I have spent a few hours trying to get my fuel gauge to give correct readings with and without the sump oil level button pushed in. After reading quite a few responses to similar questions, I decided to use an ohm-meter to measure the resistance across the level unit on the gas tank. I found the reading to be either zero or infinite with nothing in between, no matter where I positioned the float. I assume this means the unit should be replaced. Can someone confirm this before I order a new unit. Thanks.
Yes, I would say that your level sensor is bad - of course, I will assume that you checked the ohm readings on different levels of the ohm meter, just to make sure the scale you are using is viable - Tex.
Are you sure you’re not measuring the low level warning light sensor?
Think so. I’m measuring from the top connector as you view the sending unit in the tank with the float pointing toward the from of the car. The other connector on the bottom, controls the small light dot at the bottom of the gauge on the dash, when the tank is empty. I seem to recall an article which tells how to fix/repair the sensor if faulty. Are you familiar with this article??
Sorry - " toward the front of the car"
Kris is thinking what I am thinking.
You mention the oil level so presumably this is an early 120.
The green/black wire to the rear is the level sensor. The green/yellow wire to the front is the low light.
According to my wiring diagrams, its the reverse. The green yellow wire which attaches to the top connector while viewing from the rear is the level sensor. If I wire this way, attaching the green/black wire to the bottom connector, the light appears at the bottom of the gauge in the dash. Reversing the wires results in no light. So I’m connecting the ohm-meter to the green/yellow wire and ground.
Glynn,
See attached pics of my as close to 100% authentic loom, routing of wires, and connection ro Fuel Gauge Sender unit, which of course has TOP stamped on it so you mount it correct way. The Loom is from Rhode Island Wiring, the only company I recommend for 100% authentic XK wiring looms with FOUR colour trace wires as per LUCAS original, rather than most offering incorrect THREE colour trace wires, but also note relative length of green/yellow top wire, relative to the green/black bottom wire so you end up with the routing to the loom below the sender unit…as shown
If you are reusing your original braided wires, you will have no colour or discernable trace colour left, so you will need to check each wires continuity to properly identify, and label for future reference…
Re the routing of loom - see picture 2, sorry about focus, but you can see why the loom loops, as it then twists forward and up to be routed/clipped across the rear cross-member as in photo 3 and 4…
Oops, my photos are of an XK140, not XK120 - albeit Fuel Sender wiring connections are both the same…
Thanks for your response. The sending unit on my tank is on the top, under the plywood floor panels, not on the side which is where it appears in your pic. I bought a complete new, braided wiring loom a few years ago so theres no question about wire colors. If Robs pic shows a view of the sending unit while standing at the rear of the car, then the wires, as per my wiring diagram are the opposite. If I turned my sending unit around so the green-yellow wire attaches at the bottom of the pic, as in Robs pic, then of course they appear to be the same. Except that, the lug on which the green-black wire attaches controls the small dot of light on my fuel gauge when I hold the level at the empty position.
Anyway, its late and I’ll have to have another look tomorrow. Thanks to all for the comments.
Thanks - I figured your pics were not for a 120. My loom has a much shorter Green-Yellow wire than the Green-Black wire, again suggesting that the correct position for the Yellow wire is at the top, and the black wire at the bottom.
I vaguely recall that the sending unit only goes in one way on the 120, float arm forwards.
In this position, when the level is low the wiper fingers should be towards the front and make contact with the front terminal.
The terminal to the rear is the variable resistor.
I think you’re right Rob - the float should be to the front. I’ll check later and confirm. Glyn
Rob, Thanks for your help, again! I tried to include a pic of my wiring, without success (Dont see an icon for pasting). Anyway, you’re right - its an early roadster with level indicator for sump oil.
The green/yellow cable attaches to the front lug on the sending unit and controls the light dot on the fuel gauge indicating zero fuel - as you said. The level unit only goes in one way - again as you said.
I adjusted the wiring on the oil/gas switch - I had it wrong, somebody in the past had rotated the switch - and now everything works as advertised, except one item!! Right now, the oil/gas button when depressed shows the oil level in the sump over the range empty to full. Also, when not depressed, moving the fuel leveling float in the gas tank also registers properly on the fuel gauge - over the empty/full range. The only problem is when the float level is at the low position, and the light dot comes on registering zero fuel, the needle on the gauge registers full. As soon as I move the float off empty and the light dot goes out, the needle moves back to zero and then registers the correct fuel level as I move the float to max/full.
Any ideas. Thanks
I would disconnect the warning light sensor and see if level gauge still does this. If it still does go full on low level , the float rheostat is grounding out. Probably need to fix or replace the float sensor. If it acts properly after you disconnect the low level wire, your two sensor wires going to the tank are touching somewhere.
Kris is correct.
Here you can see the wipers at the halfway position.
If they go down to empty position the one wiper touches the light bar.
Notice that tiny wire passing along the top of the resistor. It should never touch the wipers.
My first guess is that yours is touching the wiper at the low end of the stroke.
Rob, you and Kris are both right to a point. When I disconnect the light sensor, the gauge still reads
full at the empty level. I noticed that one wiper touches the coil/resistor bar but the lower one
doesn’t. At empty, with light on, the gauge reads full until I press the lower wiper to the coil. Then
the light stays on and the gauge reads zero. I can’t seem to bend the lower wiper so it meets the
coil when the float is in the empty position. Does this mean I have to change the tank float
mechanism or is there another fix?? Thanks for your help - really appreciated, Glyn
Looks like you can get a new one for $125. That’s what I would do.
1 Like
Before I had mine rebuilt, I had read enough threads on here lamenting the poor quality and function of the reproductions. Maybe they’ve improved since 2016? I had mine rebuilt by West Valley Instruments of Reseda, CA. I’m happy with it.
Thanks Kris. There’s a unit on Ebay for 50 to 60 bucks so I’ll probably get one. I’ve spent a lot
of time messing about for no other reason than to “solve” the problem, Glyn