Hello everyone ;
I have a 78 F150 Ranger with the factory 2 tanks. Just replaced the fuel filter as well as the tank selector solenoid back at the rear tank since it turned out it failed. When the shop got the truck the gauge showed main tank at half full and the aux tank at full. then the switch was replaced and main tank was actually completely empty since after getting to run on aux tank. I went and filled truck both tanks - front was bone dry 18 gal. to fill and aux took maybe 3 gal.
The truck was full both tanks and now the gauge goes to above full and stays there for a long time then goes down to above half full - 140 miles - switch from main tank to aux tank and the gauge does the same thing - over full and stays there for 80 miles plus then drops to 3/4 full. The gauge was working fine before this twist . What could cause this? friend said the sending unit in the main tank could be bad but since both tanks do the same i find it difficult . The tanks were dropped a few years ago and cleaned as well as new sending units and floats with socks . the gauges, temp and oil work . oil gauge varies as the sending unit is week in the heat but works well in cold temps. will replace. Could this be the gauge in the cluster finally going bad and reading full then only going to half full tanks ? Want to say the tank switch on the dash seems to be working fine. Thanks everyone.

 

Aftermarket senders have had a long history of having questionable accuracy. Unfortunately, the factory ones aren't always much better. I've had OEM gauges not read accurately from new. Sometimes there's no rhyme or reason. After I redid my 87 F250 with 2 new tanks, a new selector valve and 2 new NOS senders, both tanks read E when they still have 1/4 left. My old 85 from new the front tank would not start to move off F until half way but when it said E it meant it. The rear tank was opposite, would fall fast and hit E about half tank. One had to remember which tank you were on, and which way was what. After awhile I got used to it, but you can imagine my surprise after driving it off the lot on the front tank, being told I had 2 full 19 gallon tanks, but apparently the front tank only was half full as the gauge dropped like a rock in less than 100 miles, then after switching to the rear tank a 100 miles later it drops to E again, and I'm wondering what's wrong thinking I'm getting 5mpg.

However, if it worked fine before you 'fixed' it, it would lead me to believe your new parts aren't sending accurate information to the gauge. Something you might check is to make sure you have good grounds at the sender and tanks. Having good grounds are just as important on fuel tanks and gauges as they are for lighting. The circuit is supposed to ground through the tank mounting bolts on the frame, but we all know what a little dirt and corrosion under these trucks can do to create electrical gremlins.

I just got into the habit of using the trip odometer to measure my miles instead of relying on the gauge totally. I know a tank will run about 200 miles, so I switch at around 180 and go fill up the other.

 

Before this the main read just under full and would go to a little below E and still have a couple gal. the Aux would read just above full stay there just a little then drop slowly till E and when at E you were out.. Now it is a gamble lol ... have been also using odometer readings with this sudden issue. each tank goes about 145 miles before fill and have a little left in both

 

Dam Gum It .... Went to add my truck and it is upside down have to figure out how to flip it ... maybe Monday have things to do lol