Jaguar XJS Overheating, Cooling Fan Problems
On the XJS, running hot is almost always too little air at idle: a seized auxiliary electric fan, a fan that gets no power, or a worn viscous clutch on the belt fan. I'd confirm the real temperature first, then make each fan work on demand, and only then pay for the radiator.
How bad is it · Garage, carefully
Drive it, but don't sit in traffic with the needle past N
A V12 coming off a fast run into a jam makes a great deal of heat. Boiling over blows hoses, and a cooked engine costs far more than any fan.
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 auxiliary electric fan has seized
Next to the belt fan sits a small electric fan that only runs when the coolant is hot or, on many cars, with the A/C. It runs so rarely that its plain bushings rust and seize; rust inside the casing can even swell and pinch the magnets onto the armature.
Fine on the open road, but the needle climbs in slow traffic or with the A/C on a hot day, and the little fan is stiff or dead when you try it — or it keeps blowing its fuse. A creaking buzz at the lights with the blades hanging loose means the plastic hub has let go. A fan frozen from disuse can go on blowing its fuse at every start even after it has been freed and spins easily.
Split the fan connector and feed it straight from the battery. If it doesn't spin, or turns slowly and labours, the motor is the problem. A stiff motor can draw several times its normal current, for example 15–20 A against about 5 A, which is why it blows fuses. If the blades are chewed or loose, check the mounting bolts and the motor bearings: a fan rubbing its shroud or a motor running hot can wreck the hub.
A stiff motor can sometimes be freed, cleaned and oiled, or rebuilt with its bushes and brush box, but on plain bushings that may only last a season. The lasting cure is a new or stronger aftermarket fan with its own relay and heavier wire; the OEM motor is C38009. The original steel shroud is usually rusty, so a kit with a new shroud and flaps is worth considering. Getting the fan out may mean tipping the radiator or drilling the shroud rivets. To free a stiff motor in place, drill a small hole in the bearing cap and work penetrating oil in. A replacement fan can need more current to start than the original, so check its draw before deciding the fuse is wrong.
No power to the fan — fuse, switch wires or relay
The fan is fed through its own fuse (10 to 30 A, depending on year and engine) and a relay on the left inner wing, switched by a thermo switch in the water pump inlet. Corrosion at the switch connectors, a splice in the light green feed or a blown fuse kills it as surely as a dead motor.
The fan spins happily on a direct battery feed but never runs on its own; or the fuse has blown. A failed fan relay often shows a melted connector.
Engine hot, ignition on: pull the two wires off the thermo switch and join them — the fan should run. The wires run down between the two fan shrouds and are easy to miss. If not, pull the fan relay and bridge sockets 30 and 87 (on late cars it's the first relay from the front); if it still won't run, slide the fuse in its clips, look for corroded ends, and check the wiring to the fan. To test the relay and motor separately, put 12 V on the white/blue wire at the relay to pull it in, or on the black/green wire to run the motor directly; unplug the motor to see whether the fuse still blows, and look for melted relay or diode sockets. The black wire at the fan is its earth: if it's corroded or its nut has been left off the stud, the fan stays dead with a good feed — test with a jumper to a clean earth. An inline fan fuse can blow unnoticed and leave the car running hot all summer.
Clean and remake the switch connectors, cut out and solder a corroded splice, renew the fuse. A relay that works only now and then is worth swapping. If the fuse keeps blowing, check the fan turns freely before fitting a bigger one. Make sure the fan relay hasn't been swapped with the sidelight relay beside it. If a previous owner tied the fan into the A/C, the fuse can blow the moment the A/C goes on: don't just fit a bigger fuse, because the wiring was sized for the original one. Undo the tie-in, or give the added load its own fused feed and relay. A failed relay and its melted connector are replaced together; on a 1990 V12 the relay is DBC2483.
A worn viscous clutch on the belt fan
The big engine-driven fan does most of the work at idle, and only as well as its viscous clutch grips. A tired clutch lets the fan freewheel just when the car stands still.
Temperature fine on the motorway, creeping above N in city traffic in hot weather. Normal at 60 mph but hot at rest points at the clutch or the airflow. The commonest way a clutch dies is losing its silicone fluid; a clutch laid flat while off the car can leave an oily puddle and come back tired.
