Fuel sending unit question

[Man With A Gun]

As the level in the fuel tank drops, does the resistance reading across the sensor in the fuel sending unit increase or decrease?

[hatzie]

Decreases.
In a perfect world.

• F=90Ω
• E=0Ω

In the real world ± 3-4Ω from the above is fine.

[Man With A Gun]

So a non functioning system with the fuel meter pegged at 3:00 ( way past full) would indicate an open circuit?

I've got the dual tanks in my '86. Left tank reads just fine. Right tank reads past full although the tank works otherwise.

Where is the likely problem? Need to replace the fuel level float sensor? Broken wire somewhere? Bad ground? Bad fuel selector switch? Bad connection?

Where specifically and what specifically should I look for?

[Man With A Gun]

I'm working on getting that fuel tank close to empty. Should I go ahead and drop the tank or is there a recommended troubleshooting procedure to start out?

[hatzie]

The first place to look is the ground wire from the sender to the frame.

Next is the sender wire from the molded connector at the valve to the Weatherpack sender connection in terminal A or C.

Dead last is the sender itself.

[Man With A Gun]

Where is the valve and where/what is the Weatherpack thing?

[Man With A Gun]

And is it possible the switch itself up inside the cab is bad?

[hatzie]

The tank valve switches the senders after 1981.

You can find it on the RH frame rail. Here's a picture from another thread. The vent is circled not the plug.


The Weatherpak plug is the connector to the valve. This is what's inside the valve and a drawing of the plug.

[Man With A Gun]

Originally Posted by hatzie

Next is the sender wire from the molded connector at the valve to the Weatherpack sender connection in terminal A or C.

So from looking at the drawing, the sender from the left tank goes to either A or C, and the sender from the opposite tank will go to the opposite of what the left tank was connected to i.e. C or A.

And the two terminals of the dash fuel gauge are connected: one side to B, and the other to the ground.

So switching the switch will connect the fuel gauge through terminal B to either the left or right tank sending unit through terminal C or A depending on switch position.

Am I correct?

[hatzie]

I agree with the first statement.
The gauge has it's own power connections inside the dash.

The gauge switch is inside the valve not on the dash like the 73-80 aux tank systems.
When you flip the dash switch the polarity of terminals D & E is reversed and the 12v DC motor inside the valve reverses till the shuttle trips both the overtravel switch stopping the vave motor and selecting the proper sender.

B is the common on the sender switch inside the valve. The wire from B goes to the gauge.

C & A on the valve are connected to the tank senders. If I drew it right A is the RH tank and C is the LH tank.

The tank senders are 0-90Ω variable resistors (specifically rheostats). One end of the sender coil is connected to the gauge and the wiper is connected to the tank. The tank is grounded to the frame by a short little chunk of wire.

Your problem is somewhere between the RH frame and terminal A on the valve.
The wire colors to the senders and the gauge in this drawing likely aren't correct but the functions are.

[Man With A Gun]

So where is the frame to tank ground as referenced in the above drawing?

Is it on the top of the tank, so you'd have to drop the tank to find it, or is it on the frame somewhere near the tank but located so you don't have to drop the tank to find it?

[hatzie]

Pull the bed back or drop the tank. The quick and dirty test would be attaching a jumper cable from the frame to the pinch weld on the tank.
Look on top of the tanks in this picture. This is a dump body truck.

[Man With A Gun]

What does a pinch weld look like and where is it?

[hatzie]

You can't miss it. It's where they welded the top to the bottom of the tank. It sticks out all the way around.

[Man With A Gun]

Oh I see. So the test wire could be connected without dropping the tank or moving the bed. I guess somehow the tank is insulated from the frame electrically, maybe by the anti squeak pads. I'll try that.

[Man With A Gun]

Is the valve visible in the above pic?

[hatzie]

Originally Posted by Man With A Gun

Is the valve visible in the above pic?

The stock location is forward of the rear cab mounts where the frame starts rising upward. You can see the frame bend just behind the valve in the valve picture. Boy that sentence just didn't flow right...

[CSHADES]

The place to start when checking the problem you have, is to disconnect the wire to the tank unit(sender) and ground the wire. If the fuel gauge reads empty when you have the wire grounded the wiring and gauges etc should be fine. The tank unit will need to be replaced. If the gauge still reads over full when the wire is grounded then you have a wiring,valve or switch problem. If the gauge reads ok on the other tank then the gauge and wiring to the switch from the gauge should be ok. It is just a matter of separating the system to narrow down the fault.

[hatzie]

Originally Posted by CSHADES

The place to start when checking the problem you have, is to disconnect the wire to the tank unit(sender) and ground the wire. If the fuel gauge reads empty when you have the wire grounded the wiring and gauges etc should be fine. The tank unit will need to be replaced. If the gauge still reads over full when the wire is grounded then you have a wiring,valve or switch problem. If the gauge reads ok on the other tank then the gauge and wiring to the switch from the gauge should be ok. It is just a matter of separating the system to narrow down the fault.

That's my test #2.
Ground the tank to the frame first.
If the tank ground is bad it'll read at 3:00 with a brand new sender.

[Man With A Gun]

Ok we grounded the tank to the frame and nothing changed in the gauge reading.

If I move the bed it looks like there are eight bolts to remove. Can the bed be rotated slightly clockwise so as to expose the passenger side fuel tank? Or just move it straight back? How heavy is the long bed? Any electrical or lines need to be disconnected before moving the bed?

[hatzie]

Two people can move the bed
There are 4 or 6 bolts IIRC and one harness plug for the tails.

Slide some lumber underneath and slide her back far enough to set the wheel openings over the bumper. 2 guys can do it but it would be easier to have 3 or 4. With 4 guys you could carry it back and set it on sawhorses.

[Man With A Gun]

What about the fuel filler lines?

[hatzie]

Originally Posted by Man With A Gun

What about the fuel filler lines?

Yes. Two clamps per side.
Make sure the axle vent is attached on the frame cross rib and not the bottom of a bed crossrail too.

Still... Not much to disconnect and hook back up compared to dropping the tank. I've slid one back onto braced 4x4 timbers all by my lonesome.

[slimneverdies]

I'm installing two new sending units on my tanks on a freshly powder coated frame. Where do I ground these units with the thick powder coat????

Thanks
Slim

[CSHADES]

When I rewire any vehicle I always treat the grounds like the car is made of fiberglass and run dedicated grounds from each device to a ground bar that I install either under hood or in the cab. Otherwise you are going to have to grind some of the powdercoat off, which takes away the advantage of powdercoat.