MF 135 fuel gauge and sender help

[Arc weld]

I need some help with a frustrating problem that is driving me nuts. The fuel gauge in my 135 UK diesel was intermittent and then would only read a little above empty. I ordered a new sending unit and it was apparently the only one listed for a 135 UK. The arm for it was too low and hit the tank bottom so I couldn't install it. Doing some looking online I see that it is for an extended range tank that is deeper. I drilled a hole to adjust the arm higher with a bolt and then drilled out the spots welds to move it. I moved the arm up and then adjusted the float arm so it works similar to the original. Mounted it in the tank and started the tractor. No change, gauges comes up a little past empty. Had a friend come over to watch the fuel gauge ( I couldn't because of the cab) and with 12 volts grounded the sending unit and the fuel gauge went all the way past full. Shut tractor off and fuel gauge slowly went to below empty. Started tractor and fuel gauge went past full. The tractor had over 3/4 of a tank so I might have it adjusted to read full at a lower setting. This is fine as I just want to know when it's getting low so it doesn't run out. I saw something about 12 volts for more than a split second will ruin the sending unit or the gauge ( I think sending unit but couldn't find the full article online). I didn't feel like pumping fuel out of the tank to see if the gauge goes lower. I thought I'll just run the tractor for a couple hours next weekend and see if the gauge goes down. Can someone tell me if I might have wrecked the new modified sending unit or the gauge? This is one of those things that makes you want to pull your hair out. The new sending unit that had to be modified was $130.00!!! If I wrecked it, how can get the %$#@* fuel gauge working properly again?

Thanks in advance for any assistance you can provide.

 

[Jerry/MT]

I always though there was small voltage regulator that operated those gauges at 5 volts, IIRC. The float arm rubs against a variable resistor with the minimum resistance when the tank is full thus putting the full voltage across a heating element that heats a bi-metal strip that drives the needle through a linkage. I'm pretty sure they are not designed for the electrical system voltage of 12V.

If I am correct, you need to make sure that you have a reduced voltage to the gauge resistor or you will fry the gauge. make sure all you grounds for that system are good. Grounds on tractors can be problematic because of the environment under the hood, vibration, dirt, oil fumes, etc.

 

[Harry in Ky]

I always though there was small voltage regulator that operated those gauges at 5 volts, IIRC. The float arm rubs against a variable resistor with the minimum resistance when the tank is full thus putting the full voltage across a heating element that heats a bi-metal strip that drives the needle through a linkage. I'm pretty sure they are not designed for the electrical system voltage of 12V.

If I am correct, you need to make sure that you have a reduced voltage to the gauge resistor or you will fry the gauge. make sure all you grounds for that system are good. Grounds on tractors can be problematic because of the environment under the hood, vibration, dirt, oil fumes, etc.

Interesting theory. Can you share your source for that information?

 

[Arc weld]

They do use less voltage. I think I fried the new sending unit but hopefully not the gauge. Maybe the old sending unit is still good? I read if you ground out the wire from the sending unit to the gauge it should read full so I'll try that to check the gauge. It would work of the voltage going to the gauge and not the full 12 volts. I'm an hour away from home with no internet or other means to look up trouble shooting when I'm trying to fix it. A fuel gauge isn't the worst thing to go wrong but is sure nice to have.

 

[Jerry/MT]

Interesting theory. Can you share your source for that information?

It's not a theory.

It came from an old Motor Repair Manual ( Motor Auto Engines and Electrical Systems-Professional Service Trade Edition -1977 ISBN.0-910992-87-S page 798). There are also other methods to do this but the old cars used the cheapest method available and this is probably it. t's also used on oil pressure displays and coolant temperature gauges.

I don't know that the more modern vehicles use this method since everything seems to have gone digital.

 

[Jerry/MT]

They do use less voltage. I think I fried the new sending unit but hopefully not the gauge. Maybe the old sending unit is still good? I read if you ground out the wire from the sending unit to the gauge it should read full so I'll try that to check the gauge. It would work of the voltage going to the gauge and not the full 12 volts. I'm an hour away from home with no internet or other means to look up trouble shooting when I'm trying to fix it. A fuel gauge isn't the worst thing to go wrong but is sure nice to have.

You probably won't fry the sending unit. It's the gauge that's pretty sensitive.

 

[generalconfusion]

try this parts supplier site in the Uk Agriline | Tractor Parts | Tractor Spares ,from memory they list 2 senders for the 135 , 1 for a 13" grille model and 1 for a 14" grille , they also have pictures of the senders

 

[Arc weld]

I have the right sending unit and gauge now. The later models with 14" grill must have the extended range fuel tank. I got A&I parts from a local JD dealer and can't believe the difference in price from the MF dealer. The MF dealer wanted $128.00 for an aftermarket sending unit, that wasn't even the correct one, in an MF box and the correct sending unit from A&I which looks a little better quality was only $36! The new gauge was $50 and $10 for shipping. Both pieces plus shipping was still cheaper than the first sending unit. The new gauge has a separate ground wire connection where the old gauge never did. I'm wondering if that could have been part of the problem all along but the gauge did work properly for close to 50 years.

 

[Arc weld]

Thought I'd give an update on the fuel gauge. Had a friend who's a mechanic help me try and figure out why the new sender and fuel gauge wouldn't work right. When the gauge was grounded it would peg the gauge past full. Tried changing the wires and checking the sending unit but no change so tried the old gauge and it did the same thing. Only other thing I could do was put the original sending unit back in and live without a fuel gauge. Surprise, surprise the fuel gauge read about 3/4 full. I put the mounts on the gauge and drove it down the slight ramp out of my shop and the gauge went down some. Backed it up the ramp and gauge read more. What the heck, I drove up the backside of one of the jumps on my MX track and the gauge went past full and when backing up the landing went to about a 1/4 tank. It appears to be fixed but I did notice the gauge jumps around from the fuel sloshing in the tank but my friend said that's normal for an older gauge. I'm not sure if it was a bad ground or maybe the gauge was stuck and playing around freed it up but it appears to be working now. I would think with a bad ground it wouldn't do anything but it used to go up just past empty and the odd time it would read higher but then come back to just over empty. The sad thing is I don't think I can return the new gauge and/or sending unit because they are electrical items. I'm even more curious as to why what is supposed to be the proper gauge and sending unit combo didn't work properly. It was kind of an expensive ordeal but I'm happy to have a working fuel gauge. Anyone else had weird things like this happen?