computer which electronically operates the flap valve motors, blower fan speed and vacuum solenoids for the external vacuum ‘motors’. This computer is diamond-shaped about 5 inches long, located on the right hand side of the airblend box. It is extremely reliable (I have never had a failure) though it is often thought to be the cause of faults.

Air Blowers (fans)

There are two fan assemblies located very low down under the dash, just above the driver’s and passenger’s ankles. They have two linked flap valves, one on top, one beneath, operated by a vacuum ‘motor’. In their ‘relaxed’ state the top valve is open which directs fresh air into the airblend box. When the vacuum motor is operated the lower flap opens and the top closes thus shutting off fresh air and feeding the airblend box with cabin air – or ‘recirculated air’. This is only used when the system is demanding maximum cold as it’s quicker to cool just the cabin air. This does lead to a common fault however – if the refrigeration system has failed, the system moves to recirculation on a hot day and so the cabin quickly becomes stuffy and humid without getting any colder. Surprisingly, the way round this is to turn the temperature up so that the system reverts to fresh air – but it still won’t do any cooling of course.

The fan speed is set by the controller automatically and is either a controlled speed according to temperature difference or full speed via a ‘high speed’ relay which overrides the automatic system. On the Mark 2 system the controlled speeds are determined by a series resistor pack controlled by relays. On the Mark 3 system, the fans have electronic speed control by a power transistor and feedback diode assembly mounted on top of the casing. This is notoriously unreliable as the components are not protected against damp and the diode in particular fails. The symptom is usually no fan operation except on high speed, or sometimes it’s high speed all the time. This fault can be repaired by it is time consuming to remove and replace the fan assembly.

Heater Matrix

This is just a small ‘radiator’ inside the airblend box, connected to the engine coolant circuit via a vacuum-operated water valve. It can develop leaks, evidenced by a sharp smell of coolant in the cabin when the system is first turned on, or wet floors and falling coolant level. Replacement is very difficult!

Hot Water Valve

Situated in the engine bay, close to the bulkhead (firewall) with two coolant pipes and a small vacuum pipe, this shuts off hot water to the heater matrix when max cold is demanded. It can become stuck open or shut but is fairly easily replaced.

Refrigeration Compressor

There are two main types on the Jaguar: The heavy, black Harrison A6 and the lighter alloy Sanden type used on R134a systems. They are driven by belt from the engine via an electric clutch mounted on the compressor shaft. Compressors can be damaged through overheating if the refrigerant falls below a certain level, so they have a device to stop their operation under fault conditions. Early cars (up to about 1988) used the A6 compressor with a high temperature switch. This switch detects the temperature of the interior of the compressor and if it exceeds a certain point it goes short circuit which in turn blows a one-time thermal fuse which opens the circuit to the compressor clutch thus switching off the compressor. Later A6 installations used a pressure switch in the same location on the backplate of the compressor which open-circuited the clutch drive if the pressure of refrigerant gas fell below a certain point (about 100psi). In either case, the compressor will not run again until the gas has been re-charged. Accordingly, the compressors are quite reliable internally, but suffer from leaking seals on the drive shaft, the pressure switch and the input and output ports. Occasionally the backplate O ring seal fails.

The Sanden compressors suffer more from complete seizure which causes an immense load on the drive belt which usually breaks. Unfortunately the Sanden compressors are significantly more expensive to replace.

The compressors need special oil, which must be compatible with the refrigerant – mineral oil for R12 and compatible systems, PAG oil for R134a systems. Ester oil can be used in either but is a lot more expensive. The compressor has a sump for oil, but a fair amount is carried around the system. Accordingly, when there is a leak, the area usually looks wet with oil.