Start it after a night off: a good clutch makes the fan roar for 20–30 seconds, then ease off. Engine hot and switched off, spin the fan by hand: more than about one turn, oil thrown from the clutch, or a fan that roars all the time means it's finished. A tired clutch can pass the spin test, so at a hot idle feel how hard the air comes out behind the shroud, ideally against a car you know is good. Only the hot test counts: fully hot, engine off, spin the fan — freewheeling is bad, a slight drag and one turn or less is good.
Fit a new clutch; the needle should return to the top of N. If the old yellow plastic fan is cracked, fit the later black fan EBC4553 with a Hayden 2665-type clutch, changed together. A new clutch is cheap, for example about $40; an aftermarket one can be smaller than the original, so fit clutch and blade together as one assembly. Store a removed clutch upright.
The blue diode pack in the fan circuit
On earlier cars a small blue box on the left inner wing, near the brake servo, looks like a relay but holds diodes. They let both the thermo switch and the A/C compressor pull in the fan relay, and give run-on after shutdown. A failed diode means the fan never comes on; a shorted one keeps it running all the time.
The switch and relay test good, yet the fan never runs, or it runs from key-on regardless of temperature.
Lift one lead of each diode and measure: it should conduct one way and not the other. Zero or open both ways means it's bad.
Replace the faulty diodes with ordinary silicon rectifiers (100 V types are fine), and the fan should come on with the A/C and run on after a hot shutdown again. A single 1N4001-type diode can stand in for the whole pack.
Later 4.0 cars — the fan ignores the A/C
On earlier cars the aux fan also runs with the A/C compressor. On the 4.0 from about 1994 only the radiator thermo switch starts it, so in slow traffic the condenser dumps its heat into the radiator before the fan helps. Wiring varies by year, and diagrams for 1989–92 cars show the fan on temperature only too, so test yours rather than trust the book.
Runs warm in traffic with the A/C on; the fan only cuts in with the needle midway between N and H.
Sit at idle with the A/C on and watch whether the aux fan runs before the gauge passes N.
Feed the fan relay from the compressor clutch relay: pin 87 of the clutch relay to pin 30 of the fan relay through a diode, arrow toward the fan relay, rated for the fan current. Raise the clutch relay fuse to suit, or it blows and the compressor stops engaging. Wrong pins leave the fan running after shutdown until the battery is flat.
An electric conversion that moves less air than the belt fan
A healthy belt fan moves a lot of air at idle. One or two modest electric fans often can't match it, especially climbing with the A/C on full. Electric fans do give full flow at idle and can rest at a cruise, which is their real advantage.
The car was converted to electric fans and now runs hot in traffic or on long climbs with the A/C on.
Compare the fans' rated airflow with what the belt fan did, and check the alternator can supply them. On the V12, cars after engine number 8S57572 have the 115 A alternator with a ribbed belt; earlier ones have a 75 A unit on a V-belt.
Refitting the belt-driven fan can bring the gauge back to N on the same climb. If you stay electric, use big fans with enough combined airflow, a relay and heavy wiring, and a stronger alternator if needed; a properly sized twin-fan kit can keep the needle at N through a summer. Expect extra work: a bigger alternator, a controller, relays, and fusible links rather than fuses for the start-up surge; a complete twin-fan setup can cost close to USD 900. Two identical fans beat against each other with a pulsing hum in the cabin, and a fan at full speed is noisy at idle.
On a converted car, the fan switch is in the wrong place
Where the switch sits decides when the fans run. In the lower hose or the water pump inlet it sees the coldest coolant, so a switch rated for engine temperature closes late or never; on the water rail it heat-soaks and keeps the fans running long after shutdown.
Since the conversion the main fan never comes on, or only after the aux fan has given up; or the fans run for half an hour after you switch off and flatten the battery.
Note where each switch or probe sits and its rating. With the engine hot, compare the real temperature at the thermostat housings with the moment each fan starts.
Match the rating to the position: a lower switch, for example 180 F, in the cold water; an engine-temperature switch in a top-hose adapter. Moving the switch off the water rail into a top-hose adapter can bring the fans to a stop within half a minute of shutdown. Fin-type probes don't read true. If a kit controller won't switch the fans off even at its highest setting, feed their relay from an ignition-on circuit so they stop with the key.
Coolant escaping at the water pump or a hidden hose
A system that keeps losing coolant runs low and then hot. The water pump seal can weep long before the bearing goes, and on the six-cylinder cars the large hose hidden under the intake can split under hard acceleration.
The level drops between drives; there are dried coolant marks at the front of the engine or on the pump casing; the pump pulley squeaks when you turn it.
