MF135 fuel gauge wiring question

[Bill Knecht]

Greetings, fellow Masseyphiles. I'm having trouble figuring out how to wire up the fuel level sensor to the fuel gauge on my 1971 MF135. Help would be most appreciated. Parts were purchased from Yesterday's Tractors.

The top picture shows the sensor with its two terminals, which I've labeled T1 and T2. There's a third, unlabeled screw with a metal strap on it that I assume is ground.

The bottom picture shows the gauge, which has no identifying labels on it. I assume terminal G4 is for lighting the gauge, which I don't care about. So, can anyone tell me "what connects to what" between sensor, gauge, battery, and ground? Normally, I'd experiment, but I don't want to blow anything out.

And, is there a way to calibrate the gauge to read accurately when empty and full?

Y'all rock. Many thanks!

 

[Jim.ME]

Greetings, fellow Masseyphiles. I'm having trouble figuring out how to wire up the fuel level sensor to the fuel gauge on my 1971 MF135. Help would be most appreciated. Parts were purchased from Yesterday's Tractors.

The top picture shows the sensor with its two terminals, which I've labeled T1 and T2. There's a third, unlabeled screw with a metal strap on it that I assume is ground.

The bottom picture shows the gauge, which has no identifying labels on it. I assume terminal G4 is for lighting the gauge, which I don't care about. So, can anyone tell me "what connects to what" between sensor, gauge, battery, and ground? Normally, I'd experiment, but I don't want to blow anything out.

And, is there a way to calibrate the gauge to read accurately when empty and full?

Y'all rock. Many thanks!

View attachment 63962

View attachment 63963

On the sender one terminal is likely insulated and would go to the sender terminal on the gauge. The other is likely ground and would have a wire to the chassis for ground if needed, some will ground through the sender mount screws and tank. A ground wire assures a steady ground. Confirm which is which with an ohmmeter by checking resistance between each terminal and mount plate. the ground terminal should have the lowest resistance to the plate, of the two.

On the gauge G4 would look like the backlight. One terminal should be a ground, check between the terminals and ground to find which one. One of the others will be power from the switch, and may be marked with an I. The other will go to the sender and they are often marked with an S. As a guess I will say the terminal on the Empty side of the gauge gets the power. Use the info at your own risk.

As for calibration, does YT sell them as matching each other based on ohm range of the sender to gauge scale reading? You might have to call and ask them. They should have wiring info on the terminals as well. You might bend the arm a bit if the float travel is not right, but you won't easily change the resistance scale. I never expect a tractor fuel gauge to be right on accurate, but more of an indicator, especially replacements.

 

[Bill Knecht]

On the sender one terminal is likely insulated and would go to the sender terminal on the gauge. The other is likely ground and would have a wire to the chassis for ground if needed, some will ground through the sender mount screws and tank. A ground wire assures a steady ground. Confirm which is which with an ohmmeter by checking resistance between each terminal and mount plate. the ground terminal should have the lowest resistance to the plate, of the two.

On the gauge G4 would look like the backlight. One terminal should be a ground, check between the terminals and ground to find which one. One of the others will be power from the switch, and may be marked with an I. The other will go to the sender and they are often marked with an S. As a guess I will say the terminal on the Empty side of the gauge gets the power. Use the info at your own risk.

As for calibration, does YT sell them as matching each other based on ohm range of the sender to gauge scale reading? You might have to call and ask them. They should have wiring info on the terminals as well. You might bend the arm a bit if the float travel is not right, but you won't easily change the resistance scale. I never expect a tractor fuel gauge to be right on accurate, but more of an indicator, especially replacements.

Hi, Jim. Thanks for the reply. Following your suggestions, I went ahead and removed the gauge & found I was wrong. It does have some info on the back. The terminal I labeled as G1 says "B", G2 = "T", G3 = "L". Any idea what those stand for? With gauge mounted on tractor, I checked resistance and got infinity between all 3 of those terminals and ground.

The sender shows my label T2 as having zero resistance to ground, while T1 shows 42 ohms on a tank w about 1 gallon of fuel in it. Can I assume that T1 is therefore the sender output, and T2 should go to ground?

 

[Jim.ME]

Hi, Jim. Thanks for the reply. Following your suggestions, I went ahead and removed the gauge & found I was wrong. It does have some info on the back. The terminal I labeled as G1 says "B", G2 = "T", G3 = "L". Any idea what those stand for? With gauge mounted on tractor, I checked resistance and got infinity between all 3 of those terminals and ground.

