Fuel tank switching, WP wire

Hello to all,
I have a 1985 VDP that won’t switch over to the right fuel tank. I went over the whole fuel change over checklist. This what I have found so far:
With the fuel switch pressed outward, I have 12 volts in and out of the inertia switch. I pulled out the fuel switch and found 12 volts on the WP wire at the switch when right side is selected.
With the fuel switch selected for the right tank, none of the three solenoid valves have power. Each return valve and change over valve has a good ground checking with a power probe and multimeter.
I found the a loom of wires running on along the right side of the rear trunk with the white/purple wire. With the power probe, I pierced the WP wire and I found no voltage, but I did get ground instead.
I found ground also at the change over valve on WP wire and at the connector at rear right light area. It’s weird to me that I have 12 volts at the fuel switch but the ground on the WP wire in the trunk area.
Thanks for any help.

Is this with the fuel selector switch plugged in? Or disconnected?


Questions arising; with the switch ‘out’ right tank is selected - and there should indeed be power on all white/purple wires. With power on white and the switch in the correct position(!) - no power on white/purple implies a faulty switch. With the switch pushed ‘in’ there is no power on the wires…

Ohming the white/purple can only be done with power/ignition off - ohming with power ‘on’ is of course meaningless. As the wires are grounded through the valves - you will read the valve coil resistance in parallel, very near 0 ohms. Also, with the switch ‘out’ you will also probably read ground back through the ign key…which confirms connections…

xj6 85 Sov Europe (UK/NZ)

Hey Doug,
I was able to open up the black rectangular connector enough on the fuel switch for me to get a reading of 12 volts on WP wire. When I press fuel switch in, I lose 12 volts on VP wire.
The fuel switch is new replaced by the PO. One reason he sold the car was that he couldn’t figure out how to get the right side fuel tank to work after replacing 2 solenoids and chasing down the wiring for years he said.

Hi Frank,
I didn’t check for ohms yet but I will.
I believe I read ground on the WP trunk wire no mater if ignition on or off and fuel tank selector in or out but I will recheck.

A great source of problems could be the switch itself.

Take the switch of and make sure all contacts do contact the way the should according to the electrical diagram.

Which ones?
Are you sure your main solenoid works electrically and mechanically?

The fuel changeover solenoids are either ON or OFF.
The return valves are NOT the same. One is NORMALLY OPEN and the other NORMALLY CLOSED. When power is applied they CHANGE STATE.

Only the sender part of the switch has continuity on both positions. The other is ON or OFF. (power in one position only).

Here is the electrical guide for the system.

Jag_SIII_FuelTank.pdf (589.4 KB)

I thought when I press fuel switch in, that would be for the left tank which it supposed to not have 12 volts to the WP wire?
OP replaced the change over valve and the right side return valve.
The left side return valve looks original.
Not sure if any solenoid valve actually work as designed since I don’t have 12 volts going to them at this time.
I will recheck the fuel switch.

Thanks for the electrical diagram motorcarman. I do have that diagram. I took both rear tires off so I can get to each return valve and change over valve in the trunk.
Again, I don’t get 12 volts on the WP wire no matter if I press the fuel switch in or out.
Also wanted to add that the fuel gauge works when switching for each tank.
In the diagram, I did find the CP9 connector by pulling away the carpet near the transmission tunnel so I test for voltage there.

Sounds like it’s an electric problem rather than the solenoids themselves.
Before you start dismantling stuff apply 12v directly on the solenoids and see if they do click.
If they do chances are that they work. They might be leaking but that’s for later.

As Motorcarman said, in one position of the switch (I think Left side) none of the solenoids have power, and on the other position they all get power.
You must find why the WP wire does not get 12V when it should.
Start from the switch and go downstream.

Once you establish that the switch does indeed switch the solenoids On and Off then you can check if the solenoids actually work as they should.

1 Like

Pressed ‘in’; ‘left’ tank selected, jdere - no power on white/purple is correct. In ‘left’ all valves are unpowered!

