Good Day all,

I've got my running issue solved (hopefully, fingers crossed), been driving the old truck everyday just to make sure I haven't missed something. However, the rear fuel gauge isn't working.

What I have is an '85 F-250 460 (with hot fuel option) manual 2x4 extended cab long bed. The truck has just under 5000 original miles, the PO left it parked for 20 years due to a running issue.

When I got it, both tanks were full of fuel. drained, removed, cleaned, reinstalled tanks. New sending units and pumps for both tanks, cleaned the fuel lines, rebuilt the carb, replaced divertor valve.

It runs and drives fine, the front tank gauge works just like it should. When I switch to the back tank the fuel gauge slowly drops to "E". I pulled the bed to have easier access to all the workings (I have a big hoist, so it's real easy for me to pull it off).

Checked all the connections, grounds and anything else that I might have over looked previously. This morning I pulled the sending unit out of the tank to check connections, so the float was in the empty position, just happened to look at the gauge and it read full. So I played with it for a bit, as long as the float is in the empty position it reads full, but if I move it to any other position it drops to "E". I retraced the wires and couldn't find anywhere the factory wiring has been buggered with. I pulled the connector at the switching valve, it has a crack in the connector itself, but otherwise it all looks good. Is it possible that the new sending unit is bad. I believe I recall the PO stating that the rear tank was empty, so that leads me to think that he might have had an issue with it when he parked it.

Thanks in advance,
Elemak

 

My second tank only reads if I shut the truck fully off, flip the switch, then turn the key back. Not sure if they are all this way.

 

It is very possible to have a bad sending unit right out of the box. To test that theory, pull the wire off the sending unit and ground it - obviously with the switch in the rear position. The gauge should go to full or thereabouts. If it does you've proven everything from the rear end of the wire through the switch to the gauge is good. That only leaves one thing - the sending unit.

 

You ever have one of those stupid moments where you can't see the forest for all the trees?

Got home and realized I hadn't checked the ohms through the sensor on the sending unit. Good thing I pulled the bed earlier.

Disconnected wiring connectors, pulled off fuel lines, removed both sending units from the tanks, pulled out the handy volt/ohm meter and found:

Front tank:
Spectre SP384H empty is 72 ohms, full is 9.5 ohms

Rear tank:
Spectre SP225H empty is 12 ohms, full is 163 ohms

That tells me that the rear sending unit is for '87 and up. Looks like I'm back on the prowl for a new sending unit for the rear tank again, only spent 3 weeks trying to track down this number. Gotta love the "Made in China" stickers on everything anymore.

Elemak

 

Yep... That's the problem...

I would put the new pump on the old sending unit and seeing if that would work for you.

I don't see anybody anywhere making the rear fuel sending unit you need.

Ford sold the sending unit with the pump. Couldn't buy it seperate.

 

I'm kicking myself now, but I threw it out, there was so much rust build-up on it that there was no way to save it, I couldn't even free up the float to get it to move. I'm gonna do some more searching, but it looks like I just won't have a rear gauge, the unit I have in works as far as pumping fuel, so its possible to still be able to use it and just keep track of the mileage.

 

Actually, your rear tank should read backwards. Full on the front is 9.5 ohms and the rear reads 12 ohms at empty - pretty close. And empty on the front is 72 ohms, and the rear goes all the way to 163 at full. So, when the rear tank is full the gauge should read below empty, and when it is empty it should read almost full.

 

Any way to remove the sender wire from the resistor and solder it on to the other end?

Or flip the resistor?

 

It might be possible to swap the wires around on the resistor. Yes. Good idea to try anyway...

It still won't read correctly because you don't know where empty will land on the gauge. It could land at 1/4 full and be empty etc...

Still would be better than reading backwards perhaps...

 

A problem that I'm running into is when the float is in any position other than empty the gauge in the dash reads empty, when the float is on empty the gauge reads full. So what I'm thinking is the difference in resistance is steep enough that even reversing resistor wiring it will still read off. (have to look into that someday)

I dropped the bed back on this morning, at some point I'll try to locate a new sending unit, just don't feel like messing with it right now, it's more of an annoyance than anything, I can drive 100 miles then switch to the front tank.

Summers coming to an end and I'll really need to get the drivers electric window working before the rain and cold weather set in. I pulled the door panel, the original window mechanism is riveted in place and ran out of time to drill them out, the way it acts is the gear is stripped, but I can't get to it to see.

Thanks for all the help,
Elemak

 

Use 1/4-20 bolts and nuts to put the mechanism back. And, you will probably find the three rubber bushings have disintegrated, allowing the motor to spin. A piece of vacuum hose the right diameter and length will solve the problem. Been there, done that.

 

O cool, thanks. sometime this weekend I'm planning on pulling it out and will update then.

 

I've got an 87 F250 SC 460 carbed hot fuel, Very similar, I found when the sending unit wire is disconnected gauge reads over full all the time. When grounded, the gauge reads empty. Just the opposite of what I'm used to.

 

They changed the fuel gauge and sending units in 1987.


As for Elemek's problem with the sender. Reversing the wires, then adding a resistor inline wouldnt let the gauge be acurate either...

hmmm...

 

The extra resistance would have to be in parallel in order to bring the overall resistance down since the rear sending unit's range is higher than that of the front. Been a while since I played with parallel resistors, but at first blush I don't think there is any value that would bring both ends into range.