Jaguar XJS Heater Not Working
On the XJS no heat is usually the vacuum heater valve on the bulkhead or a blocked matrix. I'd feel both heater hoses first: if the outlet stays cool, it's coolant flow; if both are hot, it's the flaps, servos or controls. Air left in after coolant work is the free thing to rule out.
How bad is it · Safe to drive
Safe to drive — but keep an eye on the gauge and the screen
No heat is a comfort problem in itself. A gauge that stays low, a coolant smell or a screen you can't demist needs sorting before a winter run.
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 on the bulkhead — stuck, the wrong type, or leaking
The vacuum valve at the back of the engine is normally open, and vacuum closes it — only at full cold. Old metal valves seize, stick near closed inside while the outer lever still moves, or pass water when shut. A replacement that works the other way round (open only with vacuum) gives almost no heat.
With the engine hot and the control at full heat, the hose into the valve is hot and the one out of it stays cool; or the heat can't be turned off at all.
Watch the lever while someone turns the control from full heat to full cold: it should swing about 90 degrees at the cold end. Put a hand vacuum pump on the nipple — vacuum should close it, none should open it. To prove the matrix is not the problem, swap the valve briefly for a short length of copper pipe.
Fit a normally-open plastic poppet valve with 5/8 in. ports: Stant 20101, Four Seasons 74612 (the narrow vacuum nipple) or 74803 (wider nipple, needs a short sleeve). Vacuum-test any new valve before fitting — new ones arrive leaking. A seized pivot can sometimes be freed with penetrating oil. There is no need to drain the system to change it: keep the caps tight and top up afterwards. A strainer in the line ahead of the new valve keeps debris out of it.
A blocked heater matrix
Sediment builds up inside the matrix until little coolant gets through. A new matrix in a dirty system blocks again within months.
Inlet pipe at the bulkhead hot, outlet much cooler — a healthy matrix shows only 10–20 degrees between them. Heat that fades as the fan speed goes up.
Take both hoses off at the bulkhead and run a garden hose through the matrix each way; poor flow one way, or rusty water, confirms it.
Flush both ways for about 10 minutes each, or fill it with cleaner or white vinegar overnight first. If you fit a new matrix, flush the whole system with the heater bypassed, and consider the radiator too.
Air trapped in the heater circuit
The V12's heater circuit has a high point that is hard to bleed. After any coolant work, an air pocket can sit in the matrix and stop the flow.
Heat for a few seconds, then cold; or no heat since the coolant was changed or the radiator flushed.
Front of the car raised, heater set to hot, top up at the engine filler, not the expansion tank. Open the small bleed plug at the top left of the radiator until coolant runs.
Bleed it properly; a back-flush of the matrix can push the air out. A capped tee in the heater hose at its highest point, over the right cam cover, makes the next bleed easy.
A flap servo stuck on cold
All the air is cooled first, then blend flaps send it through the matrix as needed. If a servo stops, the flaps stay on the cold side however hot the water is. On a Mk III unit a dead lower servo can freeze the upper one as well, so defrost goes too.
Both heater hoses hot, yet lukewarm or cold air; no warm defrost.
Behind the right underscuttle panel, watch the flap linkage while turning the temperature up. Unplug the servo and run it with a 9 V battery: if it moves, the control side is at fault; if not, the motor.
Dismantle the servo and free sticking brushes; run it to the hot side with a battery as a temporary fix.
The heater temperature switch on the matrix pipe
A temperature switch on the heater pipe tells the system the matrix is warm. Until it closes, the blowers stay off except on defrost. When it fails, the system acts as if there's never any heat.
Fans won't run in heat mode even with the engine fully warm; defrost still blows.
Bridge its two spade terminals: if the climate control comes alive, the sensor is dead. Or test it in hot water with a multimeter — closed hot, open cold.
Replace it — JLM2121 on a 1992 car. The sensor exists in a black and a red version to match the climate ECU, so match the colour.
Corroded climate control connectors
The small plugs on the temperature selector, flap position sensor and flap motors corrode, especially on a car that has stood. A bad contact gives the system the wrong picture, and it holds the air cold.
Heat only on defrost, or heat that comes back for a moment when you wiggle a wire.
