C1 fuel gauge malfunction

[Richard S.]

C1 fuel gauge pegs at full when ignition is turned on. Should I start with the sending unit or the gauge itself?

[Ed H.]

Re: C1 fuel gauge malfunction

One possible problem is the grounding of the sending unit. When the turn signal element is activated does the gas gauge fluctuate in time with the flasher. If so you may have to add another grounding from the sending unit to the frame. This is a classic problem with c-1s. If that does solve the problem I would go for the sending unit.

[Jimmy G.]

Re: C1 fuel gauge malfunction

Sounds like sending unit may be bad

[Richard M.]

Re: C1 fuel gauge malfunction

The beginning of John Hinckley's C2 Fuel Gauge article discusses the C1 version.

See this article which resides in the DoRD Sticky. "Fuel Gauge"

Mid-year Fuel Gauge - John Hinckley.pdf

Additional note: Key OFF. You could remove the Tan wire from the Gauge and measure the Tan terminal wire, to Ground, using a ohmmeter. Apx 30 ohms is full, 0 ohms is empty. Based on how much fuel you think is in the tank, you'll get a appropriate reading.

Here is a copy/paste of the C1/C3 version info from above. (Note that the "diagram" is missing)

The “Standard” System: The C1/C3 cars used the simple “GM Standard” onewire system that just has a tan wire from the gauge to the sending unit and aground at each end of the circuit, and diagnosing problems with this system isquite simple. As background, here’s how the “GM Standard” fuel gauge systemworks (see diagram).

Dash Gauge: The dash fuel gauge has two coils in it – the limiting coil on theleft, and the operating coil on the right, each with different resistance; the pivotingneedle has a counterweight on it that holds the needle at “empty” unlessmagnetic attraction from the operating coil moves it. The limiting coil gets 12 voltsfrom the ignition switch, and passes it to the operating coil, which is groundedthrough the gauge case.

Tank Sending Unit: The sending unit contains a wirewound variable resistor,and a contact wiper arm connected to the float rod and to ground; when the tankis empty, the wiper contact is at the beginning of the resistance, providing zeroohms to ground, and when the tank is full, the wiper contact is at the end of theresistance, providing 30 ohms (C1) or 90 ohms (C3) of resistance to ground.

How It Works: When the key is “on”, current is supplied to the limiting coil, andto the operating coil, through their common connection. From here, the currentcan go two ways – through the operating coil to ground, or through the tan wire tothe variable resistor in the sending unit to ground.

When the tank is empty, the contact wiper in the sending unit cuts out thevariable resistor entirely, so most of the current from the gauge will pass to thesending unit and directly to ground while very little current will pass through theoperating coil in the gauge, causing the gauge to show “empty”.

When the tank is full, the contact wiper in the sending unit is at the other end ofthe variable resistor, placing more resistance between the tan wire and ground;this forces more current through the operating coil in the gauge to ground, whichattracts the gauge needle and moves it to the “full” side of the gauge. This is asimple resistance circuit, dependent only on power to the gauge, variableresistance in the tank unit, and good grounds at both ends.