New Fuel Gauge Woes

[M W.]

Hi all,

I just finished installing a new 64 Fuel Guage in my car and turned the power back on and nothing. I had everything disconnected before taking the old one out which by the way was working perfectly excepted it bounced around to much for me. Any suggestions I could try next to sending the unit back is appreciated.

Cheers,
Craig Willetts

[Edward J.]

Re: New Fuel Gauge Woes

Craig Maybe Some Sort Of Electrical Connection At Gauge Or Wires Hooked To Gauge Wrong.you Can Also Try To Ground Sending Unit Wire At Tank With A Test Lamp-should Register
About1/2 Tank.( The Resistance Of A Test Light Is About 45 Ohms) This Will Tell You If Theres A Connection From Tank To Gage.

[John H.]

Re: New Fuel Gauge Woes

Use the link below for the text of a complete midyear fuel gauge system diagnostics article I wrote in the February '06 issue of "Corvette Enthusiast" magazine; the two diagrams below go with it.

http://www.lbfun.com/warehouse/tech_info/gauges%20&%20instruments/MidyearFuelGauge.pdf

[M W.]

Re: New Fuel Gauge Woes

John, does everything on that side of the cluster need to hooked up for the gauges to work? I can't imagine everything working before and after removing the old working gauge and carefully installing a new supposedly working gauge it now does not work. I did not reconnect the headlight switches thinking those did not matter. Maybe all need to be connected.

Thanks,
Craig

[John H.]

Re: New Fuel Gauge Woes

John, does everything on that side of the cluster need to hooked up for the gauges to work? Thanks,
Craig

Nope - the gauge needs the two power and sender wires connected to its terminals, and the cluster can has to be grounded (it grounds the gauge).

[Jack H.]

Re: New Fuel Gauge Woes

"However, the ’63-’67 Corvette had to be different (we don’t know why), and used a two-wire system with a powered sending unit and a Whetstone bridge voltage-divider circuit..."

John, I think I can help on the 'we don't know why' issue...

The old circuit was battery, through gauge, to variable resistance sender, to ground. Hence, the gauge was measuring absolute current flow in the branch circuit. That current would change for two reasons:

(1) The position of the float arm in the tank and the specific resistance that resulted.

and

(2) The absolute B+ voltage applied to the circuit.

With generators, the absolute B+ voltage varied pretty dramatically based on battery charge condition, battery loading (what other circuits were turned ON loading down the battery), what RPM was the engine running to spin the generator, and that specific generator's 'cut through' profile. So, in the 'good ole days' specific gauges like fuel and temperature, were known to 'bounce' giving different readings depending on the car's operation.

Things like one reading while cruising down the road at highway speed and another reading when stopped at a traffic light, foot on brake, and engine at idle affected the gauge's pointer needle position. People just lived with the variance back then...

The mid-year Whetstone bridge circuit topology was an electrical 'break through'. Study the circuit design you show in your drawing and you'll see the branch current that flows through the fuel gauge's variable side is TOTALLY INDEPENDENT of absolute B+ voltage!

It's the voltage divider action of the Whetstone bridge that dictates absolute current flow. So, this system doesn't care whether the battery is down (+8 VDC) or being aggressively charged (+15 VDC). You get the same reading all the time...

When Shark cars were introduced, modern electronics had progress significantly and designers had neat/friendly "3-legged voltage regulator" components to work with. These could be installed in the instrument cluster and PEG the supply rails of critical circuits to a constant regardless of the actually battery voltage...

So, the later cars returned to the simplified circuit topology of the 50's-early 60's with the use of these inexpensive voltage regulator IC's and the Whetstone bridge circuitry of the mid-60's was retired...