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MF 135 fuel sending unit

G

garymck

Member
My gas gauge is not working and I have tried many things, even a new sending unit. Is the terminal on the sending unit supposed to receive 12 volts of power? If so, mine is not.
 

One terminal of the gauge receives power the second terminal of the gauge has the wire that goes to the sending unit. The gauge may have full system voltage going to it, or in some cases it is a reduced voltage. Check the voltage with a voltmeter at the input and output terminals of the gauge. The sending unit provides a variable ground which makes the gauge read. Also try a jumper wire between a sending unit mounting screw and the battery ground terminal. If the sending unit body/tank loses ground the gauge won't work. Some Masseys uses an oil pressure switch to power the electrical instruments. If yours has the oil pressure switch the fuel gauge will not have power unless the engine is running, if I remember correctly.
 
What Jim said.

Gas or Diesel?

I think the Diesels are the only ones with the oil pressure switch.

Here's the skematic for a gasser.
cvphoto131888.jpg
 
I ordered a new sending unit from Massey Ferguson and I grounded the terminal on its top with the battery ground. No change. I checked the input voltage on the gauge and it reads over 12 volts. And the output to the sending unit read (as I recall) around 3 volts. No change. The tractor has a Perkins gas engine.
 
I just received new readings from my multimeter. On the supply side to the gauge, it reads about 15 volts and on the outlet to the sending unit, it reads about
4.75 volts.
 
(quoted from post at 08:48:20 07/31/22) I just received new readings from my multimeter. On the supply side to the gauge, it reads about 15 volts and on the outlet to the sending unit, it reads about
4.75 volts.
Click to expand...

At the gauge, if you unhook the wire which goes to the sender and use a jumper wire from the terminal on the gauge to ground, does the gauge needle move (with the power on to the gauge)?
 
The gauge normally reports near full no matter the amount of gas in the tank. I disconnected the wire to the sender and the gauge goes to zero. I connected a
jumper wire from the terminal on the gauge to the battery ground and the gauge immediately went to full.
 
(quoted from post at 10:32:18 07/31/22) The gauge normally reports near full no matter the amount of gas in the tank. I disconnected the wire to the sender and the gauge goes to zero. I connected a
jumper wire from the terminal on the gauge to the battery ground and the gauge immediately went to full.
Click to expand...

Eliminate the possibility of a bad wire. Make a jumper wire to go from the sending unit terminal of the gauge to the sending unit terminal and try it. Does that sending unit have two terminals? If so, are you sure the wire is hooked to the terminal going to the resistor, not the ground terminal of the sender?
 
I have just been thru a go-round with fuel gauges and sending units on 2 Masseys. One thing I learned is that the gauge itself has to be grounded. The new gauges have a ground terminal. The older ones are suppose to ground thru the body of the gauge, but sometimes rust interferes.
 
After trying everything, I decided to hook up the old sending unit with a jumper to the terminal on the gauge and to ground the unit to the battery. By moving the
float up and down, the gauge responded. I then took off the the jumper wire and replaced it with the wire that normally hooks on to the sender terminal. Same
positive response. I then did the same with my new sending unit and it worked as well. I have now put the old sending unit back in the tank and will see what
happens. Perhaps the needle in the gauge had been seized for a while? What a pain. Thanks for your comments.
 
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