With the system running, wiggle the selector and flap-sensor wires on the left side and the servo plugs. Green paste on relay bases may be factory grease, so look at the pins themselves.
Clean the pins or cut out the corroded plug and fit a new connector.
Vacuum: a loose line or a solenoid holding the valve shut
A vacuum solenoid on the climate unit feeds the heater valve. If it sticks applying vacuum, the valve stays closed and the heater is cold. Burnt or loose lines and missing one-way valves upset the flaps as well.
Vacuum present at the valve line with the control at full heat.
Put a gauge on the valve's vacuum line: full vacuum only at full cold, nothing in heat. Look for lines left off after engine work or pushed off behind the radio.
Worth trying first: unplug and cap the line at the valve, which leaves it open. Then replace the split lines or the solenoid.
Thermostats stuck open, or the wrong ones
A thermostat that sticks open keeps the coolant below temperature, so the heater can't get hot. On the V12, thermostats that don't seal the bypass port also cut the flow through the heater circuit.
The gauge sits low and the heater goes cold on a long fast run, then recovers in town.
Test both thermostats in hot water; check the gauge reaches normal within a few miles.
Fit new thermostats of the type that seals the bypass — for example Gates 33188 or Stant 13398 — and re-bleed afterwards.
Coolant leaking from the matrix or its pipe joints
The matrix or the joints where the pipes bolt to it leak inside the climate unit. Leaks are often bypassed and forgotten, and the heater then gives nothing.
A sweet coolant smell when the blower starts, a greasy film on the screen, or coolant from the A/C drains. Hoses at the bulkhead joined to each other.
Look at the pipe joints before condemning the core. Check whether the heater hoses actually run to the matrix. Before refitting a matrix, pressure it to 10–15 psi with halves of a bicycle inner tube over the pipes and see that it holds — never above about 16 psi.
Year matters: on Mk III cars (from about 1987–88, push-pull Man-Auto knob) the matrix slides out sideways without removing the dash, in a few hours. On Mk II the pipes are fixed and have to be cut. A radiator shop can recore the old matrix; aftermarket ones vary in fit. On later cars the pipes unbolt from the core with a 4 mm hex bit; renew the foam around it while it is out.
Perished foam seals let cold air round the matrix
Foam around the matrix and on the flaps crumbles with age. Air goes round the matrix instead of through it, and cold air leaks from the top and centre vents.
Warm at the feet but cool air from the centre and top vents with heat selected.
With heat selected the centre vent should give little or nothing; cold air there points at seals or flaps.
Clean the matrix face and renew the edge seals with adhesive foam so all the air passes through the core.
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.
Test the heat properly
Engine fully warm, temperature to 85, fan high: hot air at the footwells and side vents, then on defrost. Defrost defaults to full heat, so heat only there points at the flaps or the controls, not the coolant.
freeGauge and coolant level
The gauge should reach normal within a few miles. Look for a coolant smell or film on the screen.
freeFeel both hoses at the heater valve
Inlet hot and outlet cool means the valve or the matrix. Both hot but cold air means the flaps, servos or controls.
freeWatch the valve lever and test it with vacuum
It should close only at full cold. If it doesn't open, cap its vacuum line or bridge the valve with a hose joiner and see if heat comes back.
a hand vacuum pumpFlush the matrix and bleed the air
Hoses off at the bulkhead, flush both ways, refit, then bleed with the front raised.
a garden hoseFlaps, servos, sensor and plugs
Watch the linkage behind the right underscuttle, run a servo on the battery, bridge the heater temperature switch, clean the climate connectors.
a 9 V battery and a multimeterWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS Air Conditioning ProblemsIf the vents stay cool with the A/C on — the same heater valve and flaps can reheat the air.
- Jaguar XJS Overheating, Cooling Fan ProblemsIf the gauge runs high rather than low, or coolant keeps disappearing.
- XJS fuse boxes and relaysFor the climate control fuses and relays, by year.
- Jaguar XJS Blower Motor Not WorkingIf the air is warm but the fan barely blows, or works only on high or defrost.
- Jaguar XJS Climate Control ProblemsIf the heat comes and goes, Auto won't hold a temperature, or the vents and flaps won't change mode.
- 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.