Expert: Brad Sears - 8/7/2008

Question
i saw your reply to the 66 mustang. i should unplug the sending unit and ground the wire that plugs into it? i used to be able to smack the dash and it would work. that stopped. always on empty. i pulled dash out. it has cluster of wires, about 15, that plug into pod and somehow tough the circuit board. i used an ohm meter to check the continuity from the circuit board to the fuel gage. all checks out. could the gage be bad? there really aren't any wires that hook to it. just the circuit board. where should it be grounded? it looks like i have just one wire that goes from the sender to whereever. i haven't traced it yet. how do you ground just one wire?  any help on my next step would be greatly appreciated. does the sender send a signal to the gage or does the gage send it back to sender? not sure how this works.
thx, rob.

Answer
It sound like the problem, based on the smack the dash method of diagnosis, is in the dash. Key on voltage is sent to a voltage regulator on the back of the dash that keeps the gauge voltage at a constant 6 volts. This prevents gauge fluctuation when the battery and charge voltage changes, From there the voltage goes to the gauge and through its magnetic coils. The strength of the magnetic field generated to react on the magnet attached to the needle and make the needle move is controlled by the position of the float in the tank that changes a variable resistor altering the flow of current to ground. AQ simple one wire series circuit. Time to get up behind the dash with a voltmeter. The printed circuit I believe is grounded through one of the screws that holds it to the back of the dash, but my bet is that you will find low or no voltage out of the voltage regulator. Remember battery voltage in and lower or pulsating voltage out.
Brad