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fuel gauge question

Bob N.Y.

Bob N.Y.

Well-known Member
Location
Norway, N.Y.
I have a Massey Ferguson 35 with a bad fuel gauge and sending unit. The gauge says 12 volt and 30 to 240 ohms. There is one wire to the sending unit. It appears to be grounded thru the tank. Does the current go to the gauge first, then to the sending unit and ground? Does the gauge read full when the resistance at the sending unit is at the max?
 
The gauge gets voltage and sends it through the meter, then through a single wire to the sending unit. There it is grounded through a variable resistor, and float/arm that moves it like a volume control. Low resistance is usually full, high is usually empty. Jim
 
As Jim said...

A common problem is sometimes the tank or the sending unit looses it's ground.

To test the dash gauge, ignition on, disconnect the wire from the sending unit, the gauge should go to empty (some work backward). Ground the wire it should go to full (or empty). Try grounding the wire in various places to be sure you are getting a good ground.

There should be power to the ign post on back of the dash gauge with the ignition on. Some gauges need the gauge case grounded also, some do, some don't. Won't hurt to ground it just to test it.
 
To check the sending unit just hold the OHM meter on the cap and red on the wire connection move the float if the meter needle moves with the float it is good. If you touch the wire going to the sending unit to ground if the gage needle moves from one side to the other like empty to full or vice versa then it is a connection problem if one of them does not read like described then that piece is bad.
 
The sending unit is completely destroyed by rust. The gauge stays at empty no matter what I have done with it. This site lists a sending unit for a 35 gas, but no gauge. The ohm range for the sending unit is not given. I am trying to find a gauge and sending unit that match. That is proving to be quite a challenge.
 
Some of the gauges and sending unit online say high resistance is full and low is empty. Is that possible or is the description backwards. I am getting very confused!
 
European gauges sometimes do it the opposite way. How to find the truth!! get a sending unit from a car junkyard. Hook it to your gauge. Put a 12v source on the gauge supply terminal, and ground the gauge body to the other terminal. Move the float lever and watch the gauge. Determine which Ohm reading results in full reading, and which shows Low. Measure the resistance of the gauge in the tractor, and see if the reading changes in the direction expected as you use the tractor, (or drain the tank). Jim
 
My real problem is trying to order a gauge and sending unit that are compatible with each other. Most listings don't give the ohm range of the components.
 
Assuming the new gauge and new sender are compatible, you can adjust the full to empty range by adjusting the float arm length and amount of bend. You can also add 10 watt 50 ohm resistors in series to adjust the max. A potentiometer (volume control component) can be used. there is little current. The 10 watt is to keep the leads big enough to easily work with, and strong enough to resist vibration. This process is for sure not Ohm based science. Effort and tenacity are needed. Jim
 

Have you tried searching using the actual Massey Ferguson part numbers for the gauge and sending unit for your tractor? Using those numbers what you find for replacements should be compatible.
 
I have seen gauges and sending units online that are listed with the MF part number, but the ohm ranges are all over the spectrum.
 
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