Condenser Radiator

This is a large ‘radiator’ mounted in front of the engine coolant radiator, which cools the hot refrigerant gas enough to cause it to condense to a liquid. Due to its position it tends to be most damaged by stones thrown up from the road and by corrosion. It is made from aluminium and can suffer just as much salt-caused corrosion as a brass or steel radiator. If it becomes clogged externally by dust and hair (where does this come from?) it not only causes problems for the air con system but also reduces the cooling capacity of the main engine radiator. The extra heat load created by this air con radiator makes it necessary for a secondary (electric) cooling fan to be included. This is brought into action by a thermostatic switch in the engine coolant radiator or coolant circuit. It’s important that this electric fan works – it is often ignored.

Expansion Valve

In a sense this is the part that does all the work. It is a feature of liquids that when they expand into gases they must absorb heat – and so make their surroundings cooler. It is at the orifice created by the expansion valve that this conversion from liquid to gas takes place and so the cooling effect is created. The orifice is quite small and can easily be clogged, so it has a fine mesh filter in its inlet bore. This can easily become clogged – usually because the filter-dryer has started to beak up. On the Jaguar this expansion valve has a variable orifice whose aperture is controlled by the pressure and temperature of the gas exiting the evaporator. It works to make the cooling effect automatic or closed loop. On simpler cars the ‘orifice tube’ is of fixed aperture which is simple but makes for poor control of cooling. On the V12 this valve is very hard to replace as it is located at the bulkhead just behind the engine with very little room for tools and hands!


This is a cylinder about a foot long located above the coolant radiator panel, and on early cars has a glass sight gauge on the right hand end. It contains a dessicant to remove moisture from the refrigerant gas circuit that may have been introduced during servicing. It also has a fibre filter which traps debris shed by various working parts. It is a service item and should be replaced every time the system is re-gassed to ensure that residual moisture is removed. Any moisture in the system will reduce cooling capacity substantially and may freeze causing complete blockage.

Cooler Matrix (evaporator)

This is located within the airblend box and is thus very inaccessible. Fortunately it is very reliable and seldom requires replacement. Its function is to cool and dehumidify incoming air through being internally cooled by the refrigerant.

Common Faults

No cold air on warm day

As a general check of cooling, switch on engine, set a/c to maximum cool, manual (if available) and check temperature of two large pipes at the back of the compressor. The outlet pipe should be hot (careful!) and the inlet should be quite cold, possibly frosty. If both pipes are just warm then the compressor is not working or the refrigerant is not circulating.

1. Refrigerant gas has leaked out

This is the most common fault. Gas leaks out of all systems, old and new, from the various seals, hoses, joints and radiator matrixes. Most systems will lose at least half their gas over around 5 years even with no specific fault. Solution: Pressure test system with Nitrogen listening and looking for leaks. Established leaks will be evidenced by oiliness in the area, as the leaking gas carries some of the compressor oil with it. Commonest leaks are at the compressor front seal, the condenser radiator and compressor rear seals. Repair leak and re-gas.

2. Cooling circuit has blockage

The commonest blockage is at the expansion valve filter. Usual cause is internal break-up of the receiver-dryer with the particles circulating round and getting trapped in this filter. The filter can be removed (with care not to break the fragile evaporator matrix) and cleaned.

3. Compressor not running

Sometimes due to a blown fuse or faulty clutch relay but more likely due to loss of gas pressure which activates a protective switch or thermal fuse. Occasionally the frost thermostat can fail on early cars (pre-1987).

Air cool but not cold

1. Too much heat in airbox

If the coolant system is working and the compressor input pipe is very cold then the fault is likely to be that the hot water valve to the heater is stuck open. This is a common fault and can also stick shut so that there is little heating in winter. With the a/c on full cold, manual, the vacuum operated valve should contract, pulling the valve lever upwards which shuts off hot water into the airbox. This can be tested by pulling off the vacuum pipe to the valve and a slight hiss should be heard from the pipe and it should stick to your finger. When it is re-applied to the valve, the operating lever should pull up. If not then replace the valve.

2. One or both blend flaps/valves stuck

This is quite common on later cars (1987-on) with the Delanaire Mk 3 system. The two valves are cylinders running across the airbox, driven by electric motors. The motors can seize, leaving the blend valves in the wrong position – perhaps diverting hot air to the dash vents when cold is demanded. Sometimes a sharp tap with a screwdriver handle can shake the motors free and normal operation is resumed.

