Jaguar Repair: 87 XJS wont start, fuel pump relay, piston engines


Question
QUESTION: Hi...thanks for the advice.

My girlfriend as a 87 XJS  v-12 that sat covered outside in Michigan over the winter.  It ran fine last Fall.  Today it would not start.  Fuel pump was inop so I jumped it, now there is fuel pressure at the rails but still no start.  Any suggestions?   She's frantic.  Thanks

Jim

ANSWER: Hi Jim,
In a "No Start" situation you need to just go to the basics. You have started that by getting the fuel pump to run. However, you need to think from scratch. All piston engines only need 3 things to run, compression, fire and fuel. With conditions on each.
The least likely problem is compression so you can first look at the other two.
You have a symptom already, you said you had to jump the pump meaning the relay didn't do it's job. The fuel pump relay is activated by a "Main Relay" which also powers up the injectors. So if the main relay didn't activate the fuel pump relay, it would not power up the injectors either.
The fuel pump relay and main relay are in the trunk on the right side with the ECU. The relay to the front is for the pump and the relay toward the rear is the main relay.
The end result is that the main relay powers up all of the injectors and the ECU grounds them in sets of 3 at a time. Have someone spin the engine over while you put your fingers on several injectors. You can feel them "Clicking" and some times you can even hear them clicking. If they are not clicking go the the "main Relay" and check the main relay. Sometimes when a relay is not used for a long time the contacts don't connect. An old mechanics trick when a relay has failed from setting a long time is to have someone turn the key to the start position and you rap on the relay with a small wrench. This sometimes shakes the relay enough to activate it. If however, you find the relays are not activating you will need a test light and a diagram of the wires going to each relay. If you need to go that far I can supply you with what wire does what so you can just take a test light and test the circuits that are important.
let me know.
Howard


---------- FOLLOW-UP ----------

QUESTION: Howard....thanks for helping us work this out...I replaced the fuel pump relay....took the cover off the main relay which makes contact when the key is turned on.  However, the fuel pump relay does not plus there is no current to any lead going to the front (fuel pump) relay.  Anne says that she used to see oil pressure rise on the guage while initially cranking before the start. Now is doesnt. Does this engine have a oil pressure safety switch? I checked the relays by the starter relay and they all seem good.

Baffled!~!!!!!!!~~~~!!!   any ideas now....

Jim

Answer
The main relay is powered up by the Ign. sw. through the Inertia sw on the drivers door post. (white wire on terminal #86 on the main relay) Terminal # 30 should be hot with a brown wire.
When the key is on and the main relay activated, it supplies power via the pink/black wire to terminal # 86 on the pump relay. (terminal # 30 on the pump relay should be hot via a brown wire.) Terminal # 85 of the pump relay is grounded by the ECU to activate the pump relay and send power to both terminal # 87s to power up the pump and an aux air valve timer.
If you need a diagram, send me your e-mail address (fitzcharlesh@bellsouth.net)and I will e-mail one to you.
Howard