The sender shows my label T2 as having zero resistance to ground, while T1 shows 42 ohms on a tank w about 1 gallon of fuel in it. Can I assume that T1 is therefore the sender output, and T2 should go to ground?

Yes, I will say T2 would be to ground. Given T1 is the terminal with resistance on the sender, I would wire it to T on the gauge, B would be the switched power (oil pressure switch if you have a diesel, ignition switch on a gas tractor). Then L would be a ground to be sure the backlight has a ground, I would use a temporary jumper wire with a low amp inline fuse to try on the L terminal before hard wiring it to ground.

 

[Bill Knecht]

Yes, I will say T2 would be to ground. Given T1 is the terminal with resistance on the sender, I would wire it to T on the gauge, B would be the switched power (oil pressure switch if you have a diesel, ignition switch on a gas tractor). Then L would be a ground to be sure the backlight has a ground, I would use a temporary jumper wire with a low amp inline fuse to try on the L terminal before hard wiring it to ground.

Roger that. T1 to "T" on the gauge. I have a diesel, so I'll check the oil pressure switch for power, and run from there to "B". Since T2 is grounded, it sounds like a wire should go from T2 to "L" on the gauge.

I'll report back when done. It'll be a few days because I'm replacing water pump & thermostat right now, and then have to flush for rust, because the system was clogged with it.

You, Sir, are super. Can't thank you enough.

 

[Jim.ME]

Roger that. T1 to "T" on the gauge. I have a diesel, so I'll check the oil pressure switch for power, and run from there to "B". Since T2 is grounded, it sounds like a wire should go from T2 to "L" on the gauge.

I'll report back when done. It'll be a few days because I'm replacing water pump & thermostat right now, and then have to flush for rust, because the system was clogged with it.

You, Sir, are super. Can't thank you enough.

Remember some of this is best guess. If you have the time, you should call YT and see if they can tell you for certain.

It is also possible they used L for load (sender) and T for ground on the gauge. With power on (you will need to bypass the oil switch for testing if the engine isn't running), if you ground T the needle should go to full, I believe and grounding the L it won't move if my thoughts were right. If it doesn't move with power on and the T grounded but goes full scale when L is grounded, L is where the sender wire goes.

 

[Bill Knecht]

I will call them.

This from Google:
'"What does B and T stand for on a fuel gauge?

There are three electrical connections to such a fuel gauge: Terminal 'B' (connected to the ignition switch and battery positive), Terminal 'T' (which is connected to the Tank Unit) and the gauge case itself (which is unmarked but is earthed to battery negative)."

Which implies "B = Battery", "T = Tank". But, what "L" stands for beats me. We're assuming it's ground.

Time for some experimentation!

 

[Bill Knecht]

OK, back again, for them as is watchin'. Fuel gauge now works (kinda-sorta). "B" goes to 12V battery power, "T" goes to fuel tank, Bottom gauge terminal goes to ground terminal on the tank sensor. So far, I've just tested it by "B" directly to battery. Will eventually try connecting to the terminal on the fuel pressure gauge.

But, here's the oddity: Near-empty fuel tank registers 3/4 full. When I add 2 more gallons it registers same, no change. Is there any way to correct that? Would it help if I removed the sensor & posted a shot of it?

 

[Jim.ME]

OK, back again, for them as is watchin'. Fuel gauge now works (kinda-sorta). "B" goes to 12V battery power, "T" goes to fuel tank, Bottom gauge terminal goes to ground terminal on the tank sensor. So far, I've just tested it by "B" directly to battery. Will eventually try connecting to the terminal on the fuel pressure gauge.

But, here's the oddity: Near-empty fuel tank registers 3/4 full. When I add 2 more gallons it registers same, no change. Is there any way to correct that? Would it help if I removed the sensor & posted a shot of it?

View attachment 64701

It sounds like the resistance of the sender as it moves and the resistance the gauge needs to locate the needle don't match. Take the sender of and wire it to the gauge and see what the needle does as you move the sender through the full range.

Did YT confirm to you that the gauge and sender are spec'd with matching resistance ranges? Do they have documentation of resistance readings for sender positions and gauge needle positions? (X number of ohms resistance at sender for empty, 1/4, 1/2, 3/4, and full on the gauge.)

