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Discussion Starter · #1 ·
My 448 has this fuel gauge situated on the dash

It has the word METEOR on the bottom of the face.
This is the sender unit

Only the RED empty light appears to work at this time. Does anyone have any knowledge of these gauges, I've never seen one quite like it before? :headscratcher: :headscratcher: :headscratcher:
 

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That looks like it would be very handy.

I keep meaning to buy one of those float style gauges for my tank cap but I never do.


With the Onans reputation for using fuel I never ever use it without topping off the gas tank.
 

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Fuel sending units generally have a two wire hook up. One to ground and the other to the back of your gauge. If your's has one wire, then the sending unit is a self-grounding type, which works in metal tanks or must have a wire connected to ground from the mounting point of the unit.

Most sending uints for fuel (and otherwise) are operated on the principle of resistance, which directly dictates the amount of movement on the gauge, or in your case, which lights come on. For example, many Ford fuel sending units will operate from 0-90 OHMS (measured with a continuity/voltage tester) and are either wired for 0 at empty or 90 at empty.

To test your sending unit, you can remove it and hook up the two leads of your continuity tester to the hot and ground side of the sending unit, set tester at OHMS, and physically move the float up and down to see the reading. There should be a low number, somehwere around 0-10 ohms, and a high number, which will be near 90, 250, or some other number depending on manufacture specs. The main idea is that the OHMS number should change steadily from empty to full. Check your numbers a few times, and be sure that you get a consistent reading. Also make sure your sending unit is disconnected from the tractor's wiring harness, otherwise you will be reading the resistance through the gauge on the dash at the same time.

As for the gauge, hooking up a poteniometer will work well if you connect it to the back of the gauge where the sending unit hooks up, hooking the other side back to the ground at the battery. This completely takes place of the sending unit when used like this.

Good luck, I'd like to see that work for you!

Rob
 

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Discussion Starter · #5 ·
case680rob said:
Fuel sending units generally have a two wire hook up. One to ground and the other to the back of your gauge. If your's has one wire, then the sending unit is a self-grounding type, which works in metal tanks or must have a wire connected to ground from the mounting point of the unit.

Most sending uints for fuel (and otherwise) are operated on the principle of resistance, which directly dictates the amount of movement on the gauge, or in your case, which lights come on. For example, many Ford fuel sending units will operate from 0-90 OHMS (measured with a continuity/voltage tester) and are either wired for 0 at empty or 90 at empty.

To test your sending unit, you can remove it and hook up the two leads of your continuity tester to the hot and ground side of the sending unit, set tester at OHMS, and physically move the float up and down to see the reading. There should be a low number, somehwere around 0-10 ohms, and a high number, which will be near 90, 250, or some other number depending on manufacture specs. The main idea is that the OHMS number should change steadily from empty to full. Check your numbers a few times, and be sure that you get a consistent reading. Also make sure your sending unit is disconnected from the tractor's wiring harness, otherwise you will be reading the resistance through the gauge on the dash at the same time.

As for the gauge, hooking up a poteniometer will work well if you connect it to the back of the gauge where the sending unit hooks up, hooking the other side back to the ground at the battery. This completely takes place of the sending unit when used like this.

Good luck, I'd like to see that work for you!

Rob
Thanks Rob,

The gauge does have two wires. I will get to work with my multi meter tomorrow and carry out these checks. I also plan to see if the gauge itself can be opened in case it's a simple bulb replacement that is needed.
 

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I think that all rheostats used in sending units are totally enclosed. After all, having the potential for an electrical spark to take place inside the fuel tank with gasoline vapours readily available would not be a good idea.
 

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Will alcohol affect the fuel sending uinit?

Relating back to something Hydriv posted about composite carb floats, I'd say it depends on the situation. If your float is hollow core composite and it breaks down, it can fill with fuel and sink to the bottom. I've seen this alot in the automotive world, happens alot in late 80's to mid 90's cars and trucks. Also, if the float rides on a guided rod that twists a variable resistor, similar to the float-gauge-fuel cap combo mentioned by Snotrocket, the float can expand and ride too close to the outer guide rods and stick. This won't happen if your float rides on the end of a long lever which operates the sending unit, nothing for it to expand and hang up on.

As for sealed sending units, I know at least the ones in most automotive fuel tanks are open- just contacts riding on an insulated coil of wire. The contacts are the most common point of wear because they move all the time. Temp, pressure, and many fuel injection components are almost always made with sealed sending units, but it's rare to see a sealed float type sending unit unless there is some concern for wear, operation, or some sort of danger with the open type of rheostat/potentiometer. I agree that there should be a danger of combustion here, but in the 12v world, sparks are not really much of a concern. 12v DC power doesn't jump gaps well on it's own. By the time the power goes through the gauge and gets to the sending unit there is not enough there to do anything at the tank. It's like trying to take a shower when the washer, dishwasher, and kitchen sink are being used. Or like washing the car when someone is kinking the hose. There's just nothing left, the gauge uses almost all of the power before the sending unit gets any. This is generally the case, I could be wrong about cumminscanuck's sending unit having not seen it out of the tractor.
 
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