My 56 came with a stock fuel tank (in the cab) and a stock fuel level sensor. The fuel level shown on my (10-year-old) Autometer fuel gage was never correct. When it was full, it showed 3/4 tank. Then it stopped reading anything. Showed empty all the time. After making sure all the connections were OK, I reasoned that it must be the fuel level sensor had just quit.

Bought a new sensor from NPD. It was a "universal" type with crappy instructions on how to cut the rods for different height tanks. I did that and installed it.

The good news: it read a fuel level. The bad news: it pegged at full. When it ran out of fuel, it showed just over 1/2 tank.

Reasoning that the sensor was just a variable resistor, I put a 12V rheostat (RV light dimmer) in line with the sensor-gage and played with the resistance. No effect. If I turned the rheostat off, the gage went to empty. Play the **** up and down. Still no effect.

Question: Do I have a mismatch between sensor impedance and gage? If so, how would you adjust that? Or, do I need to redo the rod lengths, so the float reads closer to a true full/empty?

 

Randy, This is just a guess, but I would think that you'll need to play arround with the rod lenght. The sending units rheostat should do the trick as long as the float is in the right position. Try removing the unit and hook it up to work by hand, see what it reads when you have the float all the way up and down. Don't forget to put a temp. ground on it when testing.
Mike

 

Randy, I have the exact same setup and fuel guage as you. When I took out my tank to do my interior. I removed the sending unit. Mine was reading low and the fuel gauge needle fluctuated and never had a good tank reading. After removing the sending unit, I notice that the adjustment rod was adjusted all the way in and the rod would hit the frame in the up position, not allowing it to read correctly. I also noticed that the level was much lower in the down position and would bottom out on the bottom of the fuel tank at about 1/4 tank. I cut about a half inch off the rod, and readjusted it from the complete down position. then I raised the level to the full position making sure the rod would hit the frame and it didn't bind on the frame anymore. I also noticed that the plastic float ball was half full of gas. Couldn't find a leak or crack anywhere? I took a small drill and made a hole and drained the fuel, then melt some plastic over the whole. It seems to be working great now, at least it much closer than it was..... I noticed the top of my sending unit has a ground strap from the mounting screws to the rear cab. Don't know if that makes a difference or not? Hope this helps.

 

Randy,

The stock gage and sender used bimetallic strips and a pulsing current system - I'll assume since you've got a newer gage you are using the plain variable resistance setup on your truck.

Ed mentioned the grounding strap, and grounding is always a bug-a-boo on anything mounted remote from the battery. Make sure the sending unit has a good ground to the tank, the tank has a good ground to the cab, the cab has a good ground to the frame, and the frame has a good ground to the battery. You know the drill.

One other thing that Mike touched on. You can check the sender by removing it from the tank, attaching a ground wire to the sender flange, and then attaching the sender wire in the normal way to the sender. Obviously, be careful not to create any sparks during this part of the setup - maybe disconnect the battery for safety. Now, with power to the fuel gage, move the sender arm all the way down and then all the way up while watching the gage. This will tell you how the sender range of motion reads on the gage. From there, you should be able to make a pretty good first estimate of how to trim and/or bend the sender arm so it works inside the tank. You may have to tweak things a second time to get it to read full and empty where you want it to.

If the gage doesn't read empty with the rod all the way down, or doesn't read full with the rod all the way up, you have a mismatched sender and gage (or just a bad sender or gage). If the gage and sender are good but are mismatched, you may be able to match them by adjusting the range and minimum resistance of the sender with the following circuit.



Procedure:

• Measure the resistance of the sender with the arm at the bottom (Rbottom) and at the top (Rtop) of the range of travel. Make sure you have the sender disconnected and you measure the resistance between a clean ground on the sender (the flange for example) and the sender wire terminal.
• If Rbottom is less than Rtop
  • Move the sender to the bottom of its range. Adjust the Offset R (the offset resistor) so the gage reads above empty, then reduce it until the gage just reads empty. Adding the offset resistor to the circuit can only INCREASE the minimum reading of the gage. If the gage reads above empty without the offset resistor, you can't correct the gage/sender mismatch with this circuit.
  • Move the sender arm to the top of it's range. Adjust the Range R (the range resistor) until the gage reads below full, then increase it until the gage just reads full. Adding the range resistor can only DECREASE the amount the gage changes when you move the sender from bottom to top. If the gage reads below full without the range resistor, you can't correct the gage/sender mismatch with this circuit.
  • Repeat the two steps above until the gage reads empty when the sender is at the bottom of its travel and full when the sender is at the top of its travel. Congratulations, you're done!
• If Rbottom is greater than Rtop
  
