This is a basic "do it yourself" checklist which assumes you have minimal tools and test equipment and that your Jag, which ran well yesterday, won't start this morning. You will need basic hand tools, a 12-volt test light or voltmeter, and a jumper wire with alligator clips. I believe this info to be accurate for all 4.2 Series III cars, but the very earliest models have a combination relay instead of the separate pump relay, main relay, and diode pack.
1) Check for spark. You can buy a spark tester at any parts store which looks something like a sparkplug with an alligator clamp attached If you have one, fine. If not, remove a sparkplug wire and insert a screwdriver into the boot so that it contacts the terminal inside. Hold the screwdriver by the handle and position it about 1/4" from a metal part of the engine. Crank the engine over and watch for a spark to jump the 1/4" gap. If you have a spark, skip to Step 6. If no spark, see step 2.
2) Remove the coil wire from the distributor cap, insert screwdriver as mentioned above, and hold it about 1/4" from a metal part of the engine. Crank the engine over. If no spark, see step 3. If you DO have spark here then you have a faulty cap and/or rotor (spark is getting to the distributor cap but not getting to the sparkplugs). Remove the distributor cap and peek inside. Look for a burned center terminal, burned terminals to the plugs, cracks, or carbon tracks between the terminals. Inspect the rotor contact. If it's bad enough to create a "no start" you'll see obvious burning and/or disintegration of the contact.
3) If you don't have spark at the coil wire-to-distributor cap, then check for power getting to the ignition coil. Power here comes directly from the ignition switch via a white wire. If you have a ballast resistor, the wire will go there first. If no resistor, this wire will go directly to the "+" post of the coil. Remove the white wire and, using the test light, check for 12 volts with the key on. If you have voltage, see step 4. If no voltage, you have an open circuit. This can be confirmed by running your jumper wire from the positive battery post to the coil or ballast. If the motor starts, you have confirmed the open circuit and must now look for a wiring fault or ignition switch fault.
4) If you have 12 volts at the ballast you must check for voltage coming out of the ballast. Reattach the white wire. Using your test light, check the other ballast terminal. If the ballast is good, your test light will light up but no too brightly--- the ballast has reduced the voltage as designed. If the ballast is "open" (no continuity) your test light won't light up at all. If the ballast is OK (or you don't have a ballast) then, with the key still "on", check for voltage at both coil posts. If you have a ballast which checks good at both terminals but no voltage at the "+" coil post, then check for a wiring fault between the ballast and the coil. If you don't have a ballast you should have voltage at the "+" coil post if your wiring is OK, as mentioned earlier. You should have voltage at the coil "-" post as well. If no voltage at the "-" post, you have a bad coil.
5) If everything checks good so far but you still have no spark then you've got an amplifier problem or distributor pick-up problem. Testing of these items is beyond the scope of this checklist (requiring an ohmmeter) but do check the wiring from the amplifier for breaks.
6) You're here at this step because you've determined that your ignition system is working. Now we must check for fuel. Use extreme caution here. Be prepared for some spilled fuel and have rags, a small container, and fire extinguisher handy. Disconnect fuel supply hose at the fuel rail (at the rear of later "straight rail" cars and at the bottom of the early "loop style" rails). Some fuel may spill, have rags and container handy. A properly operating pump will produce about 34 PSI so, again, be ready ! Position fuel hose so fuel will flow into the container. (As an alternative to using a container you can hook up a fuel pressure gauge, if you have one. The usual method of attaching a fuel pressure gauge is to leave the supply hose connected to the rail but disconnect the cold start injector nozzle and use it's nipple on the fuel rail.)
8) Now, locate the fuel pump relay. It is mounted on the firewall right next to the red diode pack and has white, black, white/purple, and white/green wires attached. Remove the relay plug-in socket and determine which terminal cavity corresponds to the white/green wire. This wire goes right to the fuel pump. Using your jumper wire apply 12 volts to this wire for just a moment (you are bypassing the somewhat tricky control portion of this circuit). If the fuel pump is operational you'll get an immediate and powerful surge of fuel through the hose. If the pump does not operate, either the pump is faulty or you have a wiring fault between the relay and the pump.
9) If the pump operates with a jumper wire, plug the connector back onto the relay. The fuel pump circuit is so designed that it will operate whenever the starter motor is engaged so go ahead and engage the starter and see if you get fuel. If "yes", see Step 10. If "no", see the "Fuel Pump Circuit Diagnosis" checklist.
10) Remove the air duct from the front of the Air Flow Meter (AFM). With the key "on", move the air flap which is inside the housing of the AFM. This movement closes a switch inside the AFM and powers up the fuel pump circuit. You should get fuel in your container or a 34 PSI reading on your gauge. If not, see the "Fuel Pump Circuit Diagnosis" checklist.
11) If you have fuel pressure and good spark and still the motor won't start you should check the fuel injector circuit. The injectors receive voltage from the ballast pack (right fenderwell, across from the power steering pump). With the key "on" the two brown/slate wires goind into the ballast pack should have voltage. If they don't you have a wiring fault between the ballast pack and the "main relay" (on the firewall, white, black, brown, and brown/slate wires) or a faulty main relay. The realy can be tested this way: remove the connector. The terminal in the connector associated with the black wire should be ground. The white wire terminal should have "key on" voltage, the brown wire terminal constant battery voltage. If these are OK but there is no output to the brown/slate wire then the relay is faulty.
The injectors are grounded by the boot-mounted ECU. The "firing" event is so fast that is it difficult to check with a test light or voltmeter. The best test is to buy (inexpensive) a "node light" which plugs into the injector connector. If the light blinks while cranking the injector circuit is OK.
If you have spark, fuel, and injector pulse but your engine still does not fire we'll have to check for the third element of internal combustion: compression. Perform a compression test, bearing in mind that for the purposes of diagnosing a "no start" you're primaruly concerned that compression in the cylinders simply exists. The variance between cylinders and/or wet vs. dry test results are useful for other diagnostics but not really required here. If you have extremely low or zero compression then an internal engine problem must be investigated.
If compression (and spark, fuel, and injector pulse) is good and the car still won't run, then problems beyond the scope of this checklist must be diagnosed.