XK150 fuel tank
- Fuel level sender problems and tank diagram

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At the end of 2016, after more than three years of ownership, I finally figured out why the car's fuel gauge did not work properly. It's a long story. It's worth remembering that the presence of the spare wheel housing means the lower half of the tank has only about 2/3 the fuel capacity of the top half. I estimated that the top half of the tank holds 36 litres, and the bottom half 20L, after allowing for the fact that in my tank, there is around 5L of unuseable fuel below the fuel suction pipe.

Tank dimensions diagram in jpg format (397 kB). Same diagram as pdf, with approx. fuel capacity data, for my after-market aluminium tank.

When I bought the car, there were two fuel gauge faults:

The bulb for the low fuel level warning light was incorrectly installed - the bulb's connection to ground via the low level contact in the sender unit was open-circuited. That probem was easy to fix, although the low level indication did not ever come on, because ...

 ... the level indication was always 3/8ths full, irrespective of the amount of fuel in the tank. The sender unit itself was faulty.

Early in 2014, I fitted a new (NOS) sender unit to the tank (the tank is an aluminium replacement fitted by the previous owner). But no matter how I bent and configured the sender's wire arm, I could not get the sender to register an approximate tank fill level over the full range. I gave up as there was no obvious solution. At least the warning light came on sometimes.

In the course of fitting and testing the new sender, I confirmed that the fuel gauge was working reliably by using various resistors to simulate various tank fill levels.

In March 2016, I nearly ran out of fuel on the return leg from the National Rally in Caloundra. The car started to miss, despite the fact that the warning light was not lit. A filling station was nearby, so I dodged a bullet on that occasion.

Late in 2016, I finally set aside the time to fix the sender and the warning light function for once and for all.  I decided to measure the tank to put some science into shaping the sender's wire arm so it would function correctly. To assist with measuring and then drawing the tank, I referred to various photos of new XK150 tanks I found on the internet.

In one of the photos, I saw that the six tapped blind holes that accept the 3mm screws that hold the sender unit to the left end of the tank were oriented so that the hexagon shape that passes through the six holes had the flat side at the top and bottom, parallel to the bottom face of the tank. A close examination of the Jaguar parts manual diagram of the tank confirmed that this is the correct drilling pattern.

At that point I realised why my sender would not work correctly. The six 3mm holes in my tank were 30 degrees out of alignment. The sides of the hexagon on my tank's holes were parallel with the front and rear slanted faces of the tank. The float arm was not moving in a vertical plane; instead, the arm was moving in a plane slanted 34 degrees forwards when moving upwards. Depending on how the sender arm was bent, the float would hit the front face of the tank, or the face of the spare wheel insert in the lower half of the tank. No wonder it would not register the correct tank level.

Since I was to unable to safely drill and tap six new 3mm holes in the tank with the tank in situ, I decided to drill a new set of six holes in the sender mounting face. The new holes were drilled 20 degrees CW from the exiting holes, with just enough room to insert the standard 3mm screws adjacent to the two wiring terminals.

The plane of movement of my car's sender arm is now slanted 6 degrees to the rear of the car as the arm moves upward. Not perfect, but far better than before. The float is positioned on the front side when attached to the wire arm.  The float and arm misses the spare wheel insert at low fuel level, and there is nothing to hit to the rear once the fuel level is at 40% or more. Problem solved, at last!

Some other useful information:

The sender resistance is 4 ohms when at its lowest position, and 85 ohms at its highest position.

The sender's plastic float is submerged about 22mm when sensing the level of gasoline (RD=0.71) and is attached to its wire arm.

In the lower part of the tank, the level rises 1 cm for each 1.6 litres added. In the upper half, the level rises 1 cm for each 2.3 litres added.

To get the XK150 fuel gauge to register an accurate level over the tank's capacity range, a microprocessor is need to "condition" the sender's resistance to account for the tank's internal volume change vs height. Now there's a project!!!

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