Jaguar XJ40 Cranks But Won't Start
On the XJ40 it's usually a cheap part: a distributor rotor whose built-in resistor has failed while it still looks new, the crank sensor, a relay in the fuel pump chain, the pump or a ground. I'd split the problem first: does the pump buzz at key-on, does the rev counter move while cranking, and are the plugs wet or dry?
How bad is it · Garage, carefully
If it restarts, drive only to a garage
These faults can stop the car on the move and leave it unwilling to restart, and a failing rotor, relay or pump comes back at random. I'd carry a spare rotor, cap and crank sensor until it's 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 distributor rotor: its resistor has failed
The XJ40 rotor has a resistor sealed along its length, and every spark passes through it. When it goes high or open, a weak spark still reaches the plugs — enough for a tester, not for the engine. It looks normal.
Strong spark at the coil lead, weak or erratic at the plugs; the pump primes and fuel reaches the rail. A test lamp at a plug lead can glow on a spark far too weak to fire under compression. A fat blue spark at the coil lead but a thin orange one at the plugs points straight at the rotor and cap.
Fit a new rotor and cap — new, because cleaning an old one proves nothing. A good rotor reads roughly 1 kΩ end to end. Judge the spark with a gap tester, not a lamp. To check fuel at the same time, crack the union between the regulator and the rail while someone cranks: fuel should come out.
A new rotor, and I'd change it as the very first step.
The crankshaft position sensor, or its lead to the computer
Without its signal the management fires neither coil nor injectors. The rev counter reads the same signal, so it doubles as a tester. The sensor's metal shield can chafe against its mounting and short the signal inside, which a resistance check won't show.
The rev counter stays dead at zero while cranking instead of about 200 rpm, and the plugs come out dry; often it died in traffic without warning. A needle that only wobbles near zero doesn't convict the sensor.
Watch the rev counter while cranking. Still: check the sensor plug, then the sensor. A normal resistance reading doesn't clear it — a sensor can measure fine and still give too weak a signal, or fail only hot or only cold. If a new one changes nothing, check the screened lead to the computer and its pin at the connector. Two part numbers split by VIN during the 1991 model year and don't interchange. Back-probe at the computer only with the wiring guide for your year: the blue wire at pin 24, top row, is the sensor's earth and should read no voltage, so finding none there proves nothing. The signal is AC, not DC: across the computer's yellow connector pins 13 and 24 while cranking it should show about 1.3 V AC. Look over the sensor body and lead for chafing.
A new sensor, or a repaired lead between sensor and computer. I'd carry a spare sensor in the boot. The early sensor (DBC2139, round three-pin plug, to about VIN 629285) and the later one (DBC11501, triangular plug) differ only in the plug; a later sensor can go on an early car with its plug changed, crimped rather than soldered. Cheap aftermarket three-wire sensors fit; the later X300 type has two wires, so count them first.
A relay in the chain that powers the fuel pump
On many XJ40s the oxygen sensor heater relay energises the fuel pump relay, and both depend on the engine management main relay. A relay that clicks can still have burnt contacts, fail to pull in hot, or work loose in its socket. The coil and amplifier are fed through the ignition relay in the white base, first of the row on the bulkhead; burnt contacts there can drop their supply to about 9.5 V with a full battery.
No half-second pump buzz at key-on, or ten or twenty attempts and then it runs perfectly; worse after standing a couple of hours, or long cranking only when cold.
Swap the bulkhead relays for identical ones, pushed fully home. Bridge the pump relay's power terminals: if the pump runs, work back through the relay that drives it. A test lamp on the pump side of the relay, watched while someone cycles the key, shows whether power arrives every time. Measure the white/pink feed at the amplifier with the key on: more than a volt below the battery points at that relay. Pulling the bulkhead relays, cleaning their pins with contact cleaner and a smear of dielectric grease, then refitting them swapped, has cured months of intermittent no-starts.
