Jaguar XJ40 Heater Not Working
On the XJ40 no heat is usually the heater valve stuck shut, a thermostat stuck open, or low coolant. I'd watch the gauge and feel both heater hoses at the bulkhead first: if they are hot, the fault is inside the car — a jammed blend door, a sticking flap motor or one dead blower.
How bad is it · Safe to drive
Safe to drive — watch the gauge and the screen
A cold heater often means low coolant or a stuck thermostat, so check the level before a long run. In wet or cold weather the screen may not clear.
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.
The heater valve stuck shut or furred up
The valve sits in the heater feed and is open until vacuum closes it for full cold. Its spindle and lever rust, or the inside corrodes, and it sticks shut or chokes the flow. Year matters: cars to 1989 have it under the wiper motor, many 1990–92 cars were built without one, and on 1993–94 it is back, screwed into the rear left of the head behind the inlet manifold.
No heat from any vent with the gauge sitting at N. The feed hose at the bulkhead is hot, the return stays cool. In Heat the blowers may also wait a long time from cold, because they are held back until warm coolant reaches the matrix.
With the engine hot, feel both heater hoses where they go through the bulkhead. Pull the small vacuum pipe off the valve — with no vacuum it should open; suck on it and it should close. A quick test: pull the vacuum elbow off the valve and plug it — the valve is open without vacuum, so heat should come back within a minute if it is free. Inside the car, Manual, full heat then full cold, and watch the valve move.
Free the lever and oil it, or fit a new valve — a cheap aftermarket plastic one fits (Everco H6332 is one). Running with the valve left open and its vacuum pipe plugged gives full heat at some cost to the A/C, and a manual quarter-turn ball valve switched twice a year is a lasting cure. On 1993–94 cars the inlet manifold may have to come off to reach it. Replacement valves vary badly: several new metal ones would not close or leaked from the start, while a generic plastic vacuum valve of the same hose size sealed and has lasted. Fit new hose and clips at the same time.
The engine thermostat stuck open
If the thermostat hangs open, the coolant never gets properly hot, and the heater is only as warm as the coolant. The gauge drops only slightly, so it is easy to miss on short trips.
Heat that fades once you are moving — for example above about 40 mph — and a gauge needle that settles below N. Both heater hoses are only warm.
Watch the gauge on a longer run: it should reach the middle within a few minutes and stay there. The old thermostat can be checked in a pan of water with a thermometer.
Fit a new thermostat — two bolts and sealant, no gasket. Test the new one in hot water first, as new ones can be faulty. In a cold climate a hotter rating helps, for example 192 F in place of 176 F.
Low coolant or air in the heater matrix
The matrix is one of the highest points of the system, so it is the first place air collects. When the level drops, the heater goes cold before anything else warns you.
Heat weak and patchy, then gone. Often right after coolant work, or with a slow leak at the heater valve or its hoses. The gauge can still read normal if the level falls past its sensor. Look for the leak where it hides: a cracked metal pipe from the head to the heater can spit water onto the bulkhead under the wiper motor and pass for a leaking matrix, and the heater hose to the return pipe near the throttle body can blow off.
Check the level with the engine cold. After any coolant work, run it hot with the header tank cap off at a fast idle of about 1200–1500 rpm until the bubbles stop. To split a pipe leak from a matrix leak, join the two hoses at the bulkhead for a short run: if coolant still goes, it is not the matrix.
Top up and bleed out the air, then find the leak — the heater valve and its short hoses are the usual places on these cars.
A blocked heater matrix
Scale and rust from old coolant settle in the fine tubes of the matrix. The feed pipe is hot, the return is not, and little heat gets into the air. Crumbling foam from the flaps can also pack the air side of it.
Lukewarm air on every setting, with good coolant level and a normal gauge. The return hose at the bulkhead is clearly cooler than the feed.
Disconnect both hoses at the bulkhead and run water through the matrix from a garden hose, gently, both ways, until it comes out clear.
A flush, often with the cooling system flushed and refilled at the same time, brings the heat back. If it is too corroded to clear, a new matrix goes in from the right after the knee bolsters, glovebox and instrument cluster come out — not the whole dash. Use new O-rings on the pipes. The matrix comes out to the right without removing the console: drain, blow the rest of the coolant out at the bulkhead, undo the pipe flanges under the dash, take the climate computer off the right side and withdraw it. New O-rings on the pipes; a second-hand matrix needs fresh foam or it rattles.
Vacuum lines off, kinked or a dead solenoid
The heater valve and the vent doors are worked by vacuum, switched by solenoids at the base of the climate unit. A hose left off, a kink or a dead solenoid leaves the valve or doors in the wrong place.
Controls that seem to do nothing, demist vents that never close, centre vents that never open. Common after an A/C service, head work or anything else that disturbs the engine bay. After a head gasket job the valve's vacuum line is often the one left off.
Trace the vacuum supply hose in the engine bay and the lines to the heater valve and the climate unit. With the engine running, a solenoid that never clicks or passes no vacuum is dead. The valve's line is the yellow-striped one at the lower right-angled fitting on the bulkhead, fed from a solenoid on the underside of the climate unit; the white-striped one above goes to the inlet manifold. The two hoses are different sizes.
