Jaguar XJS 3.6 and 4.0 Won't Start
On the six-cylinder XJS a crank-but-no-start is usually one of three cheap parts: the crank position sensor, the EMS relay beside the ECU, or the yellow-base fuel pump relay in the boot. Listen for the pump, watch the rev counter while cranking and check for 12 V at the ECU before buying anything. A sensor that measures fine can still be the fault.
How bad is it · Garage, carefully
If it restarts, drive only to a garage
A failing crank sensor or relay can stop the engine on the move and leave it dead at the roadside. I'd carry a spare crank sensor until the cause is found.
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 crank position sensor
The ECU times both the spark and the injectors from this sensor at the front of the engine. When its signal goes weak, the ECU fires nothing and may not even keep the pump running, so it looks like a fuel fault.
Cranks normally with no hint of firing; often after a stutter on the road, or only in cold, wet weather. A still rev counter while cranking is a strong hint, but a needle that bobs around 200 rpm does not clear the sensor.
A resistance check proves nothing: a dead sensor can read within a few ohms of a new one, for example 1.4 against 1.37 kΩ. On a 4.0 it sits between the A/C compressor and the front pulley on one small bolt, so swapping in a known-good one is the real test.
Fit a new sensor and treat its fragile plug gently. If a new one changes nothing, try a second from another supplier: new ones can be faulty out of the box. On a high-mileage 4.0 I'd keep a spare in the boot, it's a ten-minute roadside job.
The EMS main relay next to the ECU
This relay gives the ECU its ignition-switched power. If it dies, spark, injection and the rev-counter reading all go at once, while the pump only gives its short prime.
No spark, dry plugs and no revs on the gauge while cranking. That's the classic crank-sensor picture, yet a new sensor changes nothing. Often after the car has stood.
With the key on, look for 12 V at ECU terminal 1 on the yellow connector and terminals 10 and 22 on the blue one; terminal 14 on the blue carries permanent power. Pull the relay: if nothing changes with it out, bench-test it. It should click across 85/86 and close 30 to 87.
Replace the relay: silver with a red stripe on a red base, next to the ECU in the passenger footwell. On UK right-hand-drive facelift cars the ECU and this relay sit in the left footwell behind a carpeted panel under the A-post trim, the mirror image of the manual.
The fuel pump relay in the boot, the inertia switch and the ECU earth
The pump relay is fed from the EMS relay on pin 86 and switched by the ECU earthing pin 85. The inertia switch and that ECU earth are as much a part of the circuit as the relay itself.
Silence from the boot at key on, or a pump that primes when the battery is reconnected and then won't run while cranking.
The relay is on the right of the boot at the front, behind the trim by the battery cover, over the wheel arch. Of the hidden relays there, the yellow-base one is the pump relay; book drawings for some years show a different layout, so go by the base colour. Press the inertia switch down; on the six-cylinder it's on the left front door pillar.
Replace the relay. A generic 5-pin 40 A relay works; the original Bosch 0 332 014 113 is superseded by 0 332 019 151. If the pump runs only with the socket bridged and two new relays don't help, trace the earth from ECU pin 7 on the blue connector (pink/brown wire) to relay pin 85.
Corroded pins in the right-hand under-bonnet relay bank
Green corrosion on a relay's blades breaks the feed to the injection system without any relay failing. The pump primes for a couple of seconds and that's all.
A car sold as having a bad fuel pump; a new pump and a new inertia switch change nothing; it sometimes starts after something is wiggled.
Pull each relay in the right-hand under-bonnet bank and look at its blades. Start with the white-base one in the middle, on the blue section.
Clean the relay blades and the socket, then reseat. The XJ6 go/no-go diagram leads to the equivalent relay; the XJS uses the same circuit.
The pump connector by the tank and its harness
The pump feed runs from the boot relay round the boot-lid hinge to a white connector on the right of the tank. A dirty, high-resistance joint there passes voltage to a meter but not current to the pump.
The pump is dead even with the relay bridged and all the fuses good; the voltage at the pump climbs slowly while your probes stay on it.
Measure at the pump plug with the relay bridged. Only the blue (power) and black (earth) wires serve the pump; the orange and brown are for the gauge sender. Then feed the pump directly at the boot connector to prove pump from wiring.
Unplug, clean and reseat the white connector, and clean the main body earth in the boot. Look along the harness from behind the right boot side panel to the tank: it melts on many cars.
The in-tank pump, its hose and pickup
A pump can run and still not deliver. On these cars a split rubber hose inside the tank or a pickup screen clogged with crud gives no pressure with a healthy hum.
Rail pressure of only 2 to 3 psi while cranking; or a car that refuses one day and starts the next, as the debris drops off the screen after shutdown.
Put a gauge on the rail: about 40 to 43 psi with the vacuum line on is right. Rule out the crank sensor first, because with a dead sensor the ECU won't run the pump while cranking and the fuel side looks dead too.