One test is that with the engine fully warm and the blower motors silenced by removing their fuses, one should just hear the valve motors running as one changes the temperature control from fully hot to fully cold. One motor normally runs for longer than the other so listen for both operating. On earler cars the system is vacuum operated and it is common for there to be various vacuum faults – dislodged pipes, blocked valves etc

3. Insufficient refrigeration

See ‘No cold on warm day’ above. But this can also be due to low refrigerant level, faulty expansion valve, clogged condenser radiator or a poor quality re-gas operation which has left non-condensables in the system. Also conversion to R134a gas is quite a lot less efficient than R12 or equivalent gases. See our mythbuster article.

Insufficient air, especially from centre vent

While this is a common fault, note that on pre-93 cars the centre vent only opens when the cabin temperature is much higher than the temperature set on the control knob. When the system has stabilised the centre vent will be shut

1. Stuck vent flap

This vent is always operated by a vacuum ‘motor’, so the first culprit is likely to be lack of vacuum. Check that the vacuum supply to the system from the inlet manifold (LH bank on V12) is intact. If it is, then the fault will be either a faulty vacuum motor (impossible to access without removing dash) or stuck flap – sometimes due to lack of use. Carefully removing the wooden facia will allow long fingers to feel in and up to the flap. Some pressure should cause the flap to move. Again, this is impossible to access without removing dash and airbox.

2. One or both blower motors not working.

A simple test is to set the system to full screen defrost which overides everything and both blowers should run at full speed. You should be able to hear both motors running (situated just over driver’s and passenger’s ankles). If only one appears to be running then airflow is severely restricted. Another common fault is that one motor will run only when High speed or Demist is set. This is due to the fan’s power controller circuit (mounted on top of the fan casing) having failed. (See Air Blower description above and picture).

Hot feet and cold face or vice versa

1. Stuck blend flap(s). See above.

Interior mists up after a few miles and air is stuffy

1. No dehumidification

Cooling system has failed – see causes above

2. A/C system is not running

Switched off(!) or fuse blown, or operating switch failure

3. Recirculation flaps stuck open

Unusual for both to be stuck, but the driver’s flap can get impeded by brake switch cable harness being dislodged by (large) feet! Sometimes the linkage can become detached so that the flap does not shut.

Cold draught on feet

1. Recirculation flap stuck open (see above)

2. Fresh air vent leaking (on Series 1/2/3 XJ6/12 only)

Cold water drops on passenger's feet on right hand corners

1. Condensate drains blocked

Whenever the cooling system is running, water is condensed out of the atmosphere and collects in the bottom of the air box where it drains away through two vent pipes that pass through the transmission tunnel and onto the road (hence the puddles under air con cars). The pipes can become blocked but it is much more common for the air box outlets to clog up with damp debris. While one can sometimes dislodge this with a probe from under the car, this fix will not last long. The best solution is to access the airbox by removing the (blue) air inlet pipes from the blowers, dry out the interior with an airline or hairdryer and then loosen the debris with a finger or screwdriver. Then remove debris with vacuum cleaner.

2. Water collecting in dash

While not an a/c problem it is often confused with one. On facelift cars in particular, rust holes develop in the scuttle under the lower windscreen trim. Rain gets in and collects in the various cavities under the dash. Eventually it brims over onto your feet. The only solution is to remove the screen and fix the holes.

Aircon controls don't work and cabin temperature never stabilises

1. A/C amplifier has failed.

This is very common on cars with the Delanaire Mk 2 system (ca 1975 to 1986) identifiable by the left hand control knob not having a pull out, manual operation. The a/c amp controls the airblend system by comparing the cabin air temperature with that set on the left hand knob. The relays inside the amplifier become unreliable with age. The amplifier is potted and cannot be repaired. A modern version was made for a short time by an Australian firm but are no longer available at this time (2012). If we get a lot of interest we might make some ourselves!