 

[Bill Knecht]

Jim, based on your suggestion, I figured out what was wrong. The fuel-level sending unit (part no. 189132M91) has two tiny electrical brushes that are supposed to make rheostatic contact, moving along an arc, as the fuel level changes and moves the float mechanism. Big surprise for a 52 year-old machine (not!) -- these little brushes look almost completely worn out. That explains why I was getting intermittent resistance readings (usually infinite) across the two sending unit terminals.

I've ordered a new sending unit, 60 bucks, and we'll see how that works once it gets here.

 

[Jim.ME]

Jim, based on your suggestion, I figured out what was wrong. The fuel-level sending unit (part no. 189132M91) has two tiny electrical brushes that are supposed to make rheostatic contact, moving along an arc, as the fuel level changes and moves the float mechanism. Big surprise for a 52 year-old machine (not!) -- these little brushes look almost completely worn out. That explains why I was getting intermittent resistance readings (usually infinite) across the two sending unit terminals.

I've ordered a new sending unit, 60 bucks, and we'll see how that works once it gets here.

View attachment 64809View attachment 64819View attachment 64820

Sorry, for some reason, I was thinking you had replaced both the gauge and sender, that why I was asking if they were sold as a matched pair. Yes, senders wear out from movement and can even wear through the fine resistance wire. Either way if the gauge and sender resistances have to match. Hopefully they are both made to the same spec range.

 

[Bill Knecht]

Sorry, for some reason, I was thinking you had replaced both the gauge and sender, that why I was asking if they were sold as a matched pair. Yes, senders wear out from movement and can even wear through the fine resistance wire. Either way if the gauge and sender resistances have to match. Hopefully they are both made to the same spec range.

That's my bad for not specifying. I had replaced the gauge, but not the sender. The ones I bought don't specify as being matched-pair, but I'm hoping all will be well. I bought the gauge from YT, and the sender is coming as specified by part number from the MF135 Parts Book. I'm guessing manufacturers try to standardize these things. We shall see! Sender will arrive next week, so I'll report off. Hopefully, this discussion will help others in the future.

 

[Bill Knecht]

The new fuel level sensor just arrived. Not quite as posh as the old one. One brush on the rheostat instead of the original two, and cork gasket instead of rubber. But, new one does measure infinite ohms at "tank empty", and about 4 at "tank full", so should be fine.

Tractor starts & runs w new water pump & hoses. BUT, now I see a slow fuel leak at fuel injector #1. Won't stop when I try to tighten it. Sigh...

 

[Bill Knecht]

On the sender one terminal is likely insulated and would go to the sender terminal on the gauge. The other is likely ground and would have a wire to the chassis for ground if needed, some will ground through the sender mount screws and tank. A ground wire assures a steady ground. Confirm which is which with an ohmmeter by checking resistance between each terminal and mount plate. the ground terminal should have the lowest resistance to the plate, of the two.

On the gauge G4 would look like the backlight. One terminal should be a ground, check between the terminals and ground to find which one. One of the others will be power from the switch, and may be marked with an I. The other will go to the sender and they are often marked with an S. As a guess I will say the terminal on the Empty side of the gauge gets the power. Use the info at your own risk.

As for calibration, does YT sell them as matching each other based on ohm range of the sender to gauge scale reading? You might have to call and ask them. They should have wiring info on the terminals as well. You might bend the arm a bit if the float travel is not right, but you won't easily change the resistance scale. I never expect a tractor fuel gauge to be right on accurate, but more of an indicator, especially replacements.

Jim -- when you say "power from the switch", do you mean the ignition switch?

This machine was refitted with an alternator, so the original voltage regulator's disconnected. I notice on OEM schematics, the power source for the fuel gauge was originally on the VR. This confuses me, because, when the machine is running, the "system voltage" will depend on whatever the alternator's currently outputting, correct? So, how could the fuel gauge receive a stable voltage if the alternator's output varies with demand, particularly on startup?

Maybe I'm overthinking this. Maybe the fuel gauge is just temporarily overenergized right after startup, until the alternator output replenishes the battery? If so, the fuel gauge would get too high a voltage for a minute or so, but would then settle down to a pretty stable value. Is that how you see it?

 

[Jim.ME]

Jim -- when you say "power from the switch", do you mean the ignition switch?

