Tractor Fuel Gauge/Sender Problem

[Gregm]

I am working on a friend’s fuel gauge for his tractor. Manual says wires (ground/sender) disconnected gauge goes to full and wires together goes to empty. It does both of these. The sender disconnected is showing 26 ohms (the manual says this is a quarter tank) which is a quarter tank and this matches what is in the tank based on putting a stick in the tank to measure the fuel. The gauge is not showing a quarter of tank, but almost empty.

My question, once I connect the fuel gauge wire to the sender (key in off position), the ohms drop by 10. As I increase the resistance on the sender the ohms drop increases at 67 ohms the drop is 20 ohms. Should this happen? If NO what would cause it.

Sender is grounded to metal tank. The wire from sender to cluster/fuel gauge is good. How I am measuring this. I connect the ohm meter to sender on one wire from the meter and ground the other wire to frame. This shows 26 ohms, as soon as I connect wire from the fuel gauge the meter then shows 16 ohms.

Thanks in advance for any help.

 

[Harald Kapp]

once I connect the fuel gauge wire to the sender (key in off position), the ohms drop by 10.

This can be explained by the input resistance of the gauge being in parallel to the resistance of the sensor. An input resistance of 41.6 Ohm in parallel to 26 Ohm from the sensor results in 16 Ohm. A perfect match for your observation.
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I do not understand this part:

As I increase the resistance on the sender the ohms drop increases at 67 ohms the drop is 20 ohms

Could you explain in more detail?

A simple way to tst the gauge would be a potentiometer instead of the sensor such that you can vary the resistance manually and observe the gauge's display.
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[Gregm]

Thanks for the response. I do understand about parallel resistance, but it does not explain 26 to 16 or another reading of 79 to 30.
Thanks

I am using a potentiometer to be able to get the 79 to 30.

Thanks

 

[Kiwi]

Gregm,
As Harald says you are measuring the gauge and sender resistances in parallel. Forget it, you will only confuse yourself doing this, and it won't help in diagnosing the fuel gauge system.

Note: The gauge will probably be showing the depth of the fuel in the tank, which may not be the same as the volume of fuel in the tank. A round tank with the level at 1/4 contains much less than 1/4 of the volume of a full tank.

There are three things that need to be checked to find the fault;
1 - wiring, connections, earths etc. from gauge to sender to ground. Make sure tank is securely grounded.
2 - sender resistance across range from empty to full. Make sure float is ok, and float arm doesn't catch inside tank.
3 - gauge calibration.

Looks like you have already checked the wiring, so onto the fun part.

Remove the tank sender and lay it on a flat insulated board with the float in the empty position. Connect the gauge wire and connect an earth wire to the sender body. With the key on, check the gauge reading. Then move the arm to 1/4, 1/2, 3/4, and full. Wait at least 30 seconds in each position for the gauge to stabilise. You now know how the gauge and sender are working together. Is it correct?
Disconnect the sender and measure its resistance at the points listed in the manual. Is it correct?
Set a potentiometer to the first resistance point given in the manual. Connect it to the gauge wire and check the gauge reading. Is it correct?
Disconnect the potentiometer and set it to the next point. Connect it to the gauge wire and check the gauge reading. Is it correct?
Do this for all the points given. Is the gauge reading correct across the whole range?

From these tests you should know what the problem is.

 

[BruceS]

I've just been though a similar problem & traced it to reduced tension of the little bit that slides along the windings in the sender unit. Due to wear of the pivot points there was minimal pressure being applied.
A bit of clean up (diesel is dirty you know!) & a little bending of the 'scribe' & we were good to go!
If it happens again I'll throw a new sending unit in it. (Summer time job as under a houseboat!)

 

[Gregm]

Thanks for the reply.

Here is where I am at I have removed the sender and the gauge. Using an ohm meter I have tested the sender and it meets the ohm specs outlined in the tech. manual. I have wired it up and moving the sender float up and down the gauge basically reads half of what it should. I have added some resistors to the sender and it will show more full but as expected it will not go to empty.

The problem is in the gauge, so the question now is there away to recalibrate the gauge? I have done a number of searches on the net and cannot find a way. There are rivets holding the cover on and at this point I do not want to remove them. The gauge cost $100 to replace it is from a John Deere tractor.

How it is wired, the gauge has a ground, sender and power. One wire runs from gauge to sender, one wire runs from gauge power to power source, ground of sender, gauge and power source are tied together.

Thanks in advance for any help.

 

[BruceS]

Seems pretty obvious to me that at some stage either the gauge or the sender unit was replaced with the wrong unit.
They must match. When I was trouble shooting mine I found that there appear to be 3 distinct 'types'.
Some info here.
http://www.classicinstruments.com/index.aspx?documentID=fuelgaugesandtank
You probably already found all this info?

 

[Gregm]

The owner replaced the sender a couple years back with a new one from John Deere, the tech manual says the ohm range on the sender is 0 to 93, which it is. There are 6 separate gauges in the cluster and the fuel gauge looks like the other 5, so I think it is the correct one.

In doing some more testing using resistors, at 56 ohms the gauge is 25% full, 106 ohms the gauge is 60% full and 156 ohms the gauge is 78% full. Putting an ohm meter on the gauge at the sender and the ground, with no power, I get 104 ohms.

I do not know how things work inside the gauge. Anyone have a suggestion as to what is going on?

Since it costs $100 to replace, I think the owner would allow me to open it up, but at this point I would not know what to do, so have not suggested this.

Thanks in advance.

 

[BruceS]

See if your local Auto elec chap will lend you a VDO gauge for testing?
Sounds like the sender unit is a VDO range one? 10 to 180..........
Probably cheaper to replace the sender & keep the gauge I reckon.
Has the gauge EVER worked OK with that gauge & sender unit?
Whichever way it is it's obvious you don't have a matching pair. (My opinion anyway)

 

[Gregm]

I do not think the fuel gauge has worked correctly since my friend has owned the tractor as my friend is slowly restoring it. He purchased it at auction and does not know its history. I know it has had some wiring work done to it. The sender is 0 to 93 ohms. So would I be correct in saying the gauge is in the range 0 to 180?

I am still at electronics 101 stage and do not think there is a way to do this, but is there a way to double the resistance coming from the sender.

Thanks for your help on this.

 

[Steve Wikel]

I have a x575 John Deere tractor and my fuel gauge seems to move around alot. Also have trouble with tractor shutting down and can only restart with full choke. Can the gauge be related to fuel problem? What do you think could be stalling out engine. Tried fuel filter already. Possible fuel pump? Thanks