Replace the in-tank pump; while it's out, renew the internal hose and the pickup screen. The tank can be slid back rather than fully removed to reach it.
A weak ignition coil
A tired coil still throws a spark you can see in the open air. Under compression that spark is too weak to fire the mixture.
Fuel pressure at the rail, visible spark at the plugs, no fault codes, and no catch even on starter spray.
Spray starter fluid into the air-flow meter while cranking. If it doesn't even cough, the spark is the problem: rotor, carbon brush in the cap, coil or amplifier.
Replace the coil. On a 4.0 of this age the amplifier connector may carry three wires rather than the four on the diagrams; that's normal.
The coolant temperature sensor on the 3.6
The ECU sets the starting mixture from this sensor. It can fail intermittently and still read in spec on a meter, and its injection-type plug loses grip on the pins.
No start, often on a cold morning, with spark and fuel both present.
Measure the sensor (for example about 4000 Ω cold), but don't trust a pass. Look into the plug: pins that push back under the probe don't make contact.
Replace the sensor, and renew the plug if the pins have backed out. Check the auxiliary air valve while you're there.
Injectors gummed up after a long lay-up
Varnish from stale fuel sticks the injectors shut. The engine fires on spray, has spark everywhere, and still won't run on petrol, which looks like a dead ECU.
The car has stood for a long time; it fires on starter spray into the air-flow meter, then dies.
Test the injectors electrically first. At the ECU, the crank signal reads about 1.3 V AC while cranking; the injectors are fed 12 V in two groups, 1-2-3 and 4-5-6, and the ECU switches their earth.
Take the rail off, clean the injectors and refit; expect a rough few minutes before it runs clean. Unplug and refit the ECU connectors too, as their contacts tarnish.
A flat battery and a disturbed idle setting
A battery down to 11.8 V can crank yet starve the injection. A packed-out throttle stop then gives the wrong air at start-up.
The rev counter reads about 1000 rpm while cranking, which is too fast, and a new crank sensor changed nothing.
Measure the battery at rest. Look at the throttle-stop screw for shims or tape left from earlier testing.
Charge the battery, remove anything packed under the throttle stop, then run the idle valve relearn: ignition on and off with its plug off, then again with it reconnected.
Water in the connectors after washing the engine
Pressure-washing a 4.0 drives water into the crank sensor plug, relays and plug wells. The engine cranks well, may fire for a moment on spray, then nothing.
No start straight after the engine bay was washed; spark present but perhaps weak.
Unplug the crank sensor connector and look for water before anything else; then the plug wells and the under-bonnet relays.
Worth trying: dry the bay thoroughly. A small fan heater inside it can still take a couple of days. Then prove spark, rail pressure and injector pulse one at a time.
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.
Ask what changed
Long standing, a washed engine, a new battery or recent work in the footwell or boot each point at a different version. After a long lay-up, fresh fuel and the injectors come first.
freeListen for the pump at key on
Ignition to II, wait until the brake pump stops, then off and back to II: you should hear a couple of seconds' hum from the boot. Silence means the inertia switch, the pump relay or the connector by the tank.
freeWatch the rev counter while it cranks
Dead still: crank sensor or the EMS relay. A bob around 200 rpm doesn't clear the sensor. About 1000 rpm: check the battery and the idle setting.
freeCheck for 12 V at the ECU
Key on, terminal 1 on the yellow connector and terminals 10 and 22 on the blue. Nothing there points at the EMS relay beside the ECU, before any sensor.
freeSpray starter fluid into the air-flow meter
It catches and dies: fuel side, so pump, relay, rail pressure, injectors. Not even a cough: the spark, so coil, cap, rotor, amplifier.
a canPut a gauge on the fuel rail
About 40 to 43 psi with the vacuum line on is right; a couple of psi means the pump or the hose inside the tank. To split pump from wiring, feed the pump straight from the battery, minding the polarity.
a gaugeCheck the injector pulse
A test lamp or noid light on an injector plug while someone cranks. Spark and pressure but no pulse: the crank signal or the ECU's feed.
a few poundsSwap in a known-good crank sensor
Resistance and inductance readings won't find a failing one. If you've already fitted a new one, a second from another supplier is still a fair test.
a sensorThen the coolant sensor and its plug
On the 3.6 especially. Look for pins that have backed out before buying the sensor.
a sensorWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS V12 Won't Start or Starts and DiesIf yours is the 5.3 or 6.0 V12: different ignition, different suspects.
- Jaguar XJS Won't Crank, Starter RelayIf the key gives a click or nothing and the engine doesn't turn over.
- Jaguar XJS Battery DrainIf the battery is flat by morning: long cranking flattens it too, so rule out the no-start first.
- XJS fuse boxes and relaysWhere the relays and fuses sit.
- 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.