Fuel gauges/selector switch help

[TexasNomad]

Hiya every one, I was wondering if any one has any experience the sending unit wiring.
My fuel gauge is reading both tanks on E well I know the back tank is full the saddle tank didn't have any wiring coming from the sending unit and I can not find any where on the main harness to hook up the saddle tank's sending unit to.
Is this why my gauge in the dash is not reading any thing do both tanks hook in to each others? I've never had a truck with dual tanks I spent a while under the truck to day and could not find where this hooked in to.
Any help would be awesome thanks every one =)

 

[Fellro]

Is it on the 79's? They used to run the gauge wires through the electric switch so you would get a reading on the tank the switch was drawing from. A possibility is a sunk float, the old brass floats would pinhole and fill with fuel, which would make them sink and read empty all the time.

 

[TexasNomad]

Yes sorry Its on my 79 150 7.5.
So the selector switch is that sent power to the sending units? really I didn't know that. Could the switch just be bad? I checked the floats and they where fine.

 

[Fellro]

It is possible. I don't recall for certain, but it seems to me that if you have an open circuit, the Fords read empty, so if it isn't connecting, then it could be the cause. Otherwise, if I am wrong, and the older units changed the tank sender at the switch in the dash, then it would be the problem. I'll have to do some checking to see which one controls the change.

 

[Fellro]

Well, it seems that you could have ground problems to the tanks, or a bad switch in the dash , or even a loose connection at the cluster...

 

[TexasNomad]

the aft tank is wired and grounded the forward tank has been unplugged and I dont know how to hook the forward tank up.
I looked in my hayns and my Chilton's but they didn't have any thing about how the tanks where hooked up..

 

[Fellro]

How is the front one filled, as in, do the fill necks look the same? If not, it has been added on. My 77 is that way, it actually fills through the fenderwell...

 

[TexasNomad]

Ok I dug up some wiring diagrams for my truck. The aft tank is wired right and the selector switch is what is bad I orderd a new switch to day and i'm hoping this will fix my gauge issue I hope its not the sending unit.
Now I just need to know how the forward tank it hooked up, according to my Hayens it show the forward tank is wired to the switch by its own wires but on my truck I can not find any of these wires I cant find if they had been cut and tucked the switch has the blue wire coming off but after it goes in to the main harness I lose it.. Does any one know how the front tank on a 79 F150 is hooked up?

 

[TexasNomad]

Well I think I found what the problem was.. I just lazy people and the messes they make.

 

[Mil1ion]

Had this been posted in the 4th generation truck forum I would have gotten to it sooner.

I suspect it is the sending unit not the switch.
Maybe we could have this moved to the proper forum ?

Here is the system.

 

[TexasNomad]

Thanks Million for the post, who is the mod in charge here? and could you please move this post? thanks a ton.