All valves then go their defauWith power present on white/purple
lt positions; changeover to ‘left’ tank, ‘left’ return open and ‘right’ return closed. With power, ‘right’ selected; all valves reverse positions…

But any valve may fail, powered or unpowered - identified by symptoms and simple testing…

xj6 85 sov Europe (UK/NZ)

Here is the latest. I checked for 12 volts at the fuel switch. I think the connector to the fuel switch must have been loose because I do now get 12 volts on the WP wire in the trunk area and at the changeover valve. I can’t easily check the return valves for voltage since I need to take tires off.
I do notice the left side tank has fuel coming out of the return line if fuel switch pressed in or out. No fuel is returning if I press the fuel switch in or out on the right side tank.
The changeover valve and right side return valve are both new.
The left side return valve looks original to the car. The PO left me a box marked right side return valve. Can I some how install it in a certain direction on the left side or just locate a left side return valve?

The fuel changeover solenoids are either ON or OFF.
The return valves are NOT the same. One is NORMALLY OPEN and the other NORMALLY CLOSED. When power is applied they CHANGE STATE.

This is plain wrong, jdere - fuel should only(!) return to the selected tank.

The test you perform is a standard test to verify if the return valves are working properly - which they obviously are not. Three scenarios…

  • There is no power to the return valves with switch in either position - fuel always returns to the ‘left’ tank, never ‘right’. Switch is not working/wrongly connected…

**Power on white/purple at the changeover valve, switch ‘out’, no power with switch ‘in’, implies that the switch is working. Only one white/purple wire at the switch works all valves. interconnections are ‘somewhere’ and may fail. Checking power at return valves is the best option…

***The return valves are sided, ‘left’ and ‘right’, and are not(!) interchangeable. However, it is possible, with difficulty, to physically mount a ‘left’ valve into the ‘right’ valves place - in which case; there is no return to the ‘right’ tank, irrespective of switch position. However, if there is power to the valves; both valves would be closed, no return to either valve, with ‘right’ selected…unless the ‘left’ valve is stuck open…

****The ‘right’ valve is stuck closed and the ‘left’ valve stuck open - which would mirror your symptoms perfectly, but would be ‘unusual’…

The problem is that all items may fail independently, mechanically and/or electrically - and in any combination.

Your electric tests are somewhat dubious. Disconnect white/purple from the changeover valve, and using a test lamp to verify power on the wire with switch ‘out’ (‘right’) and no power with switch ‘in’. With power on wire touch the valve terminal to check if the valve responds. If so; valve and power is OK - responding to switch operation…

After that; I’m afraid you have to access the changeover valves to do the same tests there…

The changeover switch has two entirely independent functions; power to the valves; white and white/purple. And light green/orange from fuel gauge and two light green to the tank units as gauge ground.

xj6 85 Sov Europe (UK/NZ)

As everyone said before, you can’t progress if you are not sure that all the solenoids get power when they should.
My primary suspicion is that there is something wrong with the switch and/or the contacts.
Pry the plug open, connect it to the switch and check the contacts with a multimeter and the wiring diagram at hand. Then move on to the solenoids.
No other way around it.

Thanks for everyone’s help. The latest is this: the drivers side return valve was bad.
I replaced it and everything switches over fine now. The latest problem that I’m working through is that once I got the passenger side fuel tank working, I sent a bunch of junk through the fuel lines and ended up with a no start issue. It’s seems the cold start injector was either clogged or not functioning. I replaced it and the car starts and runs. I cleaned out both fuel tanks by taking out the bottom plugs and draining all the fuel tanks. I still get brown colored fuel out of the passenger side fuel tank so next plan is remove both fuel tanks and clean them out thoroughly. i have been researching about using the Por-15 tank cleaner and tank epoxy. Does anyone recommend a different product that has been successful for them?

You suppose the fuel valve wires are only “hot” when the ignition switch is on crank or run???

Great success! Another instance of “fix things and get approriate results!”

As long as you only get “brown fuel”, but no serious residues, since you already flushed the tanks, I’d throw in a cheap one-way fuel filter, preferably before the fuel pump, but - since your tank is full and everything seems to work properly - even before the FI should work, and “flush” the tank that apparently has been unused for some time due to the broken return valve with some more fresh fills of gas. Unless you get fuel smells or other hints to a faulty tank I’d refrain from pulling it out.

Good luck


75 XJ6L 4.2 auto (UK spec)