  • Move the sender to the top of its range. Adjust the Offset R (the offset resistor) so the gage reads below full, then increase it until the gage just reads full. Adding the offset resistor to the circuit can only DECREASE the maximum reading of the gage. If the gage reads below full without the offset resistor, you can't correct the gage/sender mismatch with this circuit.
  • Move the sender arm to the bottom of it's range. Adjust the Range R (the range resistor) until the gage reads above empty, then increase it until the gage just reads empty. Adding the range resistor can only DECREASE the amount the gage changes when you move the sender from bottom to top. If the gage reads above empty without the range resistor, you can't correct the gage/sender mismatch with this circuit.
  • Repeat the two steps above until the gage reads empty when the sender is at the bottom of its travel and full when the sender is at the top of its travel. Congratulations, you're done!

I'd go to Radio Shack and get a couple of 1000 Ohm trimmer resistors for the offset and range resistors - part number 271-280, $1.29 each. I hope this helps. Have fun.

 

Randy,

As you have discovered, there is no such thing as a universal gas tank sending unit. They must be matched with the gauge. There may be a way to make this one work but that is beyond my knowledge. I know that Ford and GM used different senders and the aftermarket gauges used even a third type unless the aftermarket gauge was made for a specific brand application. So, it depends on what your Automater gauge was designed for as to what sender it needs to operate correctly. It could be that your Autometer gauge was made for a Ford sender.

Vern

 

UPDATE -

Thanks Mike, Ed, George and Vern. I finally got around to working on this problem. I bought the trimming resistors that George spec'd and was all ready to dive into calibrating the new sensor. I decided that before I tore everything apart, I would try to ensure that the setup and rod lengths were correct.

I took the old sensor and laid it out on a piece of cardboard on the driveway next to the truck. Made and attached a long test lead from the gage wire to the sensor and clipped a ground to the truck chassis. With the key on, I worked the float arm up and own and found out that the gage would read exactly full and exactly empty with the sensor position at full up/down. Good so far.

I then stuck a tape measure down the tank to determine the distance between the sensor mounting flange and the tank bottom (15"). I laid that out on the cardboard and adjusted the arms until the full sweep of the sensor met those dimensions laid out on the cardboard and tightened the adjustments. Hooked up the test leads and tried it again. Looked good.

I also noticed a possible crack in the plastic float. So, I dipped the float and arm into some liquid Varathane I happened to have around (from some furniture refinishing) and let that dry.

I put it all together. I knew I only had a couple gallons in the tank and the gage read almost empty. Went down to the gas station and filled the tank. Started the truck only to find out that the gage now read only 1/4 tank. (Bummer!). However, while driving to an errand, the gage crept slowly up to full. And...so far...it works great. I still have the new sensor and the trimmer resistors if I need them. That will be Plan B.

Thanks again all! Maybe soon, I can stop carrying that extra 2-gallon gas can around.

 

Congrats Randy. It's the pits not knowing and guessing how much gas in the tank. Since I sealed the pin hole in my floater ball (it actually was half full with gas and wouldn't come out of the pin hole) my needle doesn't bounce like it use to. I'm curious why you needle on the guage goes up so slowly? They usually read quickly. Did you put a ground wire to the sending unit and frame? Not sure if that makes a difference or not?

 

My gas gauge is inverted wen the tank its ful it reads empty wen it reachs half tank I ran out off gas. changed the sensor twice,

 

What happens to the gauge when you ground out the wire going to the sensor (it should slowly go to full, and I think I remember seeing you do not continue to ground it out after the gauge goes to full)?

 

What year/model is your truck?

 

This is a very old thread. The OP Randy Jack was one of the good guys.

 

So was Earl/George. I miss those guys. Such incredible knowledge blessed with a way to explain complicated subjects so they were easily understood. His Dilbert hanging by the necktie avatar always made me laugh.

 

91 Bronco 300 motor