This checklist will describe the operation of the fuel changeover and provide a diagnostic checklist to aid in troubleshooting. This information specifically applies to Series III 6 cylinder cars but may also be applicable to the later Series II models with fuel injection as well as fuel injected 12 cylinder cars.
The fuel system uses three solenoid operated valves to direct fuel. The dashboard switch applies or removes voltage from all three valves simultaneously. The changeover valve dictates from which tank fuel is drawn when the fuel pump is operated. The left and right return valves dictate to which tank unused fuel is returned to. When the left tank is selected there is no voltage applied and fuel is drawn from, and returns to, the left tank. When the right tank is selected, voltage is applied to all valves and reverses the flow. The left and right valves are identical except their mounting brackets which orient them in opposite directions. A single electronic event (application of 12 volts) effectively "closes" the left valve but "opens" the right. Actually, the same mechanical action takes place in both valves....only their orientation is different.
The fuel gauge reads each tank separately as the switch is changed. Wires from both tank sending units go the the changeover switch. Movement of the switch connects the proper wire to the fuel gauge to obtain a reading.
This checklist is intended to diagnose only electrical faults within this circuit. It is common that the valves may be OK electrically but be contaminated, broken, or stuck, thus rendering them inoperative. A 12 volt test light is required.
- The easiest place to start is with the valves themselves. The change-over valve is in the boot next to the fuel pump. The return valves are located in the rear wheel wells behind semi-rectangular sheet metal covers. Turn the key "on", select the right tank, and using your test light check for voltage at the white/purple wire for each valve. Your light should show 12 volts at all three valves. Now, select the left tank and make the same voltage tests. There should be no voltage at any of the valves. If these checks are OK and the valves won't switch over then the valves are mechanicallt faulty or not properly grounded. Check for proper grounding, if the ground is good then the valve(s) must be condemned.
- If voltage is present at the valves with the left tank selected then the dashboard switch is faulty and must be replaced.
- If voltage is not present at the valves when the right tank is selected then the circuit must be checked. The easiest place to start in the inertia switch mounted at the right side front kick panel (on LHD cars). With the key "on", use your test light to check for voltage at both sides of this switch. Voltage from the ignition switch enters the inertia switch on a white wire and exits the switch on a white/orange wire (turns into solid white down the line). If voltage is not present on both sides then you have a wiring fault from the ignition switch and/or an open circuit in the switch itself. Try pressing the rest button before replacing the switch. If voltage is OK here then.....
- Remove the fuel switch and check for voltage to the switch on the white wire. If no voltage here then you have a wiring/connector fault between the inertia switch and the fuel switch. If the voltage is OK going into the fuel switch.....
- Select the right tank and check for voltage at the white/purple wire. If voltage is not present then the switch is "open" and needs to be replaced. (Incidentally, if you select the left tank and the switch still carries voltage to the white/purple wire then the switch is stuck "closed" and should be replaced.) If the voltage is OK on the white/purple wire (with right tank selected) but the three valves do not receive any voltage then you have a wiring/connector fault between the changeover switch and the valve(s).
Interestingly, the electrical guide shows that this is an un-fused circuit.
Well, that's it for fuel valves 101. Jaguar did a fine job of keeping it simple.