Hi and thank you for all your last help…my truck is coming along.
My neighbor and I have all my guages working great, except for the fuel guage. The prior owner removed the front tank and put a Edelbrock electric fuel pump along the frame rail. We removed that small tank and replaced it with a 38 gallon Spectra tank. The old sending unit did not have any wires attached and was complete junk. It was sucking gas and that was it. We modified the new sending unit to the 38 gallon tank without too much problems, but it’s done and attached to the fuel pump and working perfect.

Two Questions
1). Behind the dash guage panel there is a cluster of wires that control all the guages. How do I tell which wires in this cluster are for the fuel guage? There was a group of wires near the rear tank but can’t tell where they go. The old owner appears to have done some odd wiring.
2). Would I be better off just installing an after market fuel guage and running all new wires? How would we wire an after market guage?

thank you
Larry

 

You are better off keeping the original fuel gauge. You can figure out what wires do what here in the diagrams in this link.

https://www.garysgaragemahal.com/1986-evtm.html

 

If I recall your truck had a dual tank, valve and switch for the tanks was removed. So search between the valve/switch for the wires that were likely dead headed. I don't think you'll need to go all the way to the guage cluster. It's probably bobbed off and hopefully taped up somewhere along the frame rail between the valve/switch and the dash. Then use the wiring diagram to ID the guage wire. The sender should have a two wire or maybe a three wire plug if it's set up for an in tank fuel pump. You only need the wire to the float resistor and the ground. I'm not sure where the ground wire coming out of the sender terminates, but probably to the frame near the rear tank.

 

Thank you for the link. I’ll forward this to my neighbor who knows how to read this stuff 😵‍💫

 

I’ll be out there shortly tracking that down and hopefully come up with something. My neighbor read that originally the wires of each tank were yellow/white and eventually went into one yellow/white wire and then to the instrument panel plug behind the dash. All the guages apparently share power from the same source.

 

Since the fuel sender loop is a variable resistance sensor circuit you probably want to match the guage of the existing stock wire. When you get the wire get a few aligator clips and you can do some testing prior to running your circuit and making your splices. I have several long "test leads" of that sort for general trouble shooting, checking. Cheap. They do have the clips with the insulated cover if you need that to avoid shorts.

 

Thank u we will get back on this tomorrow I believe. In the mean time I’m headed out as we speak to chase down some wire

 

When your neighbor reads those diagrams in the link, he will find out there is a yellow/white coming from the gauge in the cluster, but it changes colors after the switch or the valve. They had several different systems, it depends on what the engine was originally in the truck. Even then, Ford used the same wire colors.

So yellow/white to the gauge, and then after the switching device, darkblue/yellow to the front tank, yellow/lightblue to the rear tank.

 

Wow how do you know this? That’s very helpful thank you

 

Dave F
we tried everything using the wires that were meant for the front tank and the rear tank…zero luck. We pulled the dash wire cluster, located the yellow and white wire. We cut that wire and attached it directly to the sending unit. We then grounded the sending unit and the guage read way past full. I know my 38 gallon tank is about half full. My neighbor tested the sending unit and it’s got resistance and not reading open line so we believe it’s working…maybe not. Any suggestions would be awesome…
Thank You
Larry

 

To test the gauge, take that wire that you hooked to the yellow/white going to the gauge, and with the key on, ground the yellow/white. The gauge should swing full scale empty or full, I keep forgetting which way. Then take the yellow/white off ground and let it hang in the air. The dash gauge should swing full scale the opposite way. If this works, the gauge in the dash is good. If it still doesn't read correctly then it will be a calibration problem with the sending unit.

Most factory guages read differently even when new. When you fill the tank the gauge may stay pegged for most of the tank, and then suddenly start dropping fast, my tahoe does this. On my 89 f350, the rear sending unit is pretty much dead on. The front sending unit does the hanging near full till the very end of the tank.

Since you have a non-stock gauge setup, yours might read full till half a tank, and then start reading down toward empty from there. As long as empty is somewhat accurate, I would not worry about it. I think I would hook it up, and carry a gas can with me and see what it does. If you are going to wire it yourself, you know you need the yellow/white wire run back there, and I would also run a ground wire all the way back and find a good spot in the interior out of the weather to ground the sending unit.

 

Thank u much

 

Since you have your tank and sender installed you can't check the sending unit for resistance. If you ever remove it you can check the resistance of the sender as you swing the float from the empty position to the full. Then check the resistance of the circuit. Or check them together. You can hook it up to the dash guage and check how it reads with respect to float position. May as well hook it up now. Nothing to lose.

If I were you I'd fill the tank and run it about 300 miles and refill to check your mileage. At 10 mpg you should need to add 30 gallons at that time. Once you know your mpg you can use the odometer as a guage.

 

Thank u.
I did ground the wire that I attached to the guage and it pegged past full. Also we did out a meter on the sending unit since it was our only option because as you know we had already installed it. It did show resistance. So, I’m thinking I need to drop the tank check out the sending unit and try and determine if any of the modifications we did has failed 😵‍💫. Last resort is to put in a Bronco 33 gallon sender or simply keep track of mileage as you suggested.

 

on my 79 my sending unit wire shorted somewhere, I ran a new wire from sending unit to the cluster. was easier than trying to open the harness up and track it down.
If your sending unit is showing the proper ohms, 10-73 iirc it's good and it's a wire issue somewhere.
When I upgraded to the 38 gallon tank with the stock sending unit it will peg full up to about half full than it starts to drop. So with mine it reads full until it starts to get down to half.
I'd check the ohms on it, drive the truck around and than check the ohms again. If it changed, I would start chasing wires, if it doesn't change than think about a new sending unit.