gas gauge issue
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gas gauge issue
I have a 1974 F-100 with a 302 and 4r70w and my gas gauge will read over to full but will go to empty within 50 miles. I just replaced the sending unit but no luck. I'm going to check grounds in the morning. Any ideas or thoughts?
Logistics Pro
Joined: Jul 2005
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Try cleaning all electrical contacts. I had an '82 Mustang GT and its temperature gauge would shoot straight up to the Hot peg as soon as the engine got warm enough to lift the needle off of Cold. I removed the two nuts securing the gauge to the Printed Circuit Board and cluster back, and sanded the contact areas. It worked fine for the next 150,000 miles.
The dent gauges are weird because the gauge shafts want to spin when you loosen the contact nuts, but you can hold them still with a needle nose pliers.
There are also some connections leading back to the tank. So check and or clean them. Just follow the wire from the tank as it runs forward under the truck.
The best ground to check would be the one for the dash, and it screws to the hanging steering column support. I'm guessing it's OK if everything else is working fine.
If I'm not mistaken, the gauges always have positive juice, and the senders regulate the resistance of the ground.
Grounding the sender wire should peg the gauge.
The dent gauges are weird because the gauge shafts want to spin when you loosen the contact nuts, but you can hold them still with a needle nose pliers.
There are also some connections leading back to the tank. So check and or clean them. Just follow the wire from the tank as it runs forward under the truck.
The best ground to check would be the one for the dash, and it screws to the hanging steering column support. I'm guessing it's OK if everything else is working fine.
If I'm not mistaken, the gauges always have positive juice, and the senders regulate the resistance of the ground.
Grounding the sender wire should peg the gauge.
Fleet Mechanic
Joined: Sep 2014
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From: NE Texas
Sounds like you have a good initial plan checking grounds and connections.. Do a search for 'gas gauge problems' in the forums and you will probably find dozens with further steps you can take. (Assuming you are not actually running out of gas that fast!)
Elder User
Joined: Apr 2018
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From my experience that's pretty much "normal" behavior, each gauge is a bit different but they all tend to read "FULL" for a long time than they drop like a rock to "EMPTY".
You can try bending your float rod to correlate with the gauge, with the sending unit out of the tank and wired up move the float rod up and down while watching the needle on the gauge, if the gauge reading does not correlate to the float position bend the float rod to make the needed adjustments.
I need to do this myself but too lazy to pull the sending unit, AGAIN! so I just live with it lol.
You can try bending your float rod to correlate with the gauge, with the sending unit out of the tank and wired up move the float rod up and down while watching the needle on the gauge, if the gauge reading does not correlate to the float position bend the float rod to make the needed adjustments.
I need to do this myself but too lazy to pull the sending unit, AGAIN! so I just live with it lol.
Elder User
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If the float is not sinking and is known to be good (actually floats) the float rod will need to be bent as I noted above. This should correct your issue.
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I'll see what I can do bending it. It's just Crazy the gauge reads when you fill it up and its reading empty after about 6-10 gallons are gone. Ive never seen a gauge do that. I swapped 2 sending units into it just to make sure 1 wasn't bad. Thanks for the advice!
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Logistics Pro
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Another idear for testing this is to stick a resistor between the sender wire and a good ground. I forget the values, but there are three different resistors you can get which will (should) produce certain readings on the gauge, like 1/4, 1/2, and 3/4. Your Temperature (and Oil Pressure if you have one) gauges work the same as the Fuel gauge.
I've been guessing that the problem is in the wiring between the sender and the gauge itself, but it could also be something like the sender retaining bracket or float arm is bent or whatever, causing the potentiometer to go through its whole range too soon.
Another possibility is a bad ground between the truck and the tank, affecting the sender circuit.
Maybe drop the tank and pull the sender out and plug it into the truck, and try moving it through its range while observing how the gauge reacts.
I've been guessing that the problem is in the wiring between the sender and the gauge itself, but it could also be something like the sender retaining bracket or float arm is bent or whatever, causing the potentiometer to go through its whole range too soon.
Another possibility is a bad ground between the truck and the tank, affecting the sender circuit.
Maybe drop the tank and pull the sender out and plug it into the truck, and try moving it through its range while observing how the gauge reacts.
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