jbritt Posted March 29, 2021 Share Posted March 29, 2021 1986 f-250 XL sc 6.9l Been searching the forums for an answer but nothing has helped that I have found. Ran through the diagnostic test but it states the fuse #15 and is a 10 amp. However, the wiring diagram available here states it is fuse #3 for the selector switch. Here's the symptoms: -Truck only pulls from the rear tank -Gauge reads negative until fuel level drops to around 5-7 gallons, then begins to climb towards empty. I have yet to see what happens if I run it longer cause I tend to fill up when I notice the movement (not the usual sweep when the tank usually indicates empty) - Used to be able to run both tanks, however gauge readings have always been wonky - I have pulled both sending units and they both look to be in good condition, still unsure of a good way to test them.. - Removed and cleaned the selector switch - This part may require another thread but I am also having issues with getting power through the lg/y wire in the dome light and radio circuit. It will however blow fuse 7 every time its replaced. Will function when jumped but the wiring gets quite roasty.. not sure if it plays into the power for the dual tanks but figured I'd toss it out there. Let me know if anyone needs photos or further details as I'm sure people will, just wasn't sure what to post pictures of... Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 29, 2021 Share Posted March 29, 2021 The best way to test sending units is to pull the wire from the sending unit and ground it. With the key on the gauge should go to full scale if it is working correctly. Then put a volt/ohm meter on the sending unit. It should read somewhere between 10 ohms for full and 72 ohms for empty. But your symptoms sound like someone may have put a Bricknose sending unit in, which will read about 160 ohms full and 16 ohms empty - meaning backwards and with a different range. As for the fuse for the selector switch, this says it is #15, and it came from Documenation/Electrical/EVTM/1985 EVTM/FUEL TANK SELECTOR & GAUGES (DIESEL). Where did you see that it is #3? Link to comment Share on other sites More sharing options...
jbritt Posted March 30, 2021 Author Share Posted March 30, 2021 The best way to test sending units is to pull the wire from the sending unit and ground it. With the key on the gauge should go to full scale if it is working correctly. Then put a volt/ohm meter on the sending unit. It should read somewhere between 10 ohms for full and 72 ohms for empty. But your symptoms sound like someone may have put a Bricknose sending unit in, which will read about 160 ohms full and 16 ohms empty - meaning backwards and with a different range. As for the fuse for the selector switch, this says it is #15, and it came from Documenation/Electrical/EVTM/1985 EVTM/FUEL TANK SELECTOR & GAUGES (DIESEL). Where did you see that it is #3? I get a full sweep from the rear tank but no response from the front when grounding the wires. Rear sending unit read about 105 ohms, front unit reads open loop so I am assuming its no good. The 105 ohms leads me to believe that you are correct in your inkling that the incorrect sending unit is installed. Can you run bricknose sending units and get normal gauge function? Or does someone know where to find the correct sending units? I found the fuse info on page two of the 1986 wiring diagrams https://drive.google.com/file/d/1FQsVr3XX8DOOHDYuMAkyZ-DYuKjp1eCy/view Link to comment Share on other sites More sharing options...
jbritt Posted March 30, 2021 Author Share Posted March 30, 2021 Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 30, 2021 Share Posted March 30, 2021 I get a full sweep from the rear tank but no response from the front when grounding the wires. Rear sending unit read about 105 ohms, front unit reads open loop so I am assuming its no good. The 105 ohms leads me to believe that you are correct in your inkling that the incorrect sending unit is installed. Can you run bricknose sending units and get normal gauge function? Or does someone know where to find the correct sending units? I found the fuse info on page two of the 1986 wiring diagrams https://drive.google.com/file/d/1FQsVr3XX8DOOHDYuMAkyZ-DYuKjp1eCy/view The difference in fuse # is just a difference between the large schematics and the EVTM. I think they still point to the same fuse. If the rear sending unit reads 105 ohms then it probably is the wrong one. You can either replace it or add a Meter Match, which will map it to match the Bullnose gauge. But it might be less expensive to just replace the sending unit. On the front tank, if grounding the wire doesn't move the gauge then your selector valve may be bad. You know the wiring from the selector valve is good, so that suggests the valve is bad. Link to comment Share on other sites More sharing options...
jbritt Posted March 30, 2021 Author Share Posted March 30, 2021 The difference in fuse # is just a difference between the large schematics and the EVTM. I think they still point to the same fuse. If the rear sending unit reads 105 ohms then it probably is the wrong one. You can either replace it or add a Meter Match, which will map it to match the Bullnose gauge. But it might be less expensive to just replace the sending unit. On the front tank, if grounding the wire doesn't move the gauge then your selector valve may be bad. You know the wiring from the selector valve is good, so that suggests the valve is bad. Kinda what I was feelin was the issue, or combination of issues. Have sending units and a switch valve on the way. We'll see how she bucks once they're on. Thank you for the guidance! Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 30, 2021 Share Posted March 30, 2021 Kinda what I was feelin was the issue, or combination of issues. Have sending units and a switch valve on the way. We'll see how she bucks once they're on. Thank you for the guidance! Good luck! Link to comment Share on other sites More sharing options...
jbritt Posted April 13, 2021 Author Share Posted April 13, 2021 Good luck! New sending units and a new switch did nothing haha. Still not registering on the dash and switching the selector doesn't have any effect. Link to comment Share on other sites More sharing options...
Gary Lewis Posted April 13, 2021 Share Posted April 13, 2021 New sending units and a new switch did nothing haha. Still not registering on the dash and switching the selector doesn't have any effect. Bummer! I'd try bypassing the valve. Run a line from the tank to the switch on the HVAC panel and see if it works that way. Link to comment Share on other sites More sharing options...
jbritt Posted April 19, 2021 Author Share Posted April 19, 2021 Bummer! I'd try bypassing the valve. Run a line from the tank to the switch on the HVAC panel and see if it works that way. Messed with the connection at the switch valve a little is found that I just needed to plug it in a bit further. Not quite an exact fit from the old pigtail to the new valve body but I have fuel gauges again. It is reading a bit above full but I'll need to take it for a nice long drive to see if they are reading properly or not. Looks to have done the trick though. Link to comment Share on other sites More sharing options...
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