Jaguar XJS Climate Control Problems
On the XJS the climate control usually goes wrong at the flaps rather than in the electronics: a blend-flap servo stuck on hot, or lost vacuum that leaves the doors in their rest position. If Manual works and Auto doesn't, I'd look at the in-car sensor and its hose or fan first. Clean the connectors before buying a module or an amplifier.
How bad is it · Safe to drive
Safe to drive — as long as the screen still clears
It is a comfort fault. If the defrost won't come on or the screen mists up, sort it before driving in the wet or cold.
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.
A blend-flap servo, its feedback pot or its linkage
The control unit sets temperature by driving the blend flaps with small electric servos. A servo with stripped plastic gears, a worn feedback pot or a loose linkage leaves the flaps parked wherever it stopped — often on hot. A dirty feedback track makes the system hunt between hot and cold instead. On the Mk III the upper and lower blend flaps each have their own servo and work independently. Servos tend to jam after sitting in one position all summer or all winter.
Constant hot air, or swings between hot and cold, whatever the knob says; the servo whirrs on and on or makes no sound at all. On a Mk III: cold air pouring from the footwells with the heat on while Defrost is hot means the lower flap; no heat and a centre vent open even in Defrost means the upper one.
In Auto, sweep the knob from 65 to 85 and listen near the right footwell for the servo. Unplug it and touch a 9 V battery to the motor for a moment: if it turns, look at the control side; if the motor runs but nothing moves, look at the gears and the pivot screw on the linkage. Select Defrost: footwell air should stop within 30–60 seconds; if it doesn't, the lower flap is stuck. The lower servo sits behind the climate ECU, the upper one on the passenger side behind the glove box and the ECU, held by one bolt behind a metal brace.
Tighten the linkage pivot, or fit a used servo. On a Mk III there is no setting-up: the ECU reads the flap position from the pot and drives it home. A jammed servo can often be freed without replacing it: loosen its mounting and rock the housing while the controls drive it, or tap the case with a long wooden dowel while cycling Cold, Hot and Defrost. A seized motor can be opened and cleaned. The upper Mk III servo is the hard and costly one to reach.
Lost vacuum: a hose off, the reservoir, a vacuum servo or solenoid
Vacuum works the heater valve, the centre vent door and the fresh-air and recirculation flaps on both blowers. One split hose, a blocked reservoir or a leaking vacuum servo and those doors sit in their rest position — fresh air in, valve open, centre vent shut.
Flaps that never change mode: everything to the screen or the floor, a centre vent that never opens, recirculation that never closes, or hot air with the system off. Vents that flip to the screen under hard acceleration point at the check valve or a leaking joint, as the reservoir can't hold vacuum when manifold vacuum drops. A centre vent stuck open blowing ice-cold air at full heat is a door that has stuck; a door left unused for months can free up once it's exercised.
With the engine running and the system OFF, the recirculation flaps should close the fresh-air intake on both blowers; if they don't, follow the blue vacuum tubing at the front of the tunnel. Then test the reservoir and the check valve on the right-hand manifold, and each solenoid for continuity. The climate vacuum comes from the third outlet at the rear of the right-hand manifold, through a check valve before the bulkhead; on the 6.0 V12 look at the large rubber adapter on the manifold stub, which cracks underneath where it can't be seen. A gummed check valve can sometimes be cleared by poking a blunt tool into the correct end. Behind the right console panel and the side vent duct is a hub with white, black and green tubes: reseat each one, and see that green and black haven't been swapped.
Refit the fallen line; replace a blocked reservoir, a vacuum servo that won't hold or an open-circuit solenoid — used ones do the job. A centre vent that gives nothing at all can be a broken tee in the main vacuum supply behind the radio: the white hard line is the supply, the black one goes to the centre vent. A plastic tee is easier to fit than brass; the centre vent assembly has to come out, working through the radio opening and the console side. With the leak gone the engine can idle smoother too. Where hard-plastic lines meet soft rubber the joints part whenever the radio or servo area is disturbed; renewing the lines in solid rubber hose, with a new check valve, cures repeated drop-outs.
Cold on one side only: a blue air duct come adrift under the dash
Air from each blower reaches the heater unit through blue flexible ducts and boots under the dash, held on with tape. One flap serves both footwells, so cold air on one side only means a duct has come off, letting unheated air through.
One footwell blows cool while the other side and the other vents are fine; or the lower vents go cool after a quarter of an hour while Defrost stays hot, with no change in fan or servo noise.
Look under the dash on the cold side for a blue boot or duct hanging loose at the blower or at the heater unit.
Refit the duct and tape it to the blower housing as the factory did, or secure it with a couple of pop rivets and washers. Taking out the left-hand blower makes the job easier on the bench.
Centre vent dead because the defrost flap won't close
On the Delanair system the defrost flap and the centre vent flap are linked mechanically. The centre flap can't open until the defrost flap has closed, so a stuck defrost flap, a failed defrost actuator or no vacuum to it keeps the centre vent shut. Two 12 V vacuum solenoids behind the radio on the right switch the centre vent and the defrost door. If the defrost solenoid sticks open, the defrost door stays put and blocks the centre vent.
Cold air only from the two small end vents and nothing from the centre of the dash.
Drop the under-dash panel on the passenger side; the defrost and centre vent vacuum actuators sit at the top centre of the dash. Put a vacuum pump on each and see that it holds and moves its flap, defrost first. Reach the solenoids by taking out the panel under the glove box and the centre kick panel. Check 12 V at each coil with the A/C on, vacuum in and vacuum out; an intermittent open coil gives alternating operation. Black tube opens the centre vent, green closes the defrost door. Look at the small restrictors in the lines for clogging too.
