Jaguar XJ6 Series 3 Won't Start, Cranks But Won't Fire

Pull each relay and check it pulls in. At the main relay base: brown live at all times, black to earth, white and brown/slate live with the key on. The relays look alike, so make sure each sits in its own socket. Bridge white to white/green on the pump relay base: if the pump now runs, the pump is fine and the fault is upstream. On the V12 the main and pump relays sit on a bracket at the front, above the radiator, beside the red fan relay. If it starts and dies, earth the orange wire at the pump relay: if the engine now keeps running, the ECU has stopped switching the pump. If the pump stops the moment the key leaves crank, push the airflow meter flap in about half an inch with the key on: the pump should run — its second feed comes through a switch in that flap and the diode pack. To test or limp home, the main relay can be bridged as well: brown to brown/slate in its base with a short, heavy jumper. The injection wiring carries no fuses — the main relay's brown feed is unfused — so instead of hunting a blown fuse, measure the injector resistor pack: about 6 ohms from each injector to its brown/slate feed, measured from the injector side. An aftermarket alarm usually cuts the pump, the spark or the injector trigger, so on a car that has one check its cut-outs early.

First make sure there's fuel — the gauges can read a quarter with both tanks empty. Put a gauge on the rail: about 36 psi engine off, 28–32 running. Read the pump outlet direct, where it should be roughly 75–100 psi; below about 42 it misbehaves. If the reading falls fast once the pump stops, the non-return valve is leaking. Drain the filter into a clear jar and look at the fuel. The pump draws only from the tank the changeover valve selects, so check which one before blaming the pump. The main filter sits ahead of the spare wheel, right-hand side. A good pump gives over 100 psi; at the open filler you can hear fuel returning to the tank.

Meter on coil positive, then wobble the ballast resistor — a jumping reading is the fault. Check the white wire from the switch to the coil with the coil end disconnected; the pins in bulkhead connector CF3, engine side, right-hand side, are prone to corrosion. Old spade crimps on the coil leads can crumble as soon as you handle them.

Load-test the battery, then measure the voltage drop across each joint while cranking — for example, a bad earth at the intake manifold read 0.6 V, and 0.02 V once cleaned. Check the positive clamp at its black plastic fitting and the braided strap underneath where engine and gearbox meet, right-hand side. The injection is sensitive to low voltage: clean the earth bundle at the rear of the water rail to bare metal.

Measure the pickup — a new one reads about 2.4 ohms — and set its gap to the reluctor at 0.008–0.014 in with a brass or plastic feeler, never steel. Set static timing on cylinder 6, 6 degrees BTDC, and confirm with a timing light. A fat spark from the coil lead but none at the plugs puts the fault inside the distributor: rotor-to-cap contact, a sheared drive pin, or a body not fully seated on its drive. For an intermittent no-spark, check the wire from the amplifier to the pickup with an ohmmeter, including its bullet connector — on the V12 it hides under the cruise control bellows. The two-wire plug between the distributor and the amplifier can crack inside with nothing to see: one pin not touching kills the spark while every part tests good. Look at the rotor too — a cheap reproduction with a long centre rivet can track the spark to the shaft without warning.

Bring cylinder 6 to TDC on compression and see where the rotor points, then compare with the lead order on the cap. At the TDC mark the rotor points at whichever of the paired cylinders fires next; prove compression on that one before routing the leads.

Measure its resistance and compare with the manual's table — for example, 580–600 ohms at 2–6 °C, which the table puts at about 60 °C. Check the plug is clean and tight. A spare sensor plugged into the loom proves it. Squirt a little fuel into the intake and crank: if it fires, the fault is fuelling. With pressure and pulse proven, try another airflow meter. Across the meter plug, pins 8–9 should read about 220 ohms and pins 6–7 about 400, changing as the flap moves. The temperature sensor reads a few kilohms across its two prongs when cold.

Remove the injector and put 12 V across its terminals: it should spray, not just click. With the key held in crank, test the white/yellow feed at the injector, the thermotime switch and the diode pack. Crank with the throttle slightly open: if it then runs, the cold-start side is at fault. From cold it should spray for about 1.5–2 seconds at 18 °C. An immediate second crank doesn't fire it again, so pause between attempts. A healthy engine starts within about 3 seconds.

With the key on, both injector pins should be live; while cranking, one should drop as the ECU switches it. If not, make sure the trigger wires sit on coil negative, not positive — a wrongly labelled Haynes diagram has misled people. Then unplug and reconnect the bullet connector on the water rail with the key on and listen for a click, and check continuity on to the ECU. On the V12, look for the thin trigger wire shorting to its braided shield a few inches into the loom. The whole injection system earths at the bundle on the rear of the water rail — clean it — and at the ECU plug check the feed on brown/slate pin 10 and the earths on pins 5, 16, 17 and 35. Heat-brittle wires in the ECU-to-engine loom can be open: run a long test lead from each ECU pin to its engine-end pin and expect about 1–10 ohms. A test lamp lit across an injector plug with the key on and the engine still means a short to earth inside the ECU.

Test lamp between coil negative and earth: fully lit with the ignition on, dimming and flickering while cranking. Coil negative stuck at 12 V while cranking means the amplifier isn't switching. Spark but silent injectors: check the shielded white wire under the fuel rail, left side, from the amplifier to the ECU. Inside the AB14 box, test the Zener diode and renew the condenser before buying another module.

Find it, usually on the passenger side, and press the reset button. Then check the white feed at coil positive, the main relay and the pump relay: live at the coil but dead at the main relay points to the diode pack; dead at both relays points to the inertia switch.

Clip a spare plug, triple-gapped, to any lead and compare the spark hot and cold — a clear difference is the tell. Hot, check the injectors still click and the pump keeps running. Measure the temperature sensor at running temperature too. Check the temperature sensor and the auxiliary air valve hot, and clamp the cold start injector hose to rule it out. On the next hot start pull the vacuum hose off the fuel pressure regulator: fuel in it means a leaking diaphragm. With a gauge fitted, pinch the supply and return: pressure still falling means leaking injectors. After a short stop the auxiliary air valve can lag the engine temperature, leaving too little idle air — a touch of throttle helps it catch.

Crank with the bonnet open in the dark: tracking shows as sparks along the leads. Check that the rotor turns and that each plug lead has continuity to its cap pole. Clean the pickup and base plate with contact cleaner.

Hot idle, air filter off: push the meter flap in slightly — rising revs mean a leak. Spray brake cleaner around the hoses, the four manifold vacuum ports, the crankcase breather and the duct; a smoke test finds the rest. Confirm fuel pressure, about 36 psi while cranking.