A new fuel pump relay or oxygen sensor heater relay, reseat a loose one, clean corrosion on the linked relays, or unplug and refit the engine computer's connector to clear oxidised pins. Suspect any relay handled during a recent pump job. On a 1994 car that started one try in ten or twenty with no voltage at the pump, the engine management main relay was the fault — found by following the workshop manual's pump-circuit test for that VIN range.
The pump itself, or its connector at the tank
An old pump can stop on a dead segment and draw current without turning. The connector at the tank flange can burn and stop passing current; torn in-tank seals starve the pickup.
The relay clicks and 12 V reaches the pump plug, but no whirr; a knock on the tank sometimes brings it back.
Put a few litres in first: on a slope a nearly empty tank leaves the pickup dry. Then voltage at the pump with the relay bridged. Present but silent: tap the tank with a rubber mallet while someone cranks. Look for burnt pins at the flange connector.
A new pump and filter, or repaired terminals at the tank connector.
Grounds, harness connectors and cranking voltage
The management won't fire if its supply sags below roughly 10 V while cranking, and the coil and ignition amplifier share ground points on the bulkhead. A corroded ground or connector gives erratic spark.
Erratic rev counter while cranking, a VCM CIRCUIT 1 message, dash lights flickering, the battery gauge dropping to 9 V.
Clean and grease every ground at the bulkhead posts and the engine strap, and both battery terminals. Measure at the coil while cranking.
Clean grounds, a tight engine strap bolt, a repaired open circuit in the main harness, or secured connections where an aftermarket immobiliser was taken out — they shake loose and cut the coil feed.
Water in the distributor, a weak coil or a worn coil lead
The distributor's drain hole must sit at the bottom. If the distributor has ever been turned, the hole can end up on top and a wet week fills the cap. A tired coil makes a thin yellow spark instead of a blue one, and a worn lead from coil to cap lets the spark jump to the wrong place — both worse in the damp.
It stopped after rain or a wash, or sputtered through puddles first; a yellow spark at the plugs; arcing around the coil in the dark while someone cranks.
Cap off, look for water, dry it and spray water-dispersant. Check the coil connections are tight. Crank in the dark and watch the coil and leads; then try a borrowed coil and coil lead.
Dry the cap and it fires; repair a bad coil connection, or fit a new coil or coil-to-cap lead.
The ignition amplifier or its plug; the engine computer last
The amplifier grounds through its own body, so a new one not bolted down firmly acts dead. It can also fail inside — a supply pin shorted to its case — while the sensor, the coil feed and the rev counter all look healthy. The computer fails far less often than either.
No spark at the coil lead with 12 V present and a moving rev counter; sometimes a faint spark tracking down the engine bay at night.
Clean the amplifier plug under the coil first. Then a test lamp from coil negative to ground: pulsing while cranking means the amplifier is switching and the coil is next. Measure across the coil's own two terminals, not each to ground: 0 V there points at the amplifier.
A cleaned amplifier plug, or a new amplifier bolted down firmly. For the computer, borrow a known-good unit rather than buy one. Where a Lucas amplifier can't be found, a Bosch module of the kind used on late-1980s Volvos has been substituted; it earths through its plug, needs rewiring and must suit the coil.
The throttle sensor plug: the computer thinks the throttle is wide open
With the throttle floored, the computer deliberately cuts the injectors to clear a flooded engine. Water or a loose pin at the throttle sensor plug can feed it a permanent full-throttle signal, so it never fuels while cranking.
Good spark, the rev counter moves, the rail holds pressure, but no pulse at the injectors; often straight after an engine wash or after cleaning connectors nearby.
Unplug the throttle sensor. The computer side should give about 5 V on the supply pin; the sensor's signal should sweep from about 0.24 V closed to about 4.9 V wide open. A signal sitting at 5 V with the throttle shut is the fault.
Clean and dry both halves of the plug, squeeze the socket pins slightly for a firm grip, and re-seat it.
