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Posted by johnd [71.255.216.170] on Monday, September 17, 2012 at 20:13:22 :
In Reply to: Converting a 6V Fuel Sender to 12V posted by Steve [75.27.150.20] on Monday, September 17, 2012 at 00:04:15 :
My 2 cents:
All you need is a resistor of the right size in between the sender and the new gauge. here's how I would try to determine that, but every combination of senders and gauges would be different.
First, the sender doesn't care what the voltage is really. It's only job is to vary the amount of ground being sent up to the gauge to make the needle move.
On a workbench, hook up 12 volts to your new 12 volt fuel gauge. Connect the sender output wire to the appropriate terminal on the gauge, and remeber to ground the metal body of the sender to the same ground as the gauge. (this would normally be done by the metal tank attached to the truck frame)
Move the float up and down full stroke and see how much the gauge needle moves. I think it will read full at only half of it's stroke.
Insert a 1,000 ohm variable potentiometer in line between the fuel sender and the gauge. Do this by connecting the output of the sender to the center terminal on the potentiometer. Then run a wire from the left terminal (shaft facing you) on the potentiometer to the sender terminal on the gauge. At this point you have added a variable resistor in series to the fuel sender. Move the float to the lowest point and verify the gauge reads zero. Now move the float to the highest setting and turn the potentiometer clockwise until the gauge reads full. Carefully remove the wires from the potentiometer without moving the shaft. Put an ohm meter set to it's lowest setting on the same two terminals and read how many ohms resistor you need to add. Use about a 2 watt resistor from radio shack, etc.
A source for the potentiometer is Mcmaster Carr part # 7436k37, specify 1K when ordering.
It sounds complicated, but it's really not.
All that happens inside the gauge is the sender raises and lowers voltage of a small coil inside the gauge. The coil become an electromagnet that attracts the needle towards it. The higher the float arm is in the tank, the closer the needle gets pulled over to the full mark on the gauge.
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