Look for crusted coolant on the pump body and the front of the engine. Turn the pump pulley by hand, feel for play and listen for a squeak. Check the belt tension while you're there.
A new water pump stops the weep; on the AJ6 it's EBC4437. The seal and bearing can be pressed out and renewed, but the cast impeller cracks easily. A tired pump on an older V12 can also be worth renewing on its own: the temperatures can drop.
V12: air in the system, or a blocked overflow line
The V12's cooling system traps air easily, and a small leak on the return side lets it draw more in as it cools. An air pocket at the sender or in the heads makes the engine boil while the fans and radiator are fine. A blocked line from the expansion tank to the catch tank does the rest, forcing coolant out of the cap.
Fine for two hours, then boils out of the expansion tank within minutes of a short stop; the needle jumps rather than climbs; the tank filled only to the bottom of its neck.
Look for a weeping return hose and a pinholed crossover pipe. Clear the expansion-to-catch-tank line — the metal pipe can clog — and fit a clear hose there to watch it work. Fill the tank to the top of its neck, raise the nose and bleed until no air comes out.
Fixing a leaking return hose that was letting air in cured intermittent boiling. Bleed properly after any coolant work.
The plastic belt fan cracks and breaks up
The early yellow or white plastic fan goes brittle with age. It can fly apart at speed with no warning and chew into the radiator; a clutch that has seized and lets the fan overspeed is often behind it.
A sudden bang and clatter at speed, then coolant spraying and the needle climbing; or cracks at the roots of the blades.
Look closely at every blade for cracks, especially where it meets the hub. Before starting, spin the fan by hand: it should drag a little, not feel locked.
Worth fitting the later black fan, which is wider and moves more air, with a new clutch; on a pre-HE V12 the black fan and clutch from a six-cylinder car can be used. Recore or replace the radiator if it's been holed. Don't run a metal fan on a solid spacer without a clutch: it's fine at low revs but overspeeds as they rise.
The fan thermo switch itself has failed
The aux fan answers to a thermo switch in the water pump inlet, which reads the coolest water in the engine, coming back from the radiator. A switch that never closes leaves the fan idle however hot the engine gets; one that closes late lets the needle climb well past N first.
The fan runs the moment you join the two switch wires, but never on its own, even with the needle well past N.
Before buying anything, unplug and refit the switch connector — a poor contact there can be the whole fault. Off the car, heat the switch in a pan of water: it should be open cold and close near its stamped temperature. Tug the terminal wires and check the switch with a meter before replacing it. Note that joining the wires only proves the wiring: the coolant at the switch may simply not be hot enough yet, so heat the switch in water before condemning it.
A new switch to the original rating (92 C on, 82 C off) is worth trying. A cabin switch wired across it gets you home in the meantime. On a 1996 4.0 the switch is DAC6794. If a new switch helps for only a week, the switch was never the fault: go back to the clutch and the radiator.
A fan that won't switch off
Some run-on is normal. On earlier cars the diode pack latches the aux fan on if it was running at key-off; on later cars the fan has a permanent feed through its relay and thermo switch, so it runs with no key until the switch opens. A fan that runs from cold, or for half an hour after a short drive, is a fault: a relay stuck closed, a short in the switch wiring, a switch closed when cold, or an ignition switch that never fully turns off.
The fan starts with the key on a cold engine, runs until you disconnect the battery, or keeps going 30 to 45 minutes after you park. If the key buzzer or the radio stays live with the key out as well, look at the ignition switch.
Unplug the thermo switch: if the fan stops, the switch is closed when it shouldn't be; if it keeps running, it's the relay or, on earlier cars, the diode pack. Make sure nobody has left a bridge or a paperclip across the switch or the relay socket. Check the coolant level too.
Worth trying in this order: a new fan relay, a new thermo switch, the diode pack, then the ignition switch. A minute or two of fan after a hot run is by design and best left alone.
Radiator blocked inside, or packed with debris in front
The A/C condenser sits in front of the radiator and shares its air. Leaves, packing material and fluff collect between them, and scale builds up inside the radiator core. A clogged core also keeps hot coolant from reaching the fan switch, so the fan stays off.
Runs cool until you switch on the A/C, then climbs within a minute or two; or it gets hot after an hour despite a new clutch. Overheating at high revs rather than at idle points to restricted water flow — scale or debris in the block — rather than airflow.
