QuestionQUESTION: When the button is pushed to switch to the right tank, the guage shows below empty. The tank is full however, and then it drains through the overflow tube until it is switched back. What do you think? Thanks for your time and knowledge.
ANSWER: Hi John,
All of the XJ=6 with the two tanks use a switching valve in the trunk to draw from which ever tank you select with the dash switch. At the same time the tank sending unit is also selected by that same switch. Also, a return valve is activated (located at the bottom of each rear wheel well)
Most of the fuel pumped forward to the engine is not used and sent back to the tank that you have selected. However when one of the return valves fails and you have selected that tank, the returning fuel will be sent to the other tank. The system is designed so that if any valve or the switching valve fails (electrically) it will default to the left tank. Meaning any ELECTRICAL failure and you can use the left tank normally. However, if any valve sticks in the wrong position you can have many odd symptoms. In your case of the false reading of fuel level on the right tank you must check the wiring and the sending unit. (in behind the tail light assembly) (DON'T attempt to remove the sending unit with a full tank)
The return fuel switching valves (under a plate in the lower wheel well) on the LEFT side is normally OPEN and closed by electric. The RIGHT valve is normally CLOSED and opened by electric. The trunk tank switching valve is OPEN to the LEFT tank normally and the left side is closed off and the RIGHT side is opened when electric is applied.
If you need a wiring diagram of the tank system let me know and I will put one up on my web site for you to copy.
Howard
---------- FOLLOW-UP ----------
QUESTION: Hi, I put a test light on the green wire connecting to the sending unit, and the gas guage went up to half a tank. It's a "screwdriver like" tester with a wire. The tester did not light up, so I guess there is no power that close to the fuel tank. The green and black wires from the sending unit go to a 4 prong plug. When I unhook the plug, and then put it back, I here a clicking sound on the right side by the tank somewhere. There is power to all the purple and white wires. There are also two wires running together that are taped. One is black, the other is red with grey. They look like they are just unused, for another option, no power. So, is a ground wire off? How do I fix from here? Thanks, John.
AnswerJohn, there are only two wires that have anything to do with the fuel tank sending unit. A light green wire (from the gauge) and a black wire (ground)
To make a rough test of the gauge turn the switch to the tank that you are testing and remove the light green wire from the sending unit and while someone is watching the gauge, ONLY touch the light green wire to ground. DON'T leave it connected just touch it and remove it from the ground. The gauge should deflect fully and when you remove it from ground the needle should go all the way to below empty.
If it does that then drain the fuel from that tank and remove the sending unit. Now with a rag stuffed in the hole of the tank, connect a jumper wire to the light green wire and another to the black wire and away from the tank and the rag, connect the two jumpers to the sending unit and turn on the key and manually work the sending unit up and down and watch the gauge. If the gauge don't go up and down then the sending unit is bad.
Most likely what you are going to find is that the arm on the sending unit is stuck with rust. I have been able to free them up with WD-40 and work the arm up and down and thus save about 50% of the ones that I found stuck.
The over flowing of the tank has nothing to do with the gauge and sending unit so we will have to work on that separately AFTER you have fixed the gauge reading.
You need to do only what I say to do and not fool with any of the wiring. Let me know when you have corrected the gauge reading and I will walk you through the procedure to correct the tank fuel system.
Howard