This machine was refitted with an alternator, so the original voltage regulator's disconnected. I notice on OEM schematics, the power source for the fuel gauge was originally on the VR. This confuses me, because, when the machine is running, the "system voltage" will depend on whatever the alternator's currently outputting, correct? So, how could the fuel gauge receive a stable voltage if the alternator's output varies with demand, particularly on startup?

Maybe I'm overthinking this. Maybe the fuel gauge is just temporarily overenergized right after startup, until the alternator output replenishes the battery? If so, the fuel gauge would get too high a voltage for a minute or so, but would then settle down to a pretty stable value. Is that how you see it?

The voltage output of the alternator varies within a given range for charging, usually in the 13.6 to 14.6 volts. Your fuel gauge will be fine on system voltage and its fluctuations. On your ammeter you will see the amperage output high right after starting, but the amperage output will drop off as the battery voltage comes up. So, when your ammeter shows a high reading the battery voltage is much lower than the regulator set point. When the amp reading is lower, battery voltage is up.

If your diesel is like most MFs diesels of that vintage, I don't think you don't have an ignition switch, only a start switch (with a pull to stop cable to shut the fuel off.

In post #4, I said "B would be the switched power (oil pressure switch if you have a diesel, ignition switch on a gas tractor). That was before post #5 when you said you have a diesel. The wiring diagram I have for a 135 has a Delco Remy 10DN alternator which uses an external regulator and fuel gauge power did come from the regulator, through an inverter. If the regulator was removed, I am guessing you have either a 3-wire, or 1-wire internally regulated alternator.

The oil pressure switch is used on later tractors, which have internally regulated alternators, to power gauges. Use the oil pressure switch to turn the power on and off to the inverter (if it is still there). The fuel gauge needs to be on a switched power source, which on your tractor would be the oil pressure switch, now. It originally worked, getting power from the regulator, because it was connected to the regulator on a point that had no power when it was not charging (engine shut off).

 

[Bill Knecht]

The voltage output of the alternator varies within a given range for charging, usually in the 13.6 to 14.6 volts. Your fuel gauge will be fine on system voltage and its fluctuations. On your ammeter you will see the amperage output high right after starting, but the amperage output will drop off as the battery voltage comes up. So, when your ammeter shows a high reading the battery voltage is much lower than the regulator set point. When the amp reading is lower, battery voltage is up.

If your diesel is like most MFs diesels of that vintage, I don't think you don't have an ignition switch, only a start switch (with a pull to stop cable to shut the fuel off.

In post #4, I said "B would be the switched power (oil pressure switch if you have a diesel, ignition switch on a gas tractor). That was before post #5 when you said ou have a diesel. The wiring diagram I have for a 135 has a Delco Remy 10DN alternator which uses an external regulator and fuel gauge power did come from the regulator, through an inverter. If the regulator was removed, I am guessing you have either a 3-wire, or 1-wire internally regulated alternator.

The oil pressure switch is used on later tractors, which have internally regulated alternators, to power gauges. Use the oil pressure switch to turn the power on and off to the inverter (if it is still there). The fuel gauge needs to be on a switched power source, which on your tractor would be the oil pressure switch, now. It originally worked, getting power from the regulator, because it was connected to the regulator on a point that had no power when it was not charging (engine shut off)

The voltage output of the alternator varies within a given range for charging, usually in the 13.6 to 14.6 volts. Your fuel gauge will be fine on system voltage and its fluctuations. On your ammeter you will see the amperage output high right after starting, but the amperage output will drop off as the battery voltage comes up. So, when your ammeter shows a high reading the battery voltage is much lower than the regulator set point. When the amp reading is lower, battery voltage is up.

If your diesel is like most MFs diesels of that vintage, I don't think you don't have an ignition switch, only a start switch (with a pull to stop cable to shut the fuel off.

In post #4, I said "B would be the switched power (oil pressure switch if you have a diesel, ignition switch on a gas tractor). That was before post #5 when you said you have a diesel. The wiring diagram I have for a 135 has a Delco Remy 10DN alternator which uses an external regulator and fuel gauge power did come from the regulator, through an inverter. If the regulator was removed, I am guessing you have either a 3-wire, or 1-wire internally regulated alternator.

