Fuel Gauge Repair

[tortron]

Story goes

Fuel gauge was working fine, albeit a little off

Gradually it developed a jump, which a tap on the dash would remedy

This progressed until gauge would jump erratically all the time, until one day it was dead

Testing showed that this was the gauge itself, rather than some nice easy fix like the ground wire coming loose

Dismantled my gauge unit today and discovered the problem

As you can see, the upper coil has corroded, and indeed has broken on the left of the picture.

Having rewound my trafficators, im sure i could spend a day tediously winding 180 odd coils with a smaller than hair wire, then soldering it in place.

However, as this upper coil is only used as a shunt resistor. I figure i can just solder in an appropriate unit.

My question, is how do i work out what value resistor i need?

here are some supporting diagrams, from an MGA gauge, same set up, however the values may differ

Add a 150 ohm 2 watt resistor in series to control the current. There may be enough space in the bottom of the case to house the resistor, and you could sleeve the leads to have it connected between one of the upper posts and one end of the top coil wire (which would otherwise connect there). The 1.5 watt max heat which was originally generated by the upper coil will now be produced in the new resistor with no change of total heat inside the instrument.

It may also be noted that the upper coil serves primarily as a shunt resistor, and has little or no effect as an electromagnet in this physical configuration. As such, the upper coil could be replaced with a 2 watt 150 ohm power resistor (if you can fit it in the space allowed).

http://mgaguru.com/mgtech/electric/et213.htm

[bubblegoose]

measure thickness of coil wire and its length

then google resistance calculators

[Llama]

Or get a variable one and test it until it matches, then find the equivalent one and put it in?

I'm not quite sure if I understand how that works though...