Flooded: a short cold run, the coolant sensor or a leaking regulator
The XJ40 has no cold-start injector: the computer enriches through the injectors on the coolant sensor's word. Stop the engine a minute or two after a cold start and the extra fuel stays in the cylinders. A sensor that reads too cold, or a holed diaphragm in the fuel pressure regulator, floods it every time.
Plugs come out wet and smelling of petrol, with a good spark; often it follows a very short run from cold, or hard starting for weeks.
Hold the throttle floored while cranking, or pull the pump relay and crank, then try a normal start. Pull the vacuum hose off the regulator: any fuel in it means a holed diaphragm. Measure the coolant sensor against the table in the manual.
Clear the flood and give it a proper drive; fit new plugs if they're black and fuel-soaked, and a new coolant sensor or regulator if either fails its test.
Cam timing out, after head work or a slipped cam gear
The two camshafts look almost identical apart from a timing slot, so after a head gasket or chain job they can go back a few degrees out, or on the wrong sides. A cam gear can also slip on its coupling and stop the engine at speed; the AJ6 is an interference engine.
Spark, fuel and compression all check out, yet it won't fire or sounds out of time; it follows head work, or it stopped at speed. After a gear dropped and the engine over-revved: cranking that goes fast then slow, and spark that comes and goes.
Distributor timing alone won't show it. Take the cam cover off and check both cams sit exactly at TDC on number 1 firing stroke; never turn one cam on its own.
Re-time both cams; a slipped gear needs new cam gear parts and a check for bent valves.
Low compression
With spark and fuel both arriving, an engine that only sputters and smokes may simply not be sealing. On the AJ6 it is more often a head gasket or burnt valve seats than worn rings, which rarely go before 200,000 miles.
It catches when you pump the throttle, sputters, puffs a cloud of smoke and dies; plugs come out wet even though nothing else is wrong.
A compression test with the throttle held wide open: all six should be within 10–15% of the highest. Squirt a little oil into a low cylinder and retest — a jump means rings, no change means valves or the head gasket.
A cylinder low from worn rings meant an engine rebuild; the car was broken for parts.
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.
Key to II: listen for the pump
A half-second whirr from the rear means the relay chain and pump are alive. The 2.9 doesn't prime, so silence there is normal. On a slope, make sure there's more than a splash in the tank.
freeCrank and watch the rev counter
No movement points to the crank sensor or its wiring; about 200 rpm means its signal gets through. A needle that only wobbles near zero proves little.
freeFit a new rotor and cap
The cheapest part and the one that most often decides it. Don't reuse a cleaned one.
about $30–50Pull a plug: wet or dry?
Wet and smelling of petrol: clear the flood with the throttle floored, then look at the coolant sensor and the regulator's vacuum hose. Dry, with spark and a moving rev counter: clean the throttle sensor plug.
freeSwap the bulkhead relays, and bridge the pump relay
If the pump runs bridged, work back through the relay that energises it.
freeClean every ground; measure voltage while cranking
At the coil. Under about 10 V the management won't fire.
freeThen the coil, the amplifier and the computer
Clean the amplifier plug, then a test lamp on the coil negative; try a borrowed coil and lead. Borrow a known-good amplifier or computer rather than buy.
from cheap to expensiveAfter head work, check the cam timing
If it stopped working after a head gasket or chain job, check both cams at TDC before anything electrical.
a cam cover offWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Fuel Pump ProblemsIf the pump stays silent at key-on, or it dies on the move for lack of fuel.
- Jaguar XJ40 Rough Idle and StallingIf it starts but idles rough, hunts or stalls at junctions.
- Jaguar XJ40 MisfireIf it starts but misses or shudders under load, or cuts out in the wet.
- XJ6 (XJ40) fuse boxes and relaysOther years from the same page.
- Jaguar XJ40 Won't Crank, Just ClicksIf it won't turn over at all, just a click or silence, start there.
- 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.
