Jaguar XJS Air Conditioning Problems
On the XJS the usual story is a slow leak: once the charge is low, the pressure switch stops the compressor clutch engaging, and it looks like a dead compressor. I'd start with gauges and a check that the clutch turns, then follow 12 V to the clutch — fuses and relay are cheap. Cars to about 1987 also have a thermal limiter fuse at the compressor that melts when the gas runs low.
How bad is it · Safe to drive
Safe to drive — just leave the A/C off
Nothing here stops the car. Don't keep the compressor running by jumping it with no gas — the refrigerant carries its oil, and it will 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 refrigerant — the pressure switch keeps the clutch off
These systems lose gas slowly with age. Once the pressure drops, a switch at the compressor stops the clutch pulling in, so a low system looks exactly like a dead compressor. From 1988 it's the high-side low-pressure (HSLP) switch in the clutch earth, closing at about 25–30 psig; R134a cars from mid-1993 have a dual pressure cutout that opens below 29 psig.
The centre plate of the compressor pulley stands still with the A/C on; or the clutch clicks in and out and the air is only cool. With the charge nearly gone the floor vents can give outside air and the centre vent warm air at the same time.
Gauges on, engine off: a healthy system sits at about 70 psig. Far below that, it's the charge. Briefly bridging the pressure switch makes the clutch engage — that confirms it. Read the gauges with air across the condenser: on a hot day with poor airflow the high side reads falsely high and hides an undercharge.
Find and fix the leak, fit a new drier, pull a vacuum and charge by weight. A top-up alone works for a season at best. These systems are touchy about charge: a quarter of a pound short cools badly while the gauges look nearly right, and the stated quantity can fall short — for example, a 1995 4.0 needed about 6 oz over 1.8 lb. A completely empty system that still holds a vacuum has a leak too slow to find at once — charge it with dye and look again after a few weeks. On an empty 1994 4.0 the switch wouldn't let the clutch draw the gas in; feeding the clutch 12 V directly just while the charge went in, then reconnecting, got it working.
Fuses, relay or wiring to the clutch
The clutch feed runs through an in-line glass fuse on a brown wire and a clutch relay on the right-hand wing. A fuse can look intact and still be open, and the relay contacts wear. On some 1987–88 cars the clutch feed shares a fuse with the second cooling fan, so a fan fault — or a bigger fan fitted — kills the A/C too. Early cars (about 1983) have no clutch relay at all: the feed runs from the Ranco thermostat through the console contacts, and the voltage that reaches the clutch drops.
Full pressure on the gauges, yet no 12 V at the compressor plug with the A/C on. On an early car, a clutch and blowers that stop after about ten seconds with the engine cold or off is the coolant-temperature lockout, not a fault.
With key on and blower on, follow the voltage: battery feed into the in-line fuse, relay pin 30, out on pin 87 to the clutch. Pin 85 should get the control signal. Check the compressor plug and the relay socket for a loose wire. On early cars there is also an in-line fuse for the Ranco thermostat behind the panel under the glovebox — test it with a meter.
Replace the fuse or relay — on a 1989 car the clutch relay is the forwardmost one on the right wing panel. Clean and reseat loose relay and compressor connectors. If the fuse is shared with the cooling fan, find out why the fan blew it before fitting a new one. On an early car without a clutch relay, fitting one fed from its own fuse brought full battery voltage to the clutch.
Compressor: leaking seals, or a worn-out pump
The two hose connections on the back of the compressor seal with O-rings that harden with age. The front shaft seal leaks after long storage, and on Sanden units it should be renewed when converting to R134a. The HSLP switch underneath the compressor has its own O-ring and can empty the system in minutes. A compressor that seizes, or runs empty after a hose bursts, wears out and sends debris round the system.
The system cools after a recharge and is warm again within days or weeks. Oily dirt on the back or front of the compressor. A tired compressor takes minutes to build pressure and needs revs to cool. On the Sanden of the 4.0, dye streaks spreading round the pulley mean the shaft seal.
