Gary Lewis Posted March 9, 2023 Share Posted March 9, 2023 Gary, can you describe this process a little more? I remove the positive and negative leads to the sender. Then run a ground to the frame. If the dash reads full, the sender is bad? I don't have to do anything to the power lead? Thanks! There should be only one wire to the sender, and if it is the rear tank it is the Y/LB wire. As shown below the power comes from the fuse box in the upper left, through the resistance wire to the Instrument Voltage Regulator. From there it goes through the fuel gauge and to the Fuel Tank Selector Switch if you have two tanks. Assuming it is the rear tank it then goes on the Y/LB wire to the sender. The sender is a variable resistor with the other end tied to ground. At Full it is a 12 ohm resistor, and at Empty it is a 72 ohm resistor. But if you pull the wire off the sender and connect it to ground the gauge should go to Full and maybe beyond in a few seconds. (Don't hold it to ground for a long time as the wiring wasn't designed for that, but a few seconds is fine.) If the gauge goes to Full or beyond then the wiring, gauge, and regulator are good and the problem is the sender. Link to comment Share on other sites More sharing options...
Powerman5K Posted March 10, 2023 Author Share Posted March 10, 2023 There should be only one wire to the sender, and if it is the rear tank it is the Y/LB wire. As shown below the power comes from the fuse box in the upper left, through the resistance wire to the Instrument Voltage Regulator. From there it goes through the fuel gauge and to the Fuel Tank Selector Switch if you have two tanks. Assuming it is the rear tank it then goes on the Y/LB wire to the sender. The sender is a variable resistor with the other end tied to ground. At Full it is a 12 ohm resistor, and at Empty it is a 72 ohm resistor. But if you pull the wire off the sender and connect it to ground the gauge should go to Full and maybe beyond in a few seconds. (Don't hold it to ground for a long time as the wiring wasn't designed for that, but a few seconds is fine.) If the gauge goes to Full or beyond then the wiring, gauge, and regulator are good and the problem is the sender. Thanks! I will try this over the weekend. I only have one tank. Link to comment Share on other sites More sharing options...
Machspeed Posted March 10, 2023 Share Posted March 10, 2023 John does your tank selector valve work? The senders ground at G701(the main ground near the radio) Both senders come together at Splice 154, which also takes the lights from the rear. And then join again at a major Splice (155) I'd think if either of these splices were bad you'd have other noticable issues. So if it's both senders that are out I'd think they are individually damaged, or it's the feed wire between the instrument cluster and selector that's gone wonky. Thanks for responding, Jim! And Powerman, sorry to intrude on your post but maybe we can all learn together. Jim, my switching valve works, and I can switch from one tank to the other. In fact, that is how I keep from running out of gas completely, as I just switch tanks and then go fill the empty tank. I have no lighting issues, or for that matter, any other problems. The ground you speak of behind the radio is good, I inspected it and even cleaned it up last year when I had the dash all apart. It made no difference. Five or more years ago, my fuel gauge quit working, got nothing from either tank. As I was doing some body work to the truck, I pulled the bed and replaced both sending units. It's been a while, but I recall connecting the electrical connector to the assembly and holding the unit outside of the tank and moving the float assembly while watching the fuel gauge through the back glass. As best as I can remember, I got no movement of the gauge. I then took a long wire and grounded the sending unit to the frame, moving the float assembly and noting movement of the gauge indicator. I assumed that the assembly needed to touch the tank to make ground. I put it all back together, bed back on only to note later that the gauge did not work. Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour. Link to comment Share on other sites More sharing options...
81-F150-Explorer Posted March 11, 2023 Share Posted March 11, 2023 Thanks for responding, Jim! And Powerman, sorry to intrude on your post but maybe we can all learn together. Jim, my switching valve works, and I can switch from one tank to the other. In fact, that is how I keep from running out of gas completely, as I just switch tanks and then go fill the empty tank. I have no lighting issues, or for that matter, any other problems. The ground you speak of behind the radio is good, I inspected it and even cleaned it up last year when I had the dash all apart. It made no difference. Five or more years ago, my fuel gauge quit working, got nothing from either tank. As I was doing some body work to the truck, I pulled the bed and replaced both sending units. It's been a while, but I recall connecting the electrical connector to the assembly and holding the unit outside of the tank and moving the float assembly while watching the fuel gauge through the back glass. As best as I can remember, I got no movement of the gauge. I then took a long wire and grounded the sending unit to the frame, moving the float assembly and noting movement of the gauge indicator. I assumed that the assembly needed to touch the tank to make ground. I put it all back together, bed back on only to note later that the gauge did not work. Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour. Water is a conductor. Check to see if your frame grounds are good. It sounds like the ground gets wet, and works until it dries out. (From your description/symptom) . . . Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 11, 2023 Share Posted March 11, 2023 Water is a conductor. Check to see if your frame grounds are good. It sounds like the ground gets wet, and works until it dries out. (From your description/symptom) . . . Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour. My first '58 Chevy had intermittent wipers - when it rained they worked and when it was dry they didn't. Never did find the problem, but I knew it was a ground. However, at the time I didn't really understand about grounds and GROUNDS. Now I know that a connection can carry enough current to fool a volt/ohm meter (this was before DVM's, which are even more easily fooled), but not enough to run a motor. So I should have run a voltage-drop test. Link to comment Share on other sites More sharing options...
