The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel gauge wiring puzzle

I'm a newbie at wiring. Am wiring fuel guage on '49 GMC pickup, original gauge. There is a ballast resistor cutting voltage to coil. I ran power line off of coil to post on gauge with added Runtz reducer. I ran a new ground from gauge to back of cab with screw attachment. Ran new sender wire to post of gauge. I have 4.29 volts at one end of Runtz, 2.15 at post on gauge. Gauge reads empty when switch is on, doesn't move at all. I don't know which post is for power and which for sender, so I reversed the gauge post connections with the above setup and got reading of 1/4 tank (tank is nearly full). Needle moves when ignition switch is turned on. How do I determine which post is correct for power, sender? Is my ground correct? Is the gauge defective? The sender?
I have a 12v system but an inline ballast reducer gives 4.29 volts at the coil, from which my power line runs to the gauge. If the voltage at the gauge is only 2.15v, is that enough for the gauge?

PS: it's a 12v system, but the inline ballast reducer drops the voltage to the coil, from which I'm getting the power to go to the gauge.

Run the power line from the ignition, not the coil.

So, power wire from ignition through Runtz to fuel gauge. One wire from second terminal on fuel gauge to sending unit in tank. One wire from sending unit mounting screws, to ground.

Try this and check back in.

I think the power wire on the gauge is on the empty side, but I could be wrong.

Jeff

Thanks, Jeff. Now my dumb question: why not run the power from the coil? btw I just rewired the power to the empty side and I get the 1/4 full reading which I know to be inaccurate as the tank is near full. I do have the sender wire to the center post on the sending unit and the unit is grounded to a screw going into the cab just above the tank (tank is behind seat). Could that not be a good ground?

Well the easiest answer is that no car I can think of has ever been designed that way. I could be wrong on that though.

My assumption is that having both a Runtz and a ballast resistor will contribute to a problem with the ohms.

As far as the ground, did you use a new screw and rough up the metal? I always use star washers to cut into the metal a bit.

Technically, if everything is wired correctly, removing the wire from the sending unit should send the fuel gauge to full. Touching the sending unit wire to a ground should go all the way in the other direction to below full. If that checks out, than either the needle on your gauge is bent or the gauge might be toast.

The needle could also move in the opposite direction of what I just said when testing, I can't remember it's been a long time!

Jeff

The screw is new on the ground to the back of the cab. I roughed up the metal too. I'll try removing the runtz, and if that doesn't do it, I'll do the tests you mentioned.

Thanks

Jeff--success at last! The runtz was cutting the ohms too much, and eliminating that let me then get a valid test on the sender, which proved to be defective as well. I replaced it and the gas gauge now works perfectly.

Thanks again!