Logo
FULL SITE MENU

fuel tank sender

S

sammi

Regular Contributor
So I bought this beat up Lyman boat last year( see "more issues" thread and others) and both tanks read empty. It has the electric senders that read resistance as the sender floats up and down. I took one off to see if the float sunk. It didn't. The resistance shows 240-20 ohms as I move the float up and down. Resistance at the gauge reads 412 Therefore empty. The wiring is old and I think I'm reading 412 ohms because the wire to the tank is corroded. Does that make sense? I will replace the wires tonight because it is easy to do. I guess it could be the gauge as well but untill I get between 20-240 ohms at the gauge I'll assume bad wire.
 
240 to 20 ohms is the correct resistance reading for most common fuel tank sending units. If you are reading the resistance of the sender at the gauge, the gauge "S" terminal must be disconnected so you are only reading the sending and any bad connections in the wiring in between.
 
I hooked up a new wire to the gauge tonight. It read 124 ohms before I hooked it up at the gauge. Still reading empty. When I turn the key on the needle drops a little. You can barely see it move. HMMMM.
 
I'll run another temporary wire to ground but i think it is the gauge. The ground wire jumps from gauge to gauge and all the other gauges work fine. Time for a new set of gauges I think, as my original voltmeter didn't work right either. I replaced it with a cheap autozone gauge and it works ok but doesn't match.
 
Is it just one wire going to the sender? If so, is the tank grounded? A quick check for wiring and gauge is to short the sender wire to ground (tank or other wire). If you get full deflection at the gauge, the gauge is good and typically so is the wiring.
 
Woodieman, Thanks. Are you saying leave the gauge hooked up as it is supposed to be, then hook a wire from the sender to a good ground, either at the tank or gauge?

I just hooked up the gauge directly to the battery and a new wire to the sender. I then grounded it as you described and in went to full quickly. I don't get it. I've checked the resistance from the sender and it works fine. I've checked to make sure the float floats and it does.
 
Last edited:
The fuel gauge reads to resistance to ground through the sender. The ground connection that is looped from gauge to gauge is for the gauge lighting circuit.

If you measured 127 ohms at the sender, I'd guess it is indicating about 1/2 tank (more or less) of fuel.

From the testing you have done (eliminating the boat wiring completely) I'd guess that the gauge has failed. Not uncommon, I'm afraid. Replacement gauges from Teleflex or Faria use the 240-20 ohm standard. Some other gauge manufacturers use a different standard which would mean that you would have to replace the fuel tank sender as well.
 
I have 2 tanks and when both tanks read empty I figured it was a gauge but wanted to do as much as I could to rule out something else. I wanted to keep the gauges matching and found a NOS teleflex gauge on ebay. I know, I know, even though it is new it is still old and you never really know what you are getting but I'll give it a try.
 
You don't touch the wiring at the gauge. The jump is done at the sender. This should give you an indication of whether the the gauge and wiring to the gauge is ok. After that, it's resistance checks with the sender out and being manipulated while resistance readings are taken at the gauge end. If the resistance readings are in line at the gauge end, then it's most likely a faulty gauge. If the readings at the gauge don't check out, take readings at the sender. If they check out there, it's wiring to the gauge that's at fault. If they don't check out at the sender, a new sender is in order.
 
Last edited:
412 ohms is too much resistance. Bad wire or poor connections, corrosion etc. Don't change the gauge until you get proper resistance readings there.
 
Woodieman got it right when he asked if the tank was grounded. Since the boat was running last year I just assumed(I now, I know) the tank wiring was correct but it appears the tanks were not grounded. I ran a wire to the ground tab on the sender top to ground and the gauge appears to be working. One question. I have 2 tanks feeding one engine. I ran the ground wire from 1 tank to ground but not the other. since they are connected by copper tubing there is no need to ground the other tank right? It is already grounded since they are connected by metal.
 
Don't depend on those copper fuel fittings to ground your fuel tanks. They are meant to carry fuel, not ground current. Run a #10 green wire (stranded copper) from an absolute dead-nuts ground (ground plate, battery common negative terminals, engine blocks, etc.) to the first tank and then to the second. Crimp the fittings nice and tight.

It wouldn't hurt to run that green bonding wire to the thru-hull fittings as well.

Bill
 
Back
Top