Machspeed Posted March 11, 2023 Share Posted March 11, 2023 Water is a conductor. Check to see if your frame grounds are good. It sounds like the ground gets wet, and works until it dries out. (From your description/symptom) . . . Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour. Ohhhh, I know it's a ground somewhere.....but where? I have no other electrical issues and the ground behind the radio is good. Thoughts? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted March 11, 2023 Share Posted March 11, 2023 Ohhhh, I know it's a ground somewhere.....but where? I have no other electrical issues and the ground behind the radio is good. Thoughts? Perhaps the tabs at Connector 208A, circuit 29 or the switch itself? What else is common to both tanks and exclusive of all other circuits? 'bad ground' doesn't make sense, when everything else that joins the ground circuit at the same spot/splice is fully functional. ... and back to your scheduled program. Link to comment Share on other sites More sharing options...
81-F150-Explorer Posted March 11, 2023 Share Posted March 11, 2023 Perhaps the tabs at Connector 208A, circuit 29 or the switch itself? What else is common to both tanks and exclusive of all other circuits? 'bad ground' doesn't make sense, when everything else that joins the ground circuit at the same spot/splice is fully functional. ... and back to your scheduled program. The common ground on the 1986 is G-701. Since all those circuits work except for the gauges it has to be upstream from that eyelet connection. Easy way to check if this ground is good is to remove the fuel sender wire at the tank and ground the wire in the connector. If the gauge pegs to full, it's not the gauge or ground. If it does not peg, and the gauge is good. . . Splice S-154 would be suspect. Check for corrosion, loose, frayed, wires at this splice. Missing insulation etc. Located near the Front Fuel Gauge sender. You may have to remove the insulating tape etc. . . Note: Not all circuits that use S-154 may be malfunctioning for it to have a bad connection on one circuit. If not found there, you will have to remove the frame rail harness, unwrap it, check for abrasions along the wires. Uses a multimeter set to ohms to check for continuity among the different wires / circuits etc. Why I think it's a ground wire/splice or connection is that it tries to work when the truck is wet. Where does the wires get wet? Link to comment Share on other sites More sharing options...
Machspeed Posted March 14, 2023 Share Posted March 14, 2023 The common ground on the 1986 is G-701. Since all those circuits work except for the gauges it has to be upstream from that eyelet connection. Easy way to check if this ground is good is to remove the fuel sender wire at the tank and ground the wire in the connector. If the gauge pegs to full, it's not the gauge or ground. If it does not peg, and the gauge is good. . . Splice S-154 would be suspect. Check for corrosion, loose, frayed, wires at this splice. Missing insulation etc. Located near the Front Fuel Gauge sender. You may have to remove the insulating tape etc. . . Note: Not all circuits that use S-154 may be malfunctioning for it to have a bad connection on one circuit. If not found there, you will have to remove the frame rail harness, unwrap it, check for abrasions along the wires. Uses a multimeter set to ohms to check for continuity among the different wires / circuits etc. Why I think it's a ground wire/splice or connection is that it tries to work when the truck is wet. Where does the wires get wet? Yes, think I could probably find a fix for it if I pulled the bed, but just not interested in doing that right now. Is it not interesting that I've seen the gauge function in the rain....LOL! This makes me think it is a ground related issue. Link to comment Share on other sites More sharing options...
dionden Posted March 14, 2023 Share Posted March 14, 2023 Yes, think I could probably find a fix for it if I pulled the bed, but just not interested in doing that right now. Is it not interesting that I've seen the gauge function in the rain....LOL! This makes me think it is a ground related issue. Hi John, I am sure you know this; have a dual tank harness off vehicle. Found splice location S154 on the main harness about 1 to 2 inches towards the front of the vehicle from the junction of the front tank/main harness connection. "It had a repair" It can be accessed through the opening of the front tank and frame by the fuel filler hose from below. I have a cheesecake frame, so everything is accessible!!! Link to comment Share on other sites More sharing options...
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