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robertc
09-06-2006, 05:41 PM
Here is my question for the day. My tach went udders up and so my maintaince guy replaced it. After the replacement both fuel guages read 1/4 to 1/2 tank low when the tanks are full. They both read exactly the same, but low. I haven't had a glitch with the guages for ten years until tach replacement. Where do I start looking?

MikeM
09-06-2006, 08:55 PM
Here is my question for the day. My tach went udders up and so my maintaince guy replaced it. After the replacement both fuel guages read 1/4 to 1/2 tank low when the tanks are full. They both read exactly the same, but low. I haven't had a glitch with the guages for ten years until tach replacement. Where do I start looking?

Is the dropping resistor common to both guages, or is there one per gauge?
If there is only one, then it could effect both equally.

Anything that effects the supply voltage to the guages would effect both readings. Check the circuit breaker/power feed wire to the guages.

Both gauges probably share a common airframe ground which may have been disturbed when the tach was replaced.

Geoff Williams
09-07-2006, 11:18 AM
Robert,

Mike is on the money I think. As the tach is located close to the gauges, I would be looking at the wiring on both instruments to check for security and bonding to ground of the gauges. Grounding is supplied by the clamp that holds the fuel gauge in the panel. This clamp has a sprung leaf on its inside that touches the body of the gauge, and gets its ground from the pointed ends which contact the backside of the instrument panel.

To prove the ground (and you can reproduce the fault state with the gauges showing the incorrect reading) attach wire to a good ground point and touch it to the body of the gauge and see if you get a change in the reading. If so you might get away with just tightening the clamps a little. Also measure the volts at the end of the resistor that has the wire on it to see if you have the full battery volts during the fault condition. The resistor is attached to the back of each instrument. If the fault is still there, try disconnecting the wire that goes to the sender. This should cause the gauge to read full scale deflection to the right and maybe test the sender circuit that should read around 30 ohms for a full tank and around zero ohms for an empty tank.

Geoff