I know this sounds like a pretty trivial and maybe boring item,but it would be nice if it worked. When I got my Jag I decided that I was going to bring it back as close to as-new as I possibly can. So I made a list of everything that was wrong, both big and small. I am down to "make the gas cap door work".
Now this should be pretty straight forward. The original solenoid mechanism was frozen. Someone had simply cut the wires and used the emergency release in the trunk. So I got a replacement and it checks out on the bench.
There is a yellow wire in the area that is cut and disconnected. I have looked at the schematic for this circuit and it looks like it is a signal that should go high to activate the solenoid. I would expect the other wire(black) would go to ground. I don't see any fuses in this circuit, and no relays either. I get no signal at all on the yellow wire. I am thinking that maybe the output circuit of the chassis module is fried, or maybe the signal from the gas door button is not reaching the module. Any ideas on how to prove this? Where is the module that controls this function?

 

According to the diagram it is an Orange wire, not yellow, for the +12V signal that drives the solenoid along with a black wire to ground IN THE TRUNK.

It is controlled electronically out of the Body Processor Module (BPM), behind the glove box. It is much more likely a switch or wiriing issue rather than a BPM problem.

There is a yellow wire that is connected to ground at the DASHBOARD SWITCH to trigger the solenoid though the logic in the BPM.

 

Thank you for that "whitexkr" It was sort of dark in the trunk area, maybe it was more of an orange wire. It seems to physically be where I would expect it to be. I will check again tomorrow. Now that I know where the BPM is located I can probably trace the circuits. The trunk release works. I don't know if they have any wiring in common,but I will check inot that too. Thanks again for your input.
Lanny

 

There is a Security & Locking Control Module located in the trunk in which an Orange wire goes to the Fuel Door solenoid to operate the fuel door (along with a black ground wire). The Orange wire is located in the 16 pin connector in position 2. The trunk release wire is also an orange wire to release the trunk in position 1. You may be able to trace the orange wire back to the Security & Locking Control Module to check for power.

 

I'm sure you know this but the car is picky about when you can open the flap.
I THINK you have to be in Park, key position 2.
It won't open with ignition off or with the engine running.
It won't open with a queue of irate motorists behind you when you are at the only working pump on a Sunday morning.

 

Thanks Steve, I really did not know that it was "picky" about when it would open the gas flap. The last sentence was amusing, I would like to add it also might not open when the fuel gauge says "empty,really empty". This happened when I took delivery of the car and the trucker had jumped in his truck and made his git away. At that time I did not know about the emergency release inside the trunk. I was ready to pry it open with my pocket knife, but a kindly homeless guy came by and slipped a credit card into the slot and popped it open for me.
Why in the world would they make a simple thing like popping open a gas flap a major event? A simple wire from the switch to the solenoid would work just fine, as would a small magnet to hold the door shut.
Ah, the wonders of owning a Jaguar......

 

The design philosophy of this and most modern cars is that the on board computers know and control the state of everything. Also to save cost and allow design flexibility all switches handle only very small signal currents.

 

Well, today I tore up my car, tracing the signal from the fuel door switch toward the actual solenoid that opens the latch. I traced the signal from the switch to pin#31 on the BPM.(behind the glove box) Then I tried to trace the signal from the BPM to the SLCM module in the trunk. That was not so easy since it is evidently a data stream signal. so I jumped to the output of the SLCM (pin#2,org wire) to see if there was a signal coming out of that point. Nothing. My trunk lid still works, so I made myself a signal light that I could jump onto the output to see what a working signal looks like. There, on pin#1 I got a light for about 2 seconds that activates the trunk solenoid. So it looks like my SLCM output is dead on pin#2,or it just is not getting the signal it needs on the SCP data stream. The solenoid that was in the car when I got it was frozen, so I guess it could have taken out the module on that output.
I called the local Jag wrecking yard to see what a used module would cost. It has to be matched to the vin# in the Jag, and the cost is $250. This is a little bit much to just open the gas flap. So tomorrow I intend to access the switch on the dash, and hi-jack the yellow wire. Then I will pull a wire from the back of the dash, through the sill plate and connect it to a small relay. The relay will pass power to the solenoid when I ground the dash switch and open the gas flap. Maybe not as elegant, but simple. I really don't think I need a "committee" to decide if I can open my gas flap. Then on to trying to fix my roof latch issues.

 

Seems like a sensible plan. The relay or a transistor is definitely a good idea - as the other Steve said the dashboard switch may not be up to some amps and you don't want to fry it.