Classic/Antique Car Repair: XKE Fuel Gauge, wound resistor, zero ohms

Question
Hi Howard,

I recently purchased a 1970 XKE and the fuel gauge will not register anything higher than a quarter full.  I can get the gauge to read full by shorting the line from the sending unit.  I only did this for several seconds to ensure that the gauge would respond.  I've measured the impedance at the sending unit and get a reading of 155 ohms with a full tank.  This is compared to 24 ohms for my 1967 MGB, with a full tank.  I don't know if these two Smiths gauges are calibrated the same and should have the same relationship between resistance at the sending unit verses the tank level.

My conclusion is a faulty sending unit and possibly a stuck float.  The previous owner didn't drive it much and kept the fuel level low.

Your thoughts?  Glenn


Answer
Hi Glenn,
I have lost or disposed of my "E" type manuals but I do remember working on the sending units in "E" types. The sending units are bolted into the top of the fuel tank and easily removed. Just lift up the wood deck to gain access. You should remove one battery cable first as even a small spark at the top of a tank can be desasterous. As I remember we didn't need to look for a specific reading. We just removed the unit from the tank and put an ohm meter on the unit and moved the arm full travel. You should see a reading move progressivly through the range of travel. As I remember, zero ohms at one end of the scale but don't remember what it went up to. As I remember it didn't matter as it was a wire wound "always grounded" resistor and it either would give you a resistance reading or none, The resistance value was never off due to it being a wire wound resistor. I also remember the fuel gauge being a regulated voltage (10 volts as I remember) If the sending unit moves free and you have a progressive ohm reading then you should check the operating voltage at the gauge (temp gauge also). If the sending unit has a low fuel warning light you may have to test each terminal to see which is which. Also, we use to take the sending units apart and were able to repair some that had died. Unlike sending units of todays cars.
I hope this info will be of some use to you.

Howard