2. Blend flaps inoperative – see above

3. Cabin air temp sensor failed or blocked.

On pre-facelift XJS and Series XJ saloons the sensor is behind a hole in the front of the dash on the passenger side (RHD cars). On facelift XJS cars the sensor has a small fan and is located on the underscuttle panel over the passenger’s knees. This can become disconnected after careless servicing in the area.

4. A/C relays failed

Only on Delanaire Mk 2 systems. The relay block which controls blower speeds and some other functions can fail, or the connections work loose or are corroded.

Fault Finding

System check - Delanaire Mark 3

1. First test
Get engine warm
Turn left knob to minimum temp, right knob to Demist. After about 10 seconds hot air should be directed to the screen vents and dash outer vents. Knee vents should be shut.
Turn right knob to medium (9 o’clock position). Airflow to screen should diminish to a trickle, knee vents should open and the air should get cold.
If it is a hot day the centre dash vent should open and yield cold air. If it is a cold day, pull out left knob to Manual (still full cold) and centre vent should open – it takes about 30 seconds

If the air direction does not change from screen to knee, and/or temperature does not change, then one or both blend flap motors is stuck

2. Diagnose and fix stuck blend valves
Remove both fan fuses. Each fan motor has its own 25amp fuse, located in the fuse box on the panel in front of each fan motor. This is to make the system quiet enough to hear the flap valve motors which are small and quiet.
Remove or unhinge both under-scuttle access panels and the diamond-shaped knee vent surrounds
With a warm but not running engine, place head as close as possible to right hand side of airblend box. With ignition switch on position 2, turn right hand a/c knob to demist, and left hand to full cold. Wait for 20 seconds, then turn right hand knob to middle position. You should then hear a faint whirring sound from the blend flap electric motors. They may run for about 10 seconds. When one reaches the end of its travel it will go silent and you should be able to hear the other one still moving. Turn the right hand knob to demist again, and listen for both motors. Repeat this until you are sure of the fault – one or both motors not running.

It is possible to encourage a stuck flap motor to come back to life by striking it sharply with a screwdriver. The lower one is fairly easy to see, but the upper one is difficult. On a right hand drive car one can remove the instrument binnacle (glove box on LHD car) and see and strike the upper motor easily. Listen again for both motors to work.

If striking the motor(s) does not work you will need to remove the motor, dismantle it and clean the (probably) dirty commutator. The motor can be bench tested with a 5volt supply. It should run in both directions.

3. Diagnose and fix vacuum leak

Delanaire Mk3: All the air flow control flaps outside the blend box are operated by vacuum (rather than electric motors). These are, with their colour-coded vacuum pipes: Screen demist (green), dash centre flap (black), recirculation flaps (blue) and heater water valve (red). The vacuum to each actuator is controlled by electric vacuum valves which are in turn operated directly by the a/c ECU. If, for example, the screen demist air is flowing all the time regardless of controls, it means the vacuum to that actuator is missing. This can either be because the individual pipe has become disconnected from its electric valve (commonly occurs when a radio has been carelessly fitted), or because the entire vacuum supply is poor or leaking. This in turn will be due to it being disconnected from the rear of the right hand inlet manifold, a break in the rubber hose leading from the engine to the bulkhead hole, or a blocked non-return valve.

To diagnose loss or vacuum, having inspected the various pipe unions in the engine bay, remove the left hand knee vent, identify the white (vacuum supply) pipe and detach it. With the engine running you should hear or feel a vacuum on the end. Indeed, any hissing sounds near or in the air box with engine running indicates a leak.

Delanaire Mk2 (pre 1986) – similar in principle to Mk3 above, but there are more actuators, and the vacuum lines are controlled by mechanical vacuum valves operated directly by cams on the servo motor assembly. These valves can become clogged with dirt, and the lines can develop leaks as above.

12:08 pm KWE Cars