Jaguar XJ40 Air Conditioning Problems
On the XJ40 warm air is usually a slow leak, and the oil goes out with the gas. I'd read the sight glass and look at the back of the compressor first, then the condenser and the air through it. Cars to about 1992 left the factory on R12 and 1993–94 cars on R134a — that decides the charge and the fittings.
How bad is it · Safe to drive
Safe to drive — select ECON if the compressor knocks
A warm A/C doesn't affect how the car drives. A noisy or leaking compressor should be switched off with ECON until it's fixed, so it doesn't run dry and seize.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
Low on gas from a slow leak
These systems seep over the years, and the gas runs out a little at a time. When the charge falls the expansion valve starts to hiss and the cooling fades, first in traffic and then altogether.
A breathing or hissing sound behind the dash, slow cool-down, then warm air. A steady stream of bubbles in the receiver-drier sight glass with the blower high and the setting coldest. The compressor clicking in for half a minute and dropping out. Mostly warm, but cold again after half an hour or more on the motorway, is typical of a nearly empty system.
Look at the sight glass with the engine at about 1500 rpm. Look for oily residue at the joints and at the service-port valves; a UV dye shows small leaks. A 12 oz can is not a pound. If gas escapes every time the charging hose goes on, suspect the kit's quick coupler before the car's port.
Find and fix the leak, then charge to the figure on the label above the compressor — not just another can. The service ports on the 1993 hoses unscrew whole; they aren't sold separately by Jaguar, but Aeroquip 242-1336 and 242-1337 screw straight in.
A leak at the back of the compressor or its front seal
The compressor carries the two-wire pressure switch, a block of O-ring fittings held by a steel clamp plate, and a shaft seal behind the pulley. Each of them leaks with age, and loose housing bolts do too.
Cold for a day or three after a recharge; oil or dye dripping under the compressor; drips coming from around the pulley.
Degrease the compressor, run it, and look for the wet spot. Check the switch nut, the clamp plate for a bend, the housing bolts and the area behind the clutch.
Tighten the switch or housing bolts, fit new green O-rings and a thicker clamp plate, or renew the shaft seal — it needs circlip pliers and the seal kit. After any work on the hose fitting, check the rings went back in the right order.
No air through the condenser at idle
The condenser sits in front of the radiator and needs a strong draught. Insects and dirt between the two, a dead auxiliary fan, a lazy fan clutch or missing shroud flaps all leave the pressure high and the air warm.
Cold on the move, warm in traffic or at idle; the high side reading high.
Look between condenser and radiator with a torch. Check the auxiliary fan runs with the A/C on and the shroud flaps are there and soft. Spray cool water over the condenser and watch the vent temperature drop.
Clean the fins with coil cleaner and a gentle hose, fix the fan or its relay, and fit new rubber shroud flaps.
The blend flaps stuck at full heat, or misled in Auto
Two electric motors on the right of the climate unit turn the upper and lower blend flaps, and feedback pots report where they are. The flaps stick after a lay-up, a motor gear loses a tooth, or a demist flap breaks off and drops onto the upper door. Stuck at full heat, the cold air goes straight through the heater matrix. In Auto the flaps follow the in-car sensor, so an unplugged sensor or a clogged sensor fan sends them the wrong way.
The clutch is in and the pipes are cold, yet the face vents blow warm; hot feet with cold face vents, or the reverse; a rasp or rattle behind the dash; Manual works but Auto doesn't.
In Manual, run the 12-point vent test: Full Cool with the fan on DEFROST, High after about 45 seconds, then Full Heat. Hot air where no heat is asked for means a stuck flap. With the centre vent out and a strong torch you can see a fallen demist flap. Check the sensor fan behind the round grille under the passenger knee panel spins and is plugged in.
Tap the flap motor with a wooden rod while turning the knob from hot to cold, or cycle every control on a car that has stood. Clean the feedback pots and match them to the flap's end stop; swap a motor with a broken gear. A fallen flap can be fished out through the centre vent; refixing it for good needs the dash out.
A leaking condenser
The aluminium core corrodes through, especially in wet, salted climates, and the welded pipe joints crack. A condenser that rubs on its mounting gets a pinhole.
Oil or dye at the front of the car; a stain near the top of the condenser at the high-pressure pipe.
UV dye, or soapy water on the joints with the system charged.
A new condenser and drier; a specialist aluminium welder can repair a cracked joint, out of the car. Epoxy is only a stopgap. Some replacement condensers carry a different fitting from the original's large port, so check the unions before ordering.
The compressor itself: worn, noisy or seized
Each top-up without a repair takes oil out with the gas, and the oil level can't be checked with the compressor in the car. Valves break, bearings wear, and eventually it seizes and takes the clutch with it.
