See also the library article No Start Troubleshooting, Series III XJ6 XJ12 for more information.
The purpose of this checklist is to explain the operation of the fuel pump circuit and provide an easy-to-follow fault diagnosis procedure. The only tools needed are a 12 volt test light and a length of jumper wire. I believe it to be accurate for all Jaguar Series III XJ6 cars but some of the earliest cars may be different.
There are three main components to the system: the trunk-mounted, constant pressure electric fuel pump, a control relay, and a voltage directing diode pack. The system is so designed that the pump will operate when the starter motor is engaged and when the motor is actually running. For testing or cold-start :priming", the pump may be operated by putting the transmssion into "drive" or "reverse" and turning the key to the start position. This activates the circuit but the starter itself does not engage. Twelve cylinder XJ12 cars, by the way, are wired differently.
The diode pack is mounted on the firewall and is easily identified by it's all-red color. The fuel pump relay is normally mounted right next to the diode pack but I've seen a couple of cars where the pump relay and the main relay have been flip-flopped. So, pay attention or you may end up checking the wrong relay. The fuel pump relay has the following wire colors: white, white/purple, white/green, and black.
1) First let's see if the pump itself actually operates. Remove the connector socket from the fuel pump relay. Determine whcih connector/terminal cavity is associated with the white/green wire (this wire goes directly to the fuel pump). Attach your jumper wire to a known good 12 volt source (the battery "+" post makes good sense) and touch it to the terminal of the white/green wire. You have now bypassed the control portion of the circuit and the pump should operate (you can hear it easily of you remove the spare tire cover panel or, better yet, use a fuel pressure gauge). If the pump operates, go to Step 2. If it does not operate you have a wiring fault between the relay and the pump or a faulty fuel pump.
2) Reattach to connector to the relay. Now the fun begins. Get your test light out......
3) The black wire at relay terminal 85 should be ground at all times. If not, you have a wiring fault and/or loose ground point at the rear of the water rail. Trace out the fault or run a redundant ground.
4) The white wire at relay terminal 30 should have 12 volts whenever the key is "on". Voltage flows from the ignition switch and goes through the inertia switch. If there is voltage at the white wire, go to Step 5. If there is not any voltage at the white wire then you have a wiring/connector fault, an "open" inertia switch (try resetting with the button jumping it), or a faulty ignition switch ( try jiggling it).
5) The white/purple wire at relay terminal 86 and the white/green wire at terminal 87 should have voltage anytime the starter is engaged. Crank the engine and check for voltage at both terminals. If voltage is present at the white/purple wire (terminal 86) but not at the white/green wire (terminal 87) then the relay is faulty. If there is no voltage at terminal 86 when cranking the engine, then go to Step 6.
6) Locate the silver metal starter relay (furthest inboard of the four firewall mounted units) and, while cranking the engine, check for voltage at the white/yellow wire. If no voltage the starter relay is faulty and not sending voltage downstream to the diode pack. If voltage is present then locate the red diode pack and, again, crank the engine and check for voltage to the diode pack at terminal 3 (white/yellow wire) and at the diode pack at terminal 5 (white/purple wire). If there is no voltage at terminal 3 when cranking then you have a wiring fault between the starter relay and the diode pack. If voltage goes is present at one side of the diode but not at the other, then the diode pack is faulty. Now, if voltage leaves the diode pack but terminal 86 of the fuel pump relay does not get any voltage then you have a wiring fault bewtween the diode pack and the pump relay. I might add that, in some cases, the diode pack may have proper continuity but an internal fault can cause a voltage drop with a resultant problem downstream. You'd need a voltmeter, not a test light, to properly check this. Not too bad so far, eh?
7) Now, if the fuel pump operates when cranking but won't continue running as the engine starts to fire then we have to do some more checks. First, some background. There is an air flap inside the AFM which, when moved by engine vacuum as the engine starts, closes a micro-switch inside the AFM and powers up the fuel pump circuit. With the key "on", check for voltage into the AFM at the brown/slate wire. If no voltage then you have an open circuit between the AFM and terminal 87 of the "main" relay (next to the starter relay). If voltage into the AFM is OK then you'll have to simulate engine vacuum by manually moving the air flap inside the AFM. Simply remove the AFM front air duct and, with the key "on", move the air flap and check for voltage at the pink/blue wire.If voltage is OK, then see Step 8. If no voltage, then the internal AFM switch is faulty.
8) Voltage coming from the AFM goes to terminal 2 of the diode pack (pink/blue wire). If no voltage reaches the diode pack then you have a wiring fault between the AFM and the diode pack. Voltage exits the diode pack at terminal 5. If voltage is not present at terminal 5 then the diode pack is faulty. If voltage leaves the diode pack (white/purple wire) but does not reach terminal 86 of the pump relay then you have a wiring fault between the diode pack and the relay.