7710 fuel gauge trouble (need help trouble shooting problem)

[pharmvet]

I have a 7710 tractor with an upper and lower fuel tank. As I understand it, fuel feeds to the engine always from the lower tank. The upper tank empties via gravity into the lower tank as fuel is consumed. My tractor has a toggle switch on the dash that is used to check the fuel levels in each tank. #1 on top which apparently corresponds to the upper tank, and #2 on the bottom which apparently corresponds to the lower tank.

The upper tank reading (#1) has always worked since I bought the tractor, the lower tank reading (#2) has never worked since I bought the tractor.

The other day I had to have the lower tank dropped to fix a leaking return fuel line. I figured it would be the perfect time to replace the lower tank sending unit, which I assumed was the problem since the fuel gauge was correct for the upper tank.

Now I have a new sending unit in the lower tank and my #2 fuel reading still does not work. To add some confusion, I carried a sending unit to the dealer that did the installation (I had purchased sending unit from local dealer) and was told it was incorrect and they had to order another that matched the one they removed. They said it was hard to find and they had to use the part # off of the old one. When I returned the new sending unit to the dealer I bought it from, they found it hard to believe it was not the correct one.

QUESTION: Could the problem be in the toggle switch even though it registers correctly on the upper tank? OR
Is the problem more likely in the sending unit, and possibly someone had used an incorrect sending unit in the past and I just replaced it with an identical incorrect sending unit?

 

[Craig Clayton]

I do not know the machine but I would suggest you MARK the wires on the toggle switch and remove and test it on the bench with a Ohm meter. Look at the back of the switch and record the terminals that you see . Touch the meter between every possible combination of terminals and record the resistance as found. Move the switch to #2 position and record as found. This should give you or a person who can help a hint about the toggle switch.
Craig Clayton

 

[bigbull338]

if you had the lower tank unit replaced an it still dont work it could be 2 things.the toggle switch quit working or theres a short somewhere between the tanks.finding the prob could cost some money.

 

[mostly_gas]

What does the fuel needle do when you switch to the #2 tank? Does it behave like the key is off or does it go to some reading and never move?

 

[pharmvet]

It registers 1/4 tank no matter how much fuel is in it.

 

[Craig Clayton]

We might be barking up the wrong tree here. Have you proven the receiving unit to show full scale deflection by some means unless it is a common shared indicator.
Craig Clayton

 

[Gator6x4]

Can you switch the wires going to the tanks on the toggle switch. This would tell if the problem is in the switch if the gauge suddendly comes to life. If switching the wires has no effect then I would check with an OHM Meter the wire coming from the sending unit to the toggle switch.