Clacking or knocking that stops with the A/C off; equal pressures on both sides; the clutch slipping or smoking. A low side of about 80 psi at idle that falls only when revved, or a high side barely above the low side, means the compressor is leaking internally. A scraping that eases when the clutch engages is the pulley bearing rather than the pump.
With the engine off, turn the clutch centre by hand — a seized unit won't move. Before ordering, check the engine number: early cars used a different compressor. The 3.6 and the 4.0 take compressors of different length — a 4.0 unit won't fit the 3.6.
A rebuilt or new Sanden unit with a new drier and a flushed system. Rebuilt units often come without the Jaguar pressure switch, so move it over. Drain and measure the oil before fitting. After an R134a conversion an unflushed system can kill the compressor, and an old high-pressure hose can burst.
The pressure switch or its wire stops the clutch
The switch on the back of the compressor completes the clutch relay's earth only when there is enough pressure. On 1993–94 cars a triple switch on a small hose near the bulkhead does the job. US 1991 cars carry a one-shot superheat switch and fuse on the back of the compressor instead, which must be replaced once it opens. The switch can blow out of the compressor altogether and empty the system — on a 1989 R12 car the full charge is 40 oz with 7 oz of oil.
The clutch never pulls in, but the system still holds pressure; or a broken or loose wire at the switch. A pop under the bonnet and the charge gone at once can be the switch itself letting go.
Look at the wires on the switch. Bridge the switch, or the relay, only for a moment with a fused lead: if the compressor runs, the switch or its wiring is the fault.
Reconnect the wire or fit the correct switch — Sanden switches come in coarse and fine pitch, so a cheap one may not fit.
The clutch relay, its fuse or the clutch coil
The relay feeds the clutch. On 1993–94 cars it sits among the cube relays on the engine-bay bulkhead, in the green base; its fuse in the right-hand fuse box is shared — with the cooling fan up to 1991 (and the bonnet lamps on 1988–89 cars), with the seat lumbar pumps from 1992.
No click from the relay with the A/C selected, and no power at the clutch connector. A fuse that blows the moment the A/C goes on points at a shorted clutch coil.
Check ECON is off. Feel the relay click; test the fuse with a meter, not by eye. Feed 12 V straight to the clutch: if it pulls in, the fault is upstream. A fuse that blows at once: unplug the clutch and measure the coil — a little over 3 Ω is right; about 0.3 Ω is a shorted coil.
A new relay; the auxiliary fan relay is a different type, so don't borrow it.
The expansion valve or the receiver-drier
Debris or frozen moisture blocks the expansion valve, and high pressure can blow the relief valve on the drier, which then doesn't reseat.
Frost or ice on a pipe joint; a very high high-side reading; a puff of gas and oil under the bonnet. A relief valve that blows within a minute of forcing the clutch in after a top-up means the pressure is far too high — stop and recover the charge.
Proper manifold gauges on both sides; look for frost at the valve and the drier.
A new drier and expansion valve, O-rings, a long vacuum and a charge by weight. On an R134a conversion an R134a valve cools better than the old R12 one. The lower union at the valve is a 7/8 in UNC nut with little room: lift out the battery and tray and use a crowfoot.
The heater valve letting hot water through
The A/C cools the air and the heater matrix warms it again if hot coolant still flows. Many 1990–92 cars have no shut-off valve at all, and where there is one, it is held shut by vacuum and fails open.
Warm air whatever the setting; cold at first and warmer as the engine heats; a hot heater hose with the temperature at full cold.
Feel the hoses at the bulkhead. Check vacuum at the valve; a pipe off behind the kick panel stops it working.
Reconnect the vacuum pipe or fit a new valve; a one-way check valve in its vacuum hose helps it stay shut. On cars without one, a manual quarter-turn valve or an aftermarket vacuum valve in the heater hose works well. A heater-hose valve with smaller spigots can be fitted using a short length of hose from a valve kit.
A leaking evaporator on 1993–94 cars
The 1993–94 evaporators have a poor record, some replaced more than once. The core lives inside the dash.
Dye or oil at the evaporator drain; a short hiss from the dash every few minutes; gas lost with nothing visible under the bonnet.
Look under the left-hand lower dash for dye. Prove the leak before stripping: cap one evaporator pipe and pull a vacuum on the other.
A new core (JLM 20608, or an aftermarket one that may need bending), a new drier and fresh foam. Disarm the airbag first.
Air leaking round the evaporator or inside the dash
The foam seals inside the climate unit perish, so air bypasses the core or leaks between chambers; grime on the core face blocks the rest. The plenum joint to the dash can also come unglued.
