Fuel gauge stuck in a full position, did I just get lucky?

Rustbucket79 said:

The "happy fuel gauge" symptom seems to be a common feature in the industry.
Both my SAABs and Toyotas behaved that way.

If I remember correctly the ground wire above the tank i prone to failure and gives full tank reading? Or is that only for older models?

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Schurkey said:

The older fuel gauges are magnetically-driven. Constant power is given to one side of the gauge, which is counterbalanced by variable power to the other side.

The fuel tank sending unit works on a "0-to-90 ohm" resistor, that resistor controls the variable power to the gauge. "0" ohms produces maximum current, and therefore maximum electromagnetic power. The needle goes to "E". "90" ohms produces minimum current, minimum electromagnetic power, the needle goes to "F".

When the wire (including the sending unit ground wire) or the connections along it become corroded, resistance increases, current decreases, and the gauge reads more "Full" than is accurate. If the wire breaks completely, current flow goes to zero, and the gauge goes past Full and stays there.

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1500z71 said:

What drives the Speedometer? I’m fighting a bouncing speedometer on cold starts and cruise control

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Schurkey said:

WHAT VEHICLE?

Bad bearings/lack of lube in the speedo head. Both my motorcycles need to have the speedo heads repaired. I suppose there could be some screwy issue with the DRAC; but that would more likely result in the speed merely taking a vacation for several miles. Or days.




In the old days, when the cruise control was in-line with the speedo cable, the cruise unit could fail and cause speedo problems. Then the lower cable would twist-off, and the speedo would be dead.

Or there would be a worn speedo cable/lack of lube between cable and sheath.

But neither of those situations apply to GMT400s.

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Frank Enstein said:

My 77 Catalina ex-cop car was the exception. When the gauge read E you were walking!

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cubandeathgrip said:

Now I never have to worry about an empty tank again!



Tho..Seriously my gas gauge is stuck full it may move an eighth or a 10th of a position but it never goes past that point, just happen a few days ago thinking it may go away, just wondering what I should check for.. bad gauge or a bad sending unit?

thanks

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