Hope someone can assist; I have been troubleshooting this for quite some time. My fuel gauge is not working. I think i have narrowed it down to something with the wiring.

A mechanic verified the floats are good in the tanks. I then applied voltage to the yellow/white wire at the selector switch, and the fuel gauge jumped to full. I then applied voltage to the blue/white wire from the front tank, and nothing. So I have re-run a wire from both tanks to the selector valve switch, and at first the gauge appeared to be working on both tanks. But then now the gauge is reading 1/2 tank, then next try it reads 1/4 tank, and varies between front and rear, and I have verified I have filled both tanks. Is there something under the dash between the switch and the tanks I do not know about? My selector valve is working by the way.

If someone can tell m a wiring diagram of what connects to what it may help.

I have replaced the fuel gauge, the instrument panel internal voltage regulator, the selector switch, the instrument panel circuitboard, and I have verified the tank floats are operational. I have verified the selector valve works when flipping the selector switch. I need to rewire this gauge, so I guess i need to know how to do that. Help!

I have an 85 F150 4.9L, mechanical fuel, dual fuel tanks.

 

I have now ran a wire from the tanks to the fuel selector switch, and now the gauge works, except it only reads half full on the tanks when I fill up both tanks.

So there has to be some component in the system that is not working. I have replaced the gauge, the instrument panel circuit board, tested the sending units, replaced the selector switch, verified that the selector valve switches between tanks, and now I am totally miffed as to what else there is to look at.

 

The way to test this system is to disconnect the wire from the sending unit and ground it with the ignition in RUN or ACCY and the gauge should swing all the way to Full. Un-ground it and it should swing to Empty.

If that test passes, the wiring & gauge are good and the problem is in the sending unit(s).

I wish I had taken pictures.... the sending units are just mechanical rheostats; as the gas level changes, a brass wiper glides up & down a piece of material with a VERY fine wire wrapped around it, this produces variable resistance to ground which causes the gauge to show a reading.

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That very fine wire eventually wears through and breaks, I took one apart where that wire unwrapped itself and was floating around *****-nilly, free to contact the brass terminal any which way it felt like it.

Remove your sending units and pry the three tabs up on the triangular piece of metal at the elbow and look inside, I'll bet your problem will be staring at you in the face.

 

That test fails. So, I ran a new wire all the way from the sending unit to the fuel guage. The tank reads as normal. I tested the rear tank, it read as it should. So, I ran new positive wire from the tanks to the selector switch in the dash. The gauge reads half of what it should read. But they work, just not accurate.

So, is there something like a resistive component between the selector switch and the fuel tanks i am bypassing when running the new wire to the switch? My mechanic who tested the tank floats said they were good and I have a bad anti-slosh module, but I do not think they are on 81-85 trucks that I know of. I asked him, and he said he tryed finding the part, but could not find it, and he said it is a board on the back of the gauge. Of course, i have replaced the entire cluster components troubleshooting this, and there is nothing there.

 

The gauges operate on something like 5.1 volts supplied by the IVR (Instrument Voltage Regulator) and there are no other resistors anywhere.

Here is from the 1981 factory wiring diagram:

 

At the fuel gauge sender, take a paperclip or piece of wire and bridge the gap between the two wires in the connector. (trucks without in tank fuel pumps)

Your gauge should peg.

Do this with both sending units.

If the gauge pegs, replace the defective sending unit. (very comon problem)

If the gauge only reads half, disconnect the two wires, then take the colored wire and ground it out to bare metal. If the gauge pegs, fix the ground wires.

 

Definately a wiring issue.Thanks for the tests. Now the harness coming from the tanks has the BR-W (Tank selector valve, always works) a Y-LB (rear tank, I bypassed to the switch) a DB-Y (front tank i bypassed to the switch) and a black wire, which seems to come from the selector valve (works) to the switch.

Both tanks read, but half the value they should be. When I ground the wires to frame, the gauge pegs full.


I have no idea how the sending units are grounded. Do the grounds return to the dash or just ground to the frame? I cannot see without dropping the tanks. I think there may be an open ground then between the tanks anf the dash, but to affect both tanks the tank grounds must converge at some point and come into the cab as one wire.

I really appreciate the help! Since the tanks read halfay, I will say we are halfway to getting this fixed!

 

All grounds lead to the negative battery post.

Some of these sending units & harnesses have separate ground wires which I imagine lead inside the cab and screw to the body someplace.

Others just ground through the tank (to the body, to the battery).

 

The ground wire for both sending units, splices into one wire, and goes to the illumination connector 1222. This is located on the frame rail harness on the drivers side.

There is also a ground wire eyelit on the back half of the frame on the drivers side near the end of the frame. Make sure this ground is clean and tight. This is where your rear lights and fuel sending units ground.

It's somewhere between the connectors for the sending units and the ground wire eyelit that goes to the frame on the Black ground wire. Do you have lighting issues at the back?

 

I traced the ground wire up into the engine compartment where it gets lost with all the other wiring going into the cab so I spliced into the wire and grounded it to the frame. i am seeing better results, but I will not know for sure until I fill the tanks up again and see. Both tanks are at less than half for sure so I will have to fill up to see if the readings hold at full. Fuel is getting expensive isn't it? Wow. Well better to fill it up again now then when the prices keep going up!

 

That is another ground wire that goes inside the cab from the frame rail harness, and grounds to the dash metal just behind the radio on the right side. Might be hard to see.

Make sure this is clean and tight as well.

It shouldn't hurt anything for you to splice your ground wire to the frame like you did, although it's not my prefered method. Just make sure the connections are sealed well as we don't want it to become a problem later.

 

I signed up just to say thank you for these diagrams.

 

Just a word of advice. Don't follow and do what the original poster did and apply voltage to the sending unit wires. If you study the diagram a little bit, you will see the gauge is fed with the + voltage anytime the key is in run. The sending unit wiring is GROUND for the gauge. The sending units in the tank vary the resistance to ground for the gauge, and by the way this how the oil pressure and water temp gauges work also.

That's why it was mentioned in the previous posts to ground and then unground the sending unit wire for testing. This simulates 0 resistance to ground, and a very high resistance to ground. This will make the gauge swing back and forth to see if it's working.