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Fuel Guage

R

Red Rex

Member
1968 MF 135 diesel.
Fuel gauge pegs full regardless of fuel level in the tank when electricity is applied via the on switch.
Single wire connection on the top of the sender in the tank to the left pole on the gauge in the dash.
Ground is a metal strip from the sender base touching the tank held on by a hex cap screw holding the sender to the tank.

Pegging full under power I believe indicates lack of ground.

I then ran a ground wire from the cap screw on the sending unit to the frame and from the other pole on the gauge to frame. No success. Removed the wire from the gauge to frame, no success.

What am I missing?
 

It sounds like the gauge is getting power ok if the needle moves when you turn the key on. A quick test to tell if a gauge or sending unit is bad is to ground the wire from the sending unit and ground it (key turned on). If the gauge reads full the gauge is good and the sending unit is bad. In your case it sounds like either or gauge is bad or the wire is grounded somewhere between the gauge and sending unit. Leave the power wire on and unhook the wire that goes to the sender from the gauge. If it doesn't move when the key is turned on it is likely a bad wire. If it goes to full with the wire unhooked, bad gauge.
 

That should have said "A quick test to tell if a gauge or sending unit is bad is to [u:6210c47588]remove[/u:6210c47588] the wire from the sending unit and ground it (key turned on).
 
Thanks for the response Jim.

I could get my multi meter to test ohms on the sending unit. Would I test by connecting the center pole with one lead and the base (ground) with the other?

Testing the gauge is to apply power to one pole and ground to the other with no connection to the sender? Functiong gauge then is one that reads empty?
 

Yes on hooking the ohmmeter leads to the sending unit, as you describe, and moving the float arm you should see the ohm readings change.

The gauge needle should only go to full if the gauge has power and the sending unit terminal of the gauge is grounded. If the wire to the sender is unhooked from the back of the gauge, does it register full when the key is turned on?
 
Once the wire from the center post of the sending unit is
disconnected from the back of the gauge the needle does not
move. Resistance across the gauge reads 119 face of the
gauge says 30 to 240 ohns
 
Your tractor originally came with a Delco 10DN alternator and external voltage regulator on the dash next to the battery. The voltage regulators 4th terminal supplied the voltage to the fuel gauge. That 4th terminal had a resistance in it to lower the voltage to around 4 to 5 volts for the gauge. If you hook the gauge directly to 12V it will peg the needle. When I replaced my alternator with a 10 Si which has an internal voltage regulator so that I did not need the external regulator, I used a potentiometer to lower the voltage. The tank holds 10 gallons, so I drained the tank and filled it with exactly 5 gallons and then adjusted the pot to give one half full. I think I used a 10K ohms pot.
 
The gauge is getting 12 volts so yes, a method to step it down will likely fix the issue.

Where might I get that device you mentioned?
 
Did you use an adjustable potentiometer or just a 10K ceramic resistor? I think that may be the problem with my 205 as it always reads full as soon as it gets power.
 
I got my gauge to work. It helps to wire it correctly. The wire
from the center post of the sender connects to one side of the
gauge and a12 volt positive from a switched source connects
to the other post on the gauge. I cleaned the case of the
gauge and added an additional ground from the base of the
sending unit to the frame.

Jim, thanks for your help.
 
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