Massey 255 fuel gauge and sending unit. Aftermarket and troubleshooting.

[Floopy]

So, I bought a 255 recently and the gauges weren't working, so I'm fixing them. The tach was missing a year in the drive, a new drive and I had that going. The gas gauge didn't work, there was no power to it. It runs off the oil pressure gauge which powers everything only when it has oil pressure. Kinda nifty.

I removed all the wiring since to the lights and most of the gauge wiring. It was all hacked and spliced

I put power to the gauge and grounded it. It went to full. So, I take the hood off, remove the old sending unit and take a look. There is nothing touching at all. The end was completely corroded away. Sounds great so I ordered a sending unit.

I got my universal sending unit since they don't make the one for the tractor anymore(Dorman 55818) and flipped the part that measures over, making it right for the gauge. I test it with a meter, everything is a go.

I hook it up to the gauge wire and ground it against the tank. It goes to full and stays. I disassembled and checked the sending unit again, reassemble. Same result. I ended up putting a test light on the negative battery terminal and testing the power side. I feel a almost imperceptible tingle on my arm as I lean holding the hooked up sending unit. I test the loader where my arm was resting. No power there. I I stick it on the wire to the sending unit and get a tiny flicker on the light. I test it and I have voltage but it is barely enough to make the filament glow in the test light.

I check the gauge and look for touching wires or missing plastic standoffs. I find nothing so I remove the gauge and test with just power to it. Same result, it is sending power to the sending unit.

I assume the gauge is bad in this scenario. I have new gauges for everything but fuel, I went ahead and ordered a 2" gauge.

There isn't any reason it should be pushing power of any sort into the sending unit, right?

 

[Jim.ME]

Not sure I understand.

Power is supplied to one terminal of the gauge from the oil pressure switch. Power is sent from the sender terminal of the gauge to the sending unit. The variable resistance of the sending unit provides the ground to make the gauge needle move.

So yes, the gauge pushes power to the sending unit which is the ground for the gauge.

Some newer floating needle gauges do have a dedicated ground at the gauge as well as the power and sending unit terminals.

 

[ford4wd08]

The new gauge I installed has a ground at the gauge. I also added a ground wire in addition to the little metal tab that grounded the sending unit on the tank. I just routed the wire back to a ground on part of the pedestal frame.

 

[Floopy]

Not sure I understand.

Power is supplied to one terminal of the gauge from the oil pressure switch. Power is sent from the sender terminal of the gauge to the sending unit. The variable resistance of the sending unit provides the ground to make the gauge needle move.

So yes, the gauge pushes power to the sending unit which is the ground for the gauge.

Some newer floating needle gauges do have a dedicated ground at the gauge as well as the power and sending unit terminals.

I guess I assumed sending power inside the gas tank would be wrong. I assumed it was closer to being a multimeter. I certainly didn't expect it to send enough power to light a test light up. It seems like it would possibly be enough to spark and cause an explosion on a gas engine given it would be up in the fuel vapors on top of the gas tank.

I tested the resistance on the sending unit and it was 240-33. The gauge pegged full no matter where I moved it when I grounded the sending unit. If it was working as intended I'd expect it to at least move as I moved the sending unit float.

Am I misunderstanding the system and missing something obvious?

Power to gauge, wire to sending unit, ground sending unit. The gauges don't match and it is possible the gauge is a different type or malfunctioning. I'd assume some response somewhere between 33 and 240 resistance if some sort though. It just stays pegged on high.

Thinking back the only thing I didn't try was reversing the sending unit and power wire. Given someone was there before me it is possible it got swapped but it doesn't particularly look like it has ever been off.

 

[Jim.ME]

I have never had much of any luck matching universal sending units to tractor gauges. It is entirely possible the old gauge has problems. You will likely have better luck if you can get a gauge that says it can be used with that sending unit.

I doubt the wires on the gauge have been reversed.

 

[Floopy]

I have never had much of any luck matching universal sending units to tractor gauges. It is entirely possible the old gauge has problems. You will likely have better luck if you can get a gauge that says it can be used with that sending unit.

I doubt the wires on the gauge have been reversed.

I'd have expected some response from it regardless. I ordered a new gauge last night that is 33-240. It was only around $8, so I figured it wouldn't hurt to have one.

 

[ford4wd08]

The sending unit I bought I bought made by Tanks Inc worked on 3 different aftermarket gauges.

I didn't like the way a couple of ones I bought looked so I ended up buying a chrome bezeled one that had to ship from India. Cost for all together was maybe $50.

I would double check your wiring, you have to have power going into the tank. We are using Ohm's law to get your tank level reading. You don't have resistance without current and voltage (R=V/I).

Don't be concerned about fire risk with power going into a tank, you can put a lit cigarette out in a bowl of gasoline, it's all about the air/fuel mixture to make it ignite. The reason carburetors and fuel injection systems exist is to get the air/fuel ratios correct so it will combust.

Also power isn't the same as a spark.

 

[Floopy]

The sending unit I bought I bought made by Tanks Inc worked on 3 different aftermarket gauges.

I didn't like the way a couple of ones I bought looked so I ended up buying a chrome bezeled one that had to ship from India. Cost for all together was maybe $50.

I would double check your wiring, you have to have power going into the tank. We are using Ohm's law to get your tank level reading. You don't have resistance without current and voltage (R=V/I).

Don't be concerned about fire risk with power going into a tank, you can put a lit cigarette out in a bowl of gasoline, it's all about the air/fuel mixture to make it ignite. The reason carburetors and fuel injection systems exist is to get the air/fuel ratios correct so it will combust.

Also power isn't the same as a spark.

 

[Floopy]

The sending unit I bought I bought made by Tanks Inc worked on 3 different aftermarket gauges.

I didn't like the way a couple of ones I bought looked so I ended up buying a chrome bezeled one that had to ship from India. Cost for all together was maybe $50.

I would double check your wiring, you have to have power going into the tank. We are using Ohm's law to get your tank level reading. You don't have resistance without current and voltage (R=V/I).

Don't be concerned about fire risk with power going into a tank, you can put a lit cigarette out in a bowl of gasoline, it's all about the air/fuel mixture to make it ignite. The reason carburetors and fuel injection systems exist is to get the air/fuel ratios correct so it will combust.

Also power isn't the same as a spark.

I'd assume feeding 12 volt into it would cause sparks as it wore and had less contact with it being at the top of the fume filled tank if it had enough juice to tingle my arm or light up a test light. Maybe I'm overthinking it lol.

I cut everything out and just connected the gauge directly to power and grounded the sending unit direct to the negative on the battery. Even though I can see it changes resistance with a multimeter, it doesn't change the gauge at all even though the sending unit seems to be operating exactly as intended. I went ahead and ordered a new gauge.

 

[Floopy]

I saw a reference that the UK gauge is a 0-30 ohm. If this is the case then it explains why there is no response. I had read some things that said it was 240-33 or similar previously.

 

[ford4wd08]

I saw a reference that the UK gauge is a 0-30 ohm. If this is the case then it explains why there is no response. I had read some things that said it was 240-33 or similar previously.

From what I have found, US gauge is 33 empty - 240 full.

 

[Floopy]

From what I have found, US gauge is 33 empty - 240 full.

Which is what I was basing my tests on but I saw a reference that made in UK ones were 0-30, which would make a 240-33 sending unit stay on full all the time.