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I have a 1979 f250 highboy with a 4 speed and a 460. Now I’m not very familiar with the fuel systems on this truck I know it’s the mechanical pump on the block and it is a dual tank truck but that’s about it. It’s my father’s truck and I’m slowly bringing her back. Recently the fuel gauge has stopped working for both tanks. And when I flip the switch to change to the front tank it doesn’t work the car runs out of fuel and dies even though the front tank is full.
so these are my questions.
1. what could be the cause of the fuel gauge not working? Should I just replace both fuel senders?
2. what is wrong with switching the tanks? Is it a vacuum operated system? How does the system work?
No vacume to it. All electricial, 2 sending units (one in each in tank), one elec change over valve back on the cross member in front of the aft axle tank, and one switch on the enviromental control panel than controls the valve. Test the sending units like this....Disconnect the wire that goes to the fuel sending unit at the tank. It is a 2 prong push on elec connector and hard to reach the back one since it is so close to the bottom of the bed. To pop the plug off for the test, you can barely get up in there with your head and see the plug. While looking at it, VERY CAREFULLY use a screwdriver to pry it off. AND I MEAN VERY CAREFULLY. It is only pushed on like a 1/8". This is only if you can’t get it with your hands.
Once off ground that wire to the frame, making sure the frame is clean. Turn the ignition to the "run" position and watch the fuel gauge. If the gauge swings all the way to full, either the sending unit itself is bad or it has a bad ground. If the gauge stays on empty then either the gauge is bad or there is a wiring problem.
MORE HELP: So if your gas gauge is not working and you are questioning the gas tank sending unit or the instrument cluster VR or the fuel gage itself being bad... read this. Scott Scott posted these troubleshooting steps and I had to save them for there simplicity. Thank you Scott. "The following assumes the oil & temp gauges are working. If the other gauges work, the cluster voltage regulator is good. Ground the wire that connects to the tank sending unit. Turn the key on. The fuel gauge should go full scale. If it does go full scale, your sender is bad. If it doesn't go full scale, check the sender wire for breaks running forward along the frame. If no breaks & wire shows continuity, remove the cluster. Connect wires to the studs on the fuel gauge, power wires with a 1.5V battery. NOT A 9 VOLT Gauge should go to about 1/4 scale. If gauge doesn't move, gauge is bad, replace it.
"78/79 duel fuel tanks frame switch-Duralast/Universal 3 port fuel selector valve used with SW48 toggle switch Part Number: FSV4
Thank you for the help! I will test the Senders soon, but what about the valve between tanks? How does that work and how can I troubleshoot that? I see the little diagram you’ve attached and the steps but is there a little more detailed description you can offer?
Last edited by Air_Drewdy; May 28, 2025 at 02:47 PM.
I can't help much with issues as factory in a '79 with dual tanks. As I understand it, with the factory dual tanks, the switch over valve was electrically switched and doing that also switched feeds for level sender.
I will relate how we did mail trucks. Boss had just a few long bodies, so when he needed a new truck, he pulled the body off and sold that long truck to a drag racer or dirt racer, they made great car haulers. He also removed the saddle tank he made using two grease drums, and a manual switch valve that poked through the floor beside the seat. We always run the saddle tank first as there was no gauge to it. When the truck run that tank dry, it started to die like was out of fuel, then we just reached down and turned the valve, and in a couple secs, the truck picked up as it was now using the OEM tank which had a gauge. We didn't even take it out of gear or clutch it.
Here are some pics of the selector valve. And the elec connector for the sender. Yes easy to get to with the bed off, HARD to reach with the bed on. You can also use some long handle needle nose pliers.
With the key in the 1st position (truck NOT running) and you flip the switch, you should normally be able to hear the valve change over. Or get back there and get a hand on it and have some one flip the switch, you should be able to feel it click/vibration wise. Also make sure you power to the elec connector. Test light time.
I just skimmed over the last 2 posts.
This is how my 81 F100 system works and it looks like your 79 is the same.
As Scott said if the oil & temp gauges work then the IVR is good and I dont thnk I have see a fuel gauge go bad but there is always a first.
That 3 port valve is known to go bad. Check the fuse for the fuel valve *** I have had mine blow and the valve not work as it should.
With the switch in 1 position check for power on the wire, if no power flip it the other way if still no power you will need to check that power wire all the way back to the switch till you get power.
On the senders you can ground the wires and flipping the switch to see if the gauge moves.
You can also use a 12 volt test light and if the sender wire is good and active the light should blink.
Flip the switch to see if you get the gauge to move if wire is grounded or light blinks.
If neither like the valve power you need to trace it back to the switch.
I am going to guess both senders are not bad at this point in time.
No if you dont have power to the valve and both senders are also not getting power I would say the switch is bad in the dash.
It is a DPDT type switch so when checking wires you will find 1 wire has 12 volts, this is for the valve and another will blink or show 5 to 6 volts this is for the sending units.
You would then have 3 wires going out. 2 for the senders and 1 for the valve.
If you can find what each wire dose you can then use jumper wire to see if the valve and senders work when the switch is bypassed.
I am thinking the switch is bad but with testing you will know for sure.
Dave ----
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