Ohms a fuel gauge

Dan B · Jun 13, 2008

Is there a way to test a fuel gauge to tell what ohm range it is? Have a good gauge but dont know what sending unit to get to go with it.

Mark - IN. · Jun 14, 2008

Good morning Dan, I'd like to help you but am not sure. The only thing that comes to mind is going across the leads of the guage itself with an ohm meter, but I'm guessing that its voltage or current driven by the sending unit itself. Having never opened either, I would guess that the guage requires say either 6V or 12V to read full, but the sending unit has perhaps a variable resister (potentiometer) that when the tank is full allows either 6V or 12V to drive the needle full guage, and as the fuel level drops, increases resistanse thus decreasing voltage to the guage and dropping the meter. Unless someone has had past experience with the specific guage that you're dealing with, you might have to find it in a fuel sending unit book down at the auto parts place...and that'd be a good or best bet. I would not be surprised if a sending unit manufacturer's parts book wouldn't have a unit listed for an application...if you know what the fuel guage came out of, look it up.

Good luck.

Mark

John T · Jun 14, 2008

Dan, if you had one of those switchable resistor boxes (say 1 to a few hundred ohms) you could probably come up with the correct sender value. At low to near zero resistance the gauge should full needle deflect but then keep trying different higher resistance (perhaps 30 to 200) until it just barely deflects (i.e. on the other side). If the resistance is too high it wont move at all but the correct size (on the high end) will make it defelect just a little. Many simple senders are in the 0 to 30 or 0 to to 200 or so ohms range, so use a switchable box until you find the resistance range to make the needle swing side to side without overdriving it either way. NOTE this is for the older 2 wire gauges, some newer 3 wire gauges in order to draw less current had sort of like a center tapped ground with two internal resistors that nulled out

John T

Sam#3 · Jun 14, 2008

It's called a 'Decade box' if anyone starts shopping. Most any electronics test equipment store should have them.

dave guest · Jun 14, 2008

Fuel gauge is basically a galvanometer. You need to find current value for deflection. Take a bunch of resistors from old radioman and a volt and a half batt and put them in series one at a time with meter and using milliammeter you can determine this value. If unit is hooked up have tank half full and change resistors until you get approx. half reading. Like other said it is pretty low ohms like under 100 probably. Remember if you calibrate with volt and half batt, you must multiply resistance for circuit about 4 times for 6 volt and 8 times for 12.

Dan B · Jun 16, 2008

Thanks for all the tips. I will hav eto get some things together to test the gauge. But at least I have a better idea

Thanks again .

Ohms a fuel gauge

Dan B

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Mark - IN.

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John T

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Sam#3

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dave guest

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Dan B

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