Worth trying first: select defrost for a few seconds, then go back to the previous setting, which can bring the linkage back into step. If an actuator won't hold vacuum or the defrost flap sticks, free the flap or replace the actuator. A failed solenoid is replaced, usually as the pair. A centre flap that has seized can be worked free by hand with the walnut trim and vents out.
Auto only: the in-car temperature sensor, its hose or its fan
In Auto the unit compares the knob with cabin air drawn over a sensor; Manual ignores it. On earlier cars a small hose pulls air past it from a blower duct; on later cars a little fan behind the passenger knee bolster does it. If the sensor is unplugged, the hose has slipped off or the fan has stopped, Auto reads the wrong cabin temperature — and blows hot, over-cools or cycles.
Manual mode behaves, Auto doesn't: hot air the moment Auto is selected, slow warm-cool cycling, or full cold after a long sunny run. A sun-heated dash can fool the sensor into blowing cold for a while on a hot afternoon.
Drop the passenger knee bolster and look at the sensor: plugged in, 12 V on its feed with the ignition on, fan turning and free of lint. On cars with the hose, follow it to its tee on the blower ductwork and make sure it is seated.
Reconnect and clean the sensor and its fan, and push the aspirator hose fully home.
Corroded or loose connectors and earths on the control unit
The climate module and its switches sit in the console behind cheek panels, with several multi-pin plugs and a fused earth lead. A plug that is half home or green with corrosion gives faults that come and go, and a lost earth kills the whole system.
Faults that change from day to day; one flap or one blower behaving oddly; the lot dead at once.
Take off the console cheek panels, unplug each connector on the module and the mode switch, clean and reseat them, and check the main earths and the fused earth lead under the controls. On a Mk III look especially at the six to eight tiny connectors, often green with corrosion.
Clean the pins with contact cleaner and reseat every plug; re-crimp and secure a loose earth.
Cars to about 1987 (Delanair Mk II): the amplifier
On the Mk II system a potted amplifier under the left side of the unit turns the knob and sensor signals into power for the single cam-driven servo. It can't be repaired. When it fails the servo stops, runs to one end, or wanders at random, so heat and cold arrive at the wrong times.
No servo noise when the knob is swept, heat in summer and cold in winter, or random repositioning.
Check the in-line fuse on the amplifier earth first. Then put 9 V across the red and purple servo wires: if the servo runs end to end, the amplifier is the likely fault. The workshop manual gives resistance checks at the amplifier harness socket.
Fit a used amplifier from a Series III saloon or an aftermarket plug-in unit; test the servo for a short first, because a shorted servo can ruin the replacement.
Selector microswitches and the switch pack
Behind the knobs sit stacks of microswitches worked by cams. A switch that no longer makes contact, or a switch pack whose screws have worked loose, can leave the fan speeds stuck, the system ignoring OFF, or everything dead.
A fan speed that comes and goes, the system running with the selector at OFF, or nothing working at all.
Remove the switch pack, gently tighten the two small screws that hold it together, move the microswitch stacks towards their cams so they operate early in the travel, and check each one with a meter.
Tighten and readjust the switch pack, or replace a tired microswitch.
Late cars (Delanair Mk III): the climate control module itself
The Mk III module sits behind the right-hand cheek panel of the console. It is delicate — a momentary short can damage it — and new ones are no longer made. A failed module gives faults no single flap or sensor explains, such as the blower pulsing with the switch off.
Several unrelated faults at once that survive clean plugs, good vacuum and a working sensor.
Rule out plugs, vacuum, servos and sensor first. Then swap in a known-good module of the right part number for the build.
Fit a used module or have the original rebuilt.
Not the controls at all: a low refrigerant charge
When the refrigeration side can't deliver, the unit keeps calling for cold it never gets and the servo and heater valve end up in odd places. It looks like a control fault but goes away once the charge is right.
Air never properly cold in any mode, with bubbles in the sight glass.
Put gauges on before blaming the controls.
Find the leak and recharge.
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.
Which system have you got
Pull the temperature knob: on the Mk III (late 1987 on) it clicks out about a quarter of an inch into Manual. Cars before that have the Mk II system with a separate amplifier. Test with the engine warm — the system holds off until the coolant warms up.
freeManual against Auto
Run the same setting in both. Fine in Manual, wrong in Auto points at the in-car sensor, its hose or its fan.
freeListen for the servo
In Auto, sweep the knob from 65 to 85 and back. A whirr near the right footwell means the servo is being told to move — then look at the gears and linkage. Silence points at the amplifier or module, or at their supply.
freeVacuum
Engine running, system OFF: the recirculation flaps should close the fresh air. If not, check the hoses at the front of the tunnel, the reservoir and the check valve on the right-hand manifold.
freePlugs, earths and switches
Behind the console cheek panels, unplug, clean and reseat every connector on the module and mode switch, and check the earths. On a Mk II, check the in-line fuse on the amplifier earth.
contact cleanerDrive the servo by hand
Touch 9 V to the servo for a moment — never past its stop, or the plastic gears strip. On a Mk II use the red and purple wires at the amplifier plug. If it moves, the control side is at fault.
a 9 V batteryReplace the failed part
Used parts are the realistic route. Before fitting a replacement Mk II amplifier, check the servo motor isn't shorted.
used servo, amplifier or moduleWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Heater Not WorkingIf there is no heat at all — the heater valve on the bulkhead or a blocked matrix.
- Jaguar XJS Air Conditioning ProblemsIf the air is never cold in any mode — gas, compressor clutch and its feed.
- XJS fuse boxes and relaysFor the climate control fuses and relays, by year.
- 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.