FUEL GAUGE BOUNCE

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FUEL GAUGE BOUNCE

Postby M55RK H » Thu Dec 01, 2011 5:02 pm

Hi, guys everytime I rev the car the fuel gauge starts going up has anyone got any ideas ran out of petrol onse through it doing this.
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Re: FUEL GAUGE BOUNCE

Postby badandy61 » Sat Dec 03, 2011 8:20 pm

Try grounding the gauge body. Sometimes this will work.
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Re: FUEL GAUGE BOUNCE

Postby Charlie Tolman » Mon Dec 05, 2011 10:51 am

The voltage going to the fuel gauge, comes from a voltage stabilizing circuit, which should supply a constant 10 volts to the gauge. This might be your problem ?
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Here are some comments from Internet websites:

A '68 should have a voltage stabilizer supplying voltage to the fuel gauge. 12V goes "in" on the stabilizer's "B" terminal and 10V (choppy) comes out on the stabilizer's "I" terminal. One side of the fuel gauge should be connected to the stabilizer's "I" terminal with a light green wire. The stabilizer's 10V output is hard to measure accurately with a digital meter.

The green/black wire on the sender should go directly to one of the fuel gauge's spade lugs. On car's with the voltage stabilizer it does not matter which gauge terminal is connected to what. (On the early gauges, the gauge terminals are marked with "T" and "B" so you hook the wires up correctly... that shouldn't be applicable here).

With the ignition on, if you ground the green/black sending wire, the fuel gauge should go to full. If it does, that indicates the gauge is getting power and that the green/black wire is intact.
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Up to Oct 64 the gauge was of the undamped type. This didn't need a voltage stabiliser but was still independent of voltage changes as it used a Wheatstone Bridge arrangement of four resistors/windings - two in the sender and two in the gauge. Three were of known value, the fourth operated by the slider varied according to fuel level and its effective resistance can be calculated, or displayed on a meter or gauge, by referencing it to the three known values, and it is independent of the supply voltage.
The later damped type uses the heating effect of a current on a bimetal strip, which is totally dependant on supply voltage to give a consistent reading, hence the need for a stabilised voltage supply. As Dave implies any voltage lower than the lowest expected voltage likely to be encountered in the cars electrics will do, and it is nominally 10v. the irony is that far from being a stabilised voltage, the output of the 'stabiliser' is actually constantly switching between 0v and the cars system voltage, which is varying of course. The voltage is used to heat up another bi-metal strip in the stabiliser, which operates a contact switching the supply voltage on and off from the output (and the stabilisers heating coil). Exactly the same as in the gauge the heating effect in the stabiliser is dependent on current which is dependent on voltage. When voltage is low the bimetal strip takes longer to heat up and open the contact, so the voltage is connected to the gauge for longer, and vice-versa. The net result is that one cancels out the other, and the gauge reads consistently with a varying supply voltage.
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Fuel gauges for early Sprites need a good ground.
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The voltage stabilizer is under the dash somewhere; look for a light green wire.

Charlie T.
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Re: FUEL GAUGE BOUNCE

Postby M55RK H » Thu Mar 01, 2012 4:14 pm

Hello, Charlie could you tell me what the light green wire connects to. Thanks Mark
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Re: FUEL GAUGE BOUNCE

Postby Charlie Tolman » Fri Mar 02, 2012 1:11 am

The information in my previous note was copied from Internet postings, regarding the light green wire.

A wiring diagram in my shop manual for a Sprite III does not show the voltage stabilizer, but the diagram for a Sprite IV does show it. Official name is: bi-metal instrument voltage stabilizer.

There is a LGG wire, which I assume is a light green wire. Or, it might be a light green wire with a green tracer ? This wire connects to a terminal on the fuel gauge. (The other terminal on the fuel gauge has a GB wire, which should be green with a black tracer.)
The LGG wire also connects to terminal I (eye) on the voltage stabilizer. The other terminal (B) on the stabilizer has a green wire G.

On the fuel gauge, the terminals are labeled as B and T in the Sprite III diagram. A GB wire from terminal T goes to the fuel tank sending unit. The wiring diagram for a Sprite IV does not have the fuel gauge terminals labeled.

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