The oil pressure switch is used on later tractors, which have internally regulated alternators, to power gauges. Use the oil pressure switch to turn the power on and off to the inverter (if it is still there). The fuel gauge needs to be on a switched power source, which on your tractor would be the oil pressure switch, now. It originally worked, getting power from the regulator, because it was connected to the regulator on a point that had no power when it was not charging (engine shut off).

Jim, You're right. I just checked & it's a starter switch w two terminals only, with a fuel shutoff pull-knob close by. You've just taught me the difference between an ignition switch (which would have something like an "ACCessories" position) and a starter switch. I really appreciate your having such patience with a newbie like me.

I guess I don't quite understand the oil pressure switch. I know it's the thing that screws into the back of the oil pressure gauge itself. And, mine has two electrical blades coming out of it, but no wires currently connected to it. I had read that the OP switch was supposed to trigger a warning light if oil pressure got critically low. That would imply infinite ohms if oil pressure was OK (warning light OFF), and zero ohms if oil pressure was too low (warning light ON), wouldn't it?

But, that doesn't sound like what you're talking about. What I'm hearing is that a certain kind of OP switch functioned exactly the opposite -- zero ohms if oil pressure was OK, infinite ohms if too low. I.e., switchable 12V DC power, on only when engine is running.

Eureka!!! I just went out and tested my OP switch, and that's exactly what it does. Infinite ohms across the two blades when engine is off, zero ohms when engine on.

I have the "T" and ground terminals of the fuel gauge wired up, and it works if I apply direct 12V DC to it. SO... If I hook one blade of the OP switch to 12V DC, the other to the "B" terminal on my fuel gauge, that should light the candle of this moon rocket, yes?

 

[Jim.ME]

Jim, You're right. I just checked & it's a starter switch w two terminals only, with a fuel shutoff pull-knob close by. You've just taught me the difference between an ignition switch (which would have something like an "ACCessories" position) and a starter switch. I really appreciate your having such patience with a newbie like me.

I guess I don't quite understand the oil pressure switch. I know it's the thing that screws into the back of the oil pressure gauge itself. And, mine has two electrical blades coming out of it, but no wires currently connected to it. I had read that the OP switch was supposed to trigger a warning light if oil pressure got critically low. That would imply infinite ohms if oil pressure was OK (warning light OFF), and zero ohms if oil pressure was too low (warning light ON), wouldn't it?

But, that doesn't sound like what you're talking about. What I'm hearing is that a certain kind of OP switch functioned exactly the opposite -- zero ohms if oil pressure was OK, infinite ohms if too low. I.e., switchable 12V DC power, on only when engine is running.

Eureka!!! I just went out and tested my OP switch, and that's exactly what it does. Infinite ohms across the two blades when engine is off, zero ohms when engine on.

I have the "T" and ground terminals of the fuel gauge wired up, and it works if I apply direct 12V DC to it. SO... If I hook one blade of the OP switch to 12V DC, the other to the "B" terminal on my fuel gauge, that should light the candle of this moon rocket, yes?

Yes, that should do it. You might want to put a fuse holder in the power wire to the switch to be safe. A 10-amp fuse would be plenty for your fuel gauge. I would use a holder for the ATO (blade) type fuses.

Your oil pressure switch is a normally open switch. The contacts are open when there is no pressure and close when there is pressure, turning the gauge on.

An indicator light for no oil pressure uses a normally closed switch. No pressure the contacts are closed so the light comes on. When there is pressure the contacts open and the light goes out.

 

[Bill Knecht]

Yes, that should do it. You might want to put a fuse holder in the power wire to the switch to be safe. A 10-amp fuse would be plenty for your fuel gauge. I would use a holder for the ATO (blade) type fuses.

Your oil pressure switch is a normally open switch. The contacts are open when there is no pressure and close when there is pressure, turning the gauge on.

An indicator light for no oil pressure uses a normally closed switch. No pressure the contacts are closed so the light comes on. When there is pressure the contacts open and the light goes out.

Can't wait to try this. It's raining now, so I'll have to wait.

Man, I should send you a box of your favorite chocolate, or something like that. You've saved me countless hours trying to figure all this out.

 

[Jim.ME]

Can't wait to try this. It's raining now, so I'll have to wait.

Man, I should send you a box of your favorite chocolate, or something like that. You've saved me countless hours trying to figure all this out.

Just happy to help.

 

[Bill Knecht]

Just happy to help.

Fuel gauge works. It's AAAA--LIIIIIIIIIVE !!!