Take off the top rail and look between the condenser and the radiator. To test the core, pull the big hoses and pour water from a garden hose into the top: if it backs up instead of draining straight through, the core is blocked. Clean the fins of the oil cooler as well as the radiator and condenser. Pressure-test a new radiator before fitting it: a new one can leak at its welds, and a 'leak' on fitting can be an aluminium hose connection not tightened enough.
Worth washing the radiator and condenser from the back, renewing stiff or missing shroud flaps and sealing the gaps with foam so air has to pass through the cores. If it still runs hot, have the radiator rodded or recored — chemical cleaning alone may not be enough — and refill with coolant at the right mix with a new thermostat. A recore with a high-efficiency core is usually enough without an aftermarket kit; an old core can be partly rotten inside. Test the clutch first, though: a recore won't lower the needle if the fan isn't pulling air. A good radiator shop can restore the original brass core: re-rodded, re-finned and acid-cleaned, it can last many years. Flushing with citric acid shifts scale from the block.
V12: thermostats of the wrong type
The V12 thermostats also close a bypass. A thermostat that opens normally but is the wrong length leaves the bypass open, so too much coolant skips the radiator.
Runs hot after new cooling parts were fitted; the fans and clutch all test good.
Measure from the mounting face to the bypass disc: it should be 41 mm. Test new thermostats in hot water before fitting, and use the type with a steam relief hole.
Worth trying: the correct 41 mm bypass thermostats on both banks, with clean steam relief pipes and new caps.
The gauge reads high, the engine doesn't
The gauge reads from its own sender, separate from the fan switch. A tired sender can put the needle near H while the coolant is no hotter than normal.
The needle is high but the fan hasn't come on, and nothing boils.
Measure the thermostat housings with an infrared thermometer before trusting the needle; N corresponds to roughly 190–195 F, and gauges vary from car to car. Clean the instrument board contacts and add an extra earth for the cluster: these gauges are not always reliable.
If the readings are normal, swap the gauge sender for a known good one before anything else.
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.
Measure the real temperature first
An infrared thermometer at the thermostat housings tells you if the needle is right. If the coolant is normal, it's the sender, not the cooling system.
a cheap thermometerCheck the coolant level and the water pump
Look for dried coolant around the pump and the front of the engine, and turn the pulley by hand for play or a squeak. On a V12, fill the tank to the top of its neck and make sure the overflow line is clear.
freeCheck the belt fan and its clutch
From a cold start the fan should roar for 20–30 seconds, then ease. Hot and switched off, many free turns means a worn viscous clutch, but a tired one can still pass. Look for cracks in a yellow or white plastic fan while you're there.
freeLet it idle hot and watch the aux fan
If it never runs, feed it straight from the battery. Stiff or dead: the motor.
freeJumper the thermo switch, then the relay
Ignition on, join the two switch wires: the fan should run — if it does, suspect the switch. If not, bridge relay sockets 30 and 87, then check the fuse and connectors.
freeOn earlier cars, test the blue diode pack
Each diode should conduct one way only.
freeIf the fan won't stop, unplug the thermo switch
Fan stops: the switch is closed when it shouldn't be. Keeps running: the relay or the diode pack. If the radio also stays live with the key out, it's the ignition switch.
freeOn a converted car, check where the fan switches sit
A switch in the coldest water needs a lower rating; one on the water rail keeps the fans running after shutdown.
freeLook between the condenser and the radiator
Clear the debris and make sure the rubber flaps on the radiator cowl are all there.
freeOnly then pay for the radiator
Once the clutch is ruled out: rodding or recoring, fresh coolant and a new thermostat.
radiator shopWhat 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 A/C itself blows warm, or the condenser fan question is really an A/C one.
- Jaguar XJS Alternator Not Charging, Battery LightBefore adding electric fans, or if the voltmeter sags with the fans running.
- Jaguar XJS Misfire, Rough IdleIf a rough idle and a miss come with coolant loss, a head gasket is covered under misfire.
- XJS fuse boxesFor the fan fuse and relay; other years from the same page.
- Jaguar XJS Heater Not WorkingIf the gauge stays low and the heater is cold, rather than running hot.
- Jaguar XJS Battery DrainIf a fan that won't switch off is flattening the battery.
- Jaguar XJS Instrument Cluster, Gauges Not WorkingIf a thermometer says the engine is normal, the temperature gauge sender or the cluster earth is the suspect.
- Jaguar XJS V12 Won't Start or Starts and DiesIf it cuts out or won't restart only once hot: ignition, pump and rail checks for a hot V12.
- 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.