Add UV dye and look with a black lamp at night; a leak at the rear plate glows at once.
New rear O-rings, then evacuate and recharge. A leaking shaft seal usually means a reman compressor, a new drier and expansion valve. After a failed compressor, flush the condenser, evaporator and hoses and fit a new drier and expansion valve with the new unit; a suction-side filter catches what flushing leaves behind. On the A6, the rear O-rings can leak again because the thin single clamp bar flexes: thicker O-rings, a thicker bar and a slightly longer bolt hold them.
Schrader valves, hoses and joints
The service valves leak at the core or the base, hose crimps let go, and an O-ring pinched on the last drier change bleeds the gas out slowly.
A slow loss over months; dye at a valve cap, a crimp or a flare nut. A flare nut at the drier that turns a quarter-turn before it bites — UV dye on that union and the shroud below it gave it away.
Dye and a UV lamp on every joint; a hiss when a valve cap comes off is a leaking core. Dye at the base of the low-side valve, where the hose joins the muffler can, means a new hose rather than a valve core.
Replace the leaking valve or port, have the hose recrimped on the old ends, renew the O-rings lubricated with refrigerant oil. Charge by weight — for example 1.18 kg of R134a on a 1991 V12. On R134a cars the high-side valve is not a core: the whole port unscrews, with eight flats of 16–17 mm — for example MT0105-1.
A leaking condenser or evaporator
The condenser sits in front of the radiator and corrodes or gets holed. The evaporator is behind the dash, and its outlet fitting cracks or creases if the hose nut is forced.
Dye on the condenser face; or no dye anywhere under the bonnet while the charge keeps disappearing.
UV lamp on the condenser and at the evaporator connections at the bulkhead. If nothing glows outside, suspect the evaporator.
Replace the condenser with drier and expansion valve. An evaporator means the dash comes out — around 20 hours of work — so a cracked outlet fitting is sometimes sealed in place instead. Undo the drier-to-condenser nut gently — the aluminium pipe flare breaks off easily, and then the condenser has to come out. Early cars with the serpentine condenser can take the later parallel-flow one with new lower brackets and an adapted fitting; it cools better.
Cars to about 1987: the thermal limiter fuse at the compressor
Early cars have a superheat switch and a three-terminal thermal fuse at the compressor. When the charge runs low, the switch heats the fuse until it melts, cutting the clutch for good. 1988 and later cars have no thermal fuse; the HSLP switch does that job.
No power at the clutch on an early car, with a black three-spade plug at the compressor.
Jumper the three spades together to test: if the clutch engages, the fuse is gone. A melted fuse also means the charge is low.
Fit a new thermal limiter and fix the leak. Don't leave it bypassed: a compressor run without refrigerant runs without oil. The GM limiter is 6551258; a resettable 70 °C thermal cutout in series with the clutch is another way to keep the protection.
Overcharged, or not enough air through the condenser
Too much gas or too little airflow pushes the high side up. The air goes warm at a standstill and comes back at speed. On the 4.0 the auxiliary fan runs only on coolant temperature, not with the A/C.
Cold on the move, warm in traffic; the clutch cutting in and out after a top-up. A compressor whine after a top-up comes before the worst of it: an overfilled converted car split its suction hose next to the metal fitting.
Gauges against an R134a chart for the day's temperature. Look for a missing shroud flap and the gap between condenser and radiator.
Vent the excess slowly from the low side; refit the shroud flap and seal the condenser gap; a switch to run the auxiliary fan with the A/C.
A blockage: expansion valve, drier or a flattened pipe
Debris or drier pellets plug the expansion valve; an old drier or a pipe kinked at the last repair chokes the flow. The compressor pumps, but the gas can't get round.
Low side near zero or in vacuum while the high side stays low; a drier cold to the touch.
Gauges with the compressor running. A low side pulling vacuum points at the expansion valve or drier.
Blow out or replace the expansion valve, renew the drier, straighten or replace the flattened tube; flush if a compressor has failed.