Reconnect or replace the hose; replace the failed solenoid.
An A/C fault confusing Auto
Hot coolant reaches the matrix whenever the engine runs; it is the blend doors that decide how much heat you get. In Auto the climate computer works the doors from several sensors, so a fault on the A/C side can leave it holding them on cold. In Manual far fewer inputs are involved.
No heat and no cooling together, with both heater hoses hot, and no change whichever Auto, Manual or Econ setting you try.
Compare Auto with Manual on full heat with the compressor switched off. If the A/C is also weak, have the refrigerant pressure checked before chasing flaps.
Example: a slow refrigerant leak at a hose connector — recrimped, pressure tested and refilled, and the heater worked again along with the A/C.
The upper blend door jammed by a broken demist flap
The two demist flaps under the middle of the dash are plastic and break where they hang on their vacuum pots. A broken flap drops into the heater box and jams the upper blend door, which then can turn only a few degrees. Especially common on 1994 cars.
Both heater hoses hot, yet cold air at the screen and no heat at the face vents, even with Defrost not selected.
Take off the wooden trim and the centre vent grille, push the vent door open from below and look in with a bright light. A strip about 14 by 2 inches standing at an angle is the flap. If debris is jammed in the box and knocking won't free the motor, full access means taking the blend motors and climate computer off the right side and the matrix out, and in the worst case the dash.
Pull it out through the centre vent with long-nose pliers; cut away the thin upright on the vent door if it is in the way. That alone brings the heat back. Refitting the demist doors themselves needs the dash top off.
A sticking blend flap motor
Two small electric motors on the right of the heater box turn the blend flaps, with position pots on the left. Their gears gum up from sitting in one position, and the flap stays on cold. The lower flap feeds the footwell and rear vents.
Cold feet with warm face air, or the other way round. A flap that only moves when you help it.
Remove the panel under the glovebox and rap each motor sharply with a wooden dowel or a screwdriver handle while someone swings the temperature from full cold to full hot. On 1990 cars a small one-screw panel in the glovebox gives access to the upper flap motor for tapping.
A knock often frees it for good. Otherwise take the motor off (a couple of screws), clean and grease the gears, bench-test it on 12 V and refit. A position pot that has slipped on its spindle can be loosened and turned back. Swing the controls every few days to keep them free.
Only one blower running
The two blowers face each other and push air through the heater box together. With one dead, the other's air escapes back through it and little reaches the matrix.
Warm air only at speed, when ram air does the pushing. A weak flow that the fan speed barely changes.
In Manual on High, pull each blower fuse in turn and listen at each footwell.
Get both blowers running — the blower page below covers the fuse box joints, transistor and brushes.
Auto misled by a broken sensor wire
The incoming-air sensor rides on the right blower's recirculation flap, and its wires flex every time the flap moves. Once they break, Auto can hold the blend flaps on cold.
Cold air in winter on Auto, while Manual or Defrost gives heat.
Compare Auto with Manual on full heat. If only Auto is cold, check the wires at the sensor on the right blower.
Repair the broken wires; it needs the right blower out.
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.
Gauge and coolant level
Check the level cold and watch the gauge on a run. A needle that settles below N is the thermostat, and the heater can't be hotter than the coolant.
freeFeel both heater hoses at the bulkhead
Engine hot, feel the two hoses where they go through the bulkhead below the wiper motor. Both cool: heater valve or thermostat. Feed hot, return cool: the valve half shut or a blocked matrix. Both hot: the problem is inside the car — flaps, blowers or controls.
freeRun the Defrost test in Manual
Select Manual and Defrost. Hot air should reach the screen within about 30 seconds, with none from the footwell. Air at the feet means the lower flap is stuck; cold air at the screen means the upper one is. Then Manual, medium fan, full cold to full heat: the centre vents should shut and the side vents run hot within about 45 seconds.
freeMake sure both blowers run
On High, pull each blower fuse in turn and listen. One dead blower is enough to leave the heater weak.
freeHeater valve and vacuum
Pull the vacuum pipe off the valve and see whether the return hose gets hot. If it does, trace the vacuum side; if it doesn't, free the valve or replace it.
free to cheapLook for a broken demist flap, then rap the flap motors
Look through the centre vent for a broken demist flap jamming the upper blend door. If it is clear, knock the flap motors behind the glovebox while working the temperature control.
freeFlush the matrix, then replace it
Flush through the bulkhead hoses both ways until clear. If the return still stays cool, the matrix comes out from the right side of the heater box.
free to flush; dear to replaceWhat 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 one or both blowers are dead or only run on high.
- Jaguar XJ40 Air Conditioning ProblemsIf the trouble is the opposite — warm air when you want it cold.
- XJ6 (XJ40) fuse boxesWhich fuse feeds each blower — other years from the same page.
- Jaguar XJ40 Overheating, Radiator and Cooling FansIf the gauge climbs instead of staying low.
- 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.