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MG MGB Technical - Fuel Gauge Question

The fuel gauge has stopped working ( reads empity) on my 66 B. I tested both wires connected to the float assy. and found no power. Shoulden't one have 12 volts running throught it? If so, does any one know where it picks up the power? Any other thoughts on what might be amiss? Thanks to all....
Steve O

One lead (black wire) to the sending unit is a ground lead, on most cars it runs thru the harness and is connected to ground (car body) in the trunk. The other lead goes to the gauge. The gauge has two terminals, one to the sending unit and one to the voltage stabalizer mounted behind the dash. Grounding the lead to the sending unit should cause the gauge to swing across the range. If it dosen't check that the gauge is getting 10 volts from the voltage stabalizer. Never connect a power lead to the sending unit, it is a coil of fine wire directly submersed in the fuel, there would be a large noise and a ball of flame.
John H

Wiring diagrams are available at http://www.advanceautowire.com click on stock schematics.
Kimberly

12v comes through the stabiliser and the gauge to the sender, so this should be detectable on the green/black wire at the sender. Grounding this wire should send the gauge rapidly towards the F position, don't leave a ground on this wire any longer than necessary to check gauge movement as it does push more current through the gauge and stabiliser than normal.

The original stabiliser works by switching 12v on and off about once per second (after taking 2 or 3 seconds to 'warm up' when first switching on the ignition. This *averages* about 10v over time, don't expect to see 10v when measured with a voltmeter, unless someone has done an after-market frig.

There is a 2-way bullet conenctor between the rear harness and the main harness in the mass of conenctors by the fusebox, if you don't see 12v on the geen/black at the sender with the ignition on check both sides of this connector. If not on either side here then the problem is either a disconnection at the gauge - check both sides of this and if on the green/black side but not the light-green/green side the gauge is open-circuit or its conenctions are bad.

If on neither side of the gauge check both the I(nstrument) and B(attery) terminals of the stabiliser as well. If on the B but not the I the stabiliser is open-circuit, if on neither check back towards the green circuit fuse. However on a 66 the green circuit comes to the stabiliser first then branches off to the indicators and the heater fan, so if these are working the supply to the stabiliser itself must be OK.
Paul Hunt

PS. Whilst I wouldn't connect a direct 12v to the sender terminal for other reasons (it would prove nothing and push more current through the sender than normal) it is unlikely to cause a fireball. For that to happen there would have to be a spark, and that would have to occur in a relatively narrow range of fuel to oxygen ratios whereas what is normally in a connected-up fuel tank is far to 'rich' to explode.

Even at its lowest resistance the sender will only pass about 1/3rd of an amp which isn't enough to burn the sender wire out to cause a spark, and even if it did so it would be 'under water' anyway. With lower fuel levels when the sender winding is in an atmosphere rather than liquid fuel the resistance is higher and so the current much lower, down to a few tens of milliamps even less lilely to burn out the sender winding and cause a spark.
Paul Hunt

Sounds confusing but I will give it ago...Thanks to all!
Steve O

This thread was discussed between 20/05/2008 and 25/05/2008

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