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Old 04-05-2007, 06:39 PM
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Originally Posted by goshawks00
Hooked up the fuel gauge through a 6 volt resistor as all the original gauges were 6 v. the rest of the car is 12v.
I got no signal... tested the wire going to the sending unit and the test light flashed. Switched the fuel guage to 12 v and guage is working but registering full, even though I have a measued 1/2 tank ( 12 gals in a 24 gal tank). Is there a rule of thumb about adding resistors to bring the guage and tank into sync?
Doc here,

First, there are no 6 volt resistors..Resistors are measured in ohms and watts to determine how many amps at what voltage they will be operating at..That rule of thumb is Called Ohms law..(and a little math)..

IF you meant, your gauge WAS a 6 volt system, AND you Converted your Car to 12 volts , and used a dropping resistor on the power wire, it would be : 6 volts out, with a draw of about an amp, = 6 ohms..At about 6 watts (unless my confuser slipped a digit) from 12 volts..

IF the gauge was 12 volt to begin with and you added a resistor to the power wire..It won't work at all..

A Calibration resistor needs to be installed on the sender unit wire ONLY , not the power wire. The Sender wire is a RESISTANCE to ground through the sender which IS a Variable resistor mounted on a float aparatus..The higher the float arm moves up, the more resistance (or less depending on the sender configuration) the more resistance the gauge See's, the more the needle drives upward..( "F" ) ..the lower the float, the reverse becomes true.. ( "E" )..

IF your gauge IS supposed to be 12 volts, remove the resistor from the power wire. and reconnect as per stock.

To test your sender, remove the sender wire, and watch the gauge, it should peg at full, If it does, then ground the sender wire, it should peg at empty or opposite the last reading, If you have this the system is whole and working.

If not check the Sender wire for bad connections or cuts, be sure the tank sender has a solid ground wire to the frame too..If the wire is good check (or replace ) the gauge.

If it is reporting in error, you need to find the PROPER resistance of the sender unit / Gauge..usually 0 to 30, 0 to 60 , 0 to 90, or 0 to 120 Ohms , up to 290 ohms, (your parts guy should be able to tell you by the make and year of the vehicle, or post it here, I can look it up..)

NEXT, get a Linear taper potentiometer, (like an old radio volume control ) (rat shak ,$3,00) Say, your system is 0 to 60 Ohms , get your linear taper (pot) within a range of 0 to 100 ohms at 5 watts..

locate an easy access area that you can mount the pot along the sender wire from the tank..cut the sender wire..

Strip and attach the wire from the tank and solder it to the left terminal AND the center terminal of the pot..insulate with shrink tubing.

Strip and attach the wire going to the Gauge to the right terminal. Solder and Insulate with Shrink tubing.

That's it,.... Now to Calibrate..Drain (or run it out of gas) , and add 2 gallons of fuel. Monitor your gauge, it should be about "E"..

If not , Turn the pot in one direction or the other, watch the gauge, one way will go up the other will go down. as you turn the pot knob..

Set it for "DEAD ON" "E"..this way , you will ALWAYS have 2 gallons to "limp home on " when you hit empty..and hopefully won't ever run dry.

Lastly, when done get some Silly~Cone and Seal the WHOLE pot up..front , sides, top bottom and KNOB (so it won't turn ) and so it won't get water inside the pot..and your done!

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