Weird Fuel Gauge Issue....

[6LMonster]

Does anyone know why this would happen? (See attached Pic)

The fuel gauge is reading extremely full and not moving down. Only happening with the front tank, as the rear tank is reading normal. Bad sending unit???

Also, a secondary issue is that the truck is running a little rough right now?!? Maybe an injector? Would a bad sending unit cause the truck to run rough?

Thanks,

 

[bushpilot]

sending unit or a bad connection / ground

 

[johndeerebones]

The fuel senders have less ohms of resistance when full than empty if I remember right...

IE about 20 ohms full and 120 or 130 empty. with that in mind I would say you have a wire shorted, or gauge is bad. Does it do it both tanks? Your fuel valve could be bad too, I think the switching of the gauge between tanks goes on down there...

 

[BJS]

Yes the fuel tank level goes through the selector switch.

Tank selector valve test procedures (copied from FSMs)

looking at the connector on the pump with the slot for the connector alignment on top. you should see 6 connectors, or at least places for 6 the one on the far left may or may not be there since it is not used.

From left to right pins are
pin # - contact # (wire color/stripe color)
1- not used (no wire)
2- 789 (BR/W)
3- 786 (R)
4- 673 (DB/Y)
5- 29 (Y/W)
6- 675 (Y/LB)

Ok now that you know which pins are what

Test Rear tank Circuit
Apply power to 789(BR/W) (+) and 786 (R) (-)
conductivity between 29 (Y/W) and 675 (Y/LB)
no conductivity between 29 (Y/W) and 673 (DB/Y)

Test Front Tank Circuit
Apply power to 786 (R) (+) and 789 (BR/W) (-)
conductivity between 29 (Y/W) and 673 (DB/Y)
no conductivity between 29 (Y/W) and 675 (Y/LB)

Ok that's the end of the tank selector valve testing

Now to check the fuel sender to make sure that the signal is even getting to the tank selector switch

On the wiring harness use the same numbering scheme as above

675 (Y/LB) rear tank sender
673 (DB/Y) Front tank sender

Measure resistance to ground, Full tank is 145 ohms Empty tank is 22.5 ohms.

 

[95_stroker]

Yes the fuel tank level goes through the selector switch.

Tank selector valve test procedures (copied from FSMs)

looking at the connector on the pump with the slot for the connector alignment on top. you should see 6 connectors, or at least places for 6 the one on the far left may or may not be there since it is not used.

From left to right pins are
pin # - contact # (wire color/stripe color)
1- not used (no wire)
2- 789 (BR/W)
3- 786 (R)
4- 673 (DB/Y)
5- 29 (Y/W)
6- 675 (Y/LB)

Ok now that you know which pins are what

Test Rear tank Circuit
Apply power to 789(BR/W) (+) and 786 (R) (-)
conductivity between 29 (Y/W) and 675 (Y/LB)
no conductivity between 29 (Y/W) and 673 (DB/Y)

Test Front Tank Circuit
Apply power to 786 (R) (+) and 789 (BR/W) (-)
conductivity between 29 (Y/W) and 673 (DB/Y)
no conductivity between 29 (Y/W) and 675 (Y/LB)

Ok that's the end of the tank selector valve testing

Now to check the fuel sender to make sure that the signal is even getting to the tank selector switch

On the wiring harness use the same numbering scheme as above

675 (Y/LB) rear tank sender
673 (DB/Y) Front tank sender

Measure resistance to ground, Full tank is 145 ohms Empty tank is 22.5 ohms.

Good solid info Brandon, thanks.

 

[94f450sd]

Good solid info Brandon, thanks.

yes thank you!!!makes life a bit easier to track down that problem with dropping tanks