Fuel tank gauge is off?

[55Trucker]

I just replaced my fuel tank sending unit (after having gotten stuck without fuel because it wasn't working reliably over the summer - everything on it was very corroded). When I hooked up the new sending unit, the fuel gauge registered just above 3/4 tank when I had the sender in the "full" position, and below empty when I had it in the empty position. So I put a voltmeter on the gas tank leads and found 10.2 volts across them. However, the battery itself read a little over 12 volts across the posts. I then ran a wire directly to the negative pole of the battery to my voltmeter, and measured the voltage across it and the + sender lead from the fuel gauge. It was 10.2 volts too, so it seems the ground wire to the sender is good.

My question is, do you think I have a problem with the gauge, or the +12V accessory line on the boat? Things like the boat stereo work, and I imagine they wouldn't at 10.2V. Any pointers on where to look next? Thank you for your help.

 

[dtfeld]

These work on resistance. Hook your MM up to the sender and see what the resistance reading are It should be around 240 in the empty position and around 30 in the full position. If these reading are correct, the gage itself might be off. They are programmable fuel gages if you want some improved accuracy.

 

[Lazy Daze]

What happens at the gauge when you short the sender's pink wire to the ground (at sender)? Gauge should read full. You can short at the gauge, too.

 

[markrsimon]

My guess - the voltage you see at the sender unit is provided by, or more specifically, fed through the gauge. Both the gauge and the sender have resistance - so voltage (as read by a meter) is divided across the devices - thus your not seeing the full 12v at the sender.

Measuring resistance (as dtfeld suggests) without any wires connected is the best way.

In addition - you can see if you can adjust the stops (https://www.mgexp.com/article/how-to-adjust-the-fuel-gauge-sender.140). Alternatively you may have the wrong sender if you are that far off.

 

[55Trucker]

I shorted the two wires at the tank, and the gauge registered a hair below full. I am sending the replacement sending unit back as defective. It is supposed to be a 240/30 sender. In the full position it registers 35, and empty 230. But the interesting thing is that as I slide the arm it intermittently shows 0 ohms, so there's something wrong with it. Thank you everyone for your help.