Cold air reheated: heater valve, flaps or the in-car sensor
On the Delanair system all air goes through the evaporator and is then reheated as needed. A heater valve that doesn't close, or flap foam that has crumbled, mixes hot air back in. Old foam over the evaporator face cuts airflow. In Auto the system reads the in-car sensor: if it's unplugged or its small fan stops, it thinks the cabin is cold and calls for heat.
Gauges and pipes look right — the suction pipe is cold — yet the vents are only cool; weak flow from the centre vent. Cold in manual, but hot or swinging between warm and cool in Auto.
Pinch the heater hose and drive: if it turns cold, it's the heater valve or the blend flaps. Check the vacuum line to the heater valve. Switch to manual (on earlier cars, pull the temperature knob out): if the cold comes back, check the in-car sensor above the glovebox — its plug, air tube and fan.
Tighten or replace the heater-valve vacuum line, renew the flap foam, clean the evaporator face; some bypass the heater matrix in hot climates. A heater valve whose diaphragm won't hold vacuum gets a new valve — a generic plastic one, about $10, fits early and facelift cars; a valve stuck with dirt can be cleaned and lubricated. Reconnect the in-car sensor and clean its fan.
The compressor clutch: shorted coil or slipping plate
A partly shorted coil draws twice the current and blows the fuse it shares with other circuits. A slipping plate — a broken rivet on a drive strap, or a pressure switch with a bad earth cycling the clutch fast — gets hot and can burn the rubber damper.
A fuse that blows the moment the clutch relay energises, but holds with the compressor unplugged. Or a shake through the cabin each time the compressor cuts in. On an old A6 the plate can tear off its rubber at high revs — a clunk when it drops out, burning rubber and smoke when it engages.
Unplug the clutch and measure the coil: about 3–4 ohm on a Sanden. For example, a reading of 1.5 ohm meant a failed coil. Paint marks across plate and pulley under a timing light show slip. Set the clutch gap to about 0.4–0.8 mm.
Replace the coil — on a Sanden it can be done without opening the refrigerant circuit; keep the flyback diode. A slipping plate is a clutch-disc swap with a loan tool.
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.
A/C on, engine running: does the clutch turn?
The centre plate of the pulley should spin with it and the suction pipe go cold. Check the A/C really is switched on.
freeGauges on the service ports
Engine off, about 70 psig static is healthy. Low pressure means a leak — the switch is doing its job.
a gauge setFollow 12 V to the clutch
In-line glass fuse on the brown wire, the clutch relay, the compressor plug; on cars to about 1987, the thermal limiter too. 12 V straight to the clutch is a test only — never run it empty.
a multimeterDye and a UV lamp
Start at the back of the compressor, then the Schrader valves, hoses, condenser and the bulkhead connections.
a can with dyeRepair, new drier, vacuum, charge by weight
Vacuum held overnight before charging. Skip sealer cans.
parts plus a shop chargeCold at speed, warm in traffic
Check the charge isn't too high, the condenser gap and shroud flap, the auxiliary fan; then pinch the heater hose to rule out reheat. Warm in Auto only: switch to manual to rule out the in-car sensor.
freeThe 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, 1995 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
- XJS fuse boxes and relaysWhere the A/C fuses and relays sit, by year.
- Jaguar XJS Won't Crank, Starter RelayThe same relay checks — pins 30, 85, 86, 87 — if the starter relay beside it is the problem.
- Jaguar XJS Alternator Not Charging, Battery LightIf the battery light or a low voltmeter is the real complaint.
- Jaguar XJS Heater Not WorkingIf the trouble is the opposite — cold air with the heat turned up.
- Jaguar XJS Blower Motor Not WorkingIf the air is cold but there's almost no flow — one blower may be dead.
- Jaguar XJS Climate Control ProblemsIf it's cold in Manual but blows hot in Auto, or swings between hot and cold.
- Jaguar XJS Overheating, Cooling Fan ProblemsIf the engine runs hot with the A/C on, or the aux fan never comes on in traffic.
- 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.