Pressures look right and the pipes are cold, yet the vents never get really cold; both blowers roar but little reaches the centre vents.
With the core out, look at the foam and both faces of the core.
Clean the core, reseal it with rigid foam and re-glue the plenum joint.
Weak airflow: vacuum to the vents and doors, or a slow blower
The centre vent flap and the recirculation doors run on engine vacuum through small solenoids. A split hose, a cracked plastic T or a dead solenoid leaves the centre vents shut or the doors on fresh air, so the system cools hot outside air. A tired blower runs at half speed.
Cold suction pipe but only lukewarm vents; air going to the screen instead of the face; vents that switch over when you accelerate, as vacuum drops; little air from one side. A centre vent that flaps open and shut in rhythm is the vacuum module cup with its moulded hooks broken.
Listen to each blower on full. Check vacuum at the reservoir, the check valve and both sides of each solenoid; the T behind the radio housing and the hose by the check valve are known breaks. Some cars carry an unused spare solenoid next to the recirculation one. Look in the fresh-air intake: a rodent nest stops the flaps moving and starves the airflow. A corroded blower relay plug slows one blower.
Renew the split hose or T, move the wiring and hoses to the spare solenoid, free the doors, overhaul the blower. A broken centre-vent actuator can come out and the flaps be propped open for summer. A broken vacuum cup can be sealed to its diaphragm with silicone and the spring left out; a cracked actuator body can be bonded, but find out why no vacuum reaches it.
A knock with the A/C on: belts before the compressor
The compressor adds load to the drive. Belts that have settled after fitting, a slipping crank damper or a seized fan clutch make noises that sound like a dying compressor.
Knocking or squeal with the A/C on, quiet with it off, and the air still cold.
Belt tension; a chalk line across the crank damper's ring and hub.
Re-tension the belts; a new damper or fan clutch if one has gone.
The wrong charge after an R134a conversion
R134a runs at higher pressure and needs less than the R12 figure. Too much gas raises the suction pressure and cuts the cooling; too much oil does the same.
Pressures that look high, poor cooling straight after a conversion, the vents never below the low 60s °F.
About 35 psi on the low side is right for R134a. A gauge shows pressure, not quantity; only weighing the charge in is certain.
Worth trying: recover and charge by weight to about 80–90 per cent of the R12 figure. For example, about 28 oz where the R12 label says 40 oz.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Is the compressor turning?
ECON off, A/C on, engine running: the centre of the clutch should spin with the pulley.
freeRead the sight glass and listen
Steady bubbles or a hiss behind the dash mean low charge. Cars to about 1992 left the factory on R12; 1993–94 cars on R134a.
freeClear glass but warm air? Test the flaps
In Manual, run the 12-point vent test; hot air where no heat is asked for means a stuck blend flap. Tap its motor on the right of the climate unit while turning the knob. In Auto, make sure the in-car sensor is plugged in and its fan spins.
freeLook for the leak
Oily residue at the back of the compressor, behind the pulley, along the condenser; on 1993–94 cars, dye at the evaporator drain.
free, or a UV dye kitClutch not pulling in? Switch, relay, fuse
Wires at the pressure switch, the relay click, the fuse with a meter, then 12 V straight to the clutch for a moment.
freeCondenser and airflow
Clean between condenser and radiator, check the auxiliary fan and the shroud flaps.
freeHeater valve, vacuum and recirculation doors
Heater hoses cold with the setting at full cold; centre vents open and the doors on recirculation with the A/C on; vacuum at the reservoir and the solenoids.
freeRepair, then charge by weight
New drier every time the system is opened, a long vacuum, the charge weighed in to the label figure.
a drier and a rechargeOnly then the compressor or the evaporator
Check the engine number before ordering a compressor; prove an evaporator leak before taking the dash out.
a compressor or a coreThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
Circuit descriptions from the Jaguar fuse tables, 1993 edition. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Blower Motor Not WorkingIf little or no air comes out at all.
- Jaguar XJ40 FUSE 1 or FUSE 2 FAIL WarningIf the A/C drops out together with the radio and blower, or a fuse blows when it's switched on.
- Jaguar XJ40 Battery Drain, Flat After a Few DaysIf the compressor clutch stays engaged with the A/C off and the battery goes flat.
- XJ6 (XJ40) fuse boxesWhere the right-hand fuse box sits — other years from the same page.
- Jaguar XJ40 Heater Not WorkingIf the heater gives no heat at all — a different fault from warm A/C.
- Jaguar XJ40 Overheating, Radiator and Cooling FansIf the engine runs hot in traffic as well as the A/C going warm.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.