I have a 2005 c5500 rollback duramax and the fuel gauge stays on emety and it will not pump fuel from the rear tank I have ohmed both sendings units and they seem to be working I am in need of any other known trouble spots and a schematic

The fuel level sending unit is integrated in the fuel pump, which is located in the tank. The age of your vehicle warrants replacement of the fuel pump as a unit. Never try to repair a sending unit Just make sure the float for the sending unit is free to swing when installing the fuel pump into the tank. Also empty water out of fuel pump well of the fuel tank

Bad sending unit, bad ground, bad gauge, I have seen sending unit go bad they will work part way.

Check your parts sites. It is true that some fuel pumps are unitized with the fuel gauge. But there are also some with replaceable pumps which means the gauges are also separate.

There are also differences in the connectors with some having flat pins, broader flat pins, and round pins in the connection.

But dropping the tank is a must. Also be aware that there can be a plastic screen on the pump and it may fall apart and jam the float.

Nissan Car Altima & Sentra 1999-06 Repair Guide
Fuel Pump - Removal & Installation

1. Before servicing the vehicle, refer to the Precautions Section.
2. Relieve the pressure from the fuel system.
3. Remove or disconnect the following:
   NOTE When the fuel sending unit needs to be removed, pull the tab upwards. The tab is located on the sending unit, opposite the end of the float. After the tab is pulled, the sending unit will lift straight out of the tank bracket.
   Negative battery cable Rear seat and the access cover Fuel pump electrical connector Fuel lines from the fuel pump assembly Locking ring Fuel gauge assembly Fuel tube and connector from the fuel gauge
   Fuel pump by pinching the 2 locking tabs together. Lift the fuel pump assembly straight upward and out of fuel tank. O-ring and discard
4. Place a clean rag in the hole to keep out dirt.
   

To install:

1. Remove the rag
2. Install or connect the following:
   NOTE Verify that the mark on the fuel tank and the components are aligned when installing the pump and fuel gauge sending unit.
   New O-ring and fuel pump Electrical connection and fuel tube to the fuel gauge sending unit Fuel sending unit into the tank
3. If equipped with a threaded lock ring torque the ring to 22-26 ft. lbs. (30-35 Nm). If equipped with a lock ring held by six retaining screws, torque the screws to 18-22 inch lbs. (2-2.5 Nm).
   Fuel lines and fuel pump electrical connector. Always install new clamps on the fuel lines. Negative battery cable
4. Start the engine, check for leaks and repair if necessary.
   Fuel pump access cover Rear seat
   

The fuel pump is located in the fuel tank on all vehicles. In-tank fuel pumps are accessible by lifting up the rear seat to gain access to the inspection cover.

1. Before servicing the vehicle, refer to the Precautions Section.
2. Relieve the fuel system pressure.
3. Remove or disconnect the following:
   Negative battery cable Rear seat from the vehicle Inspection cover that is located under the rear seat Inlet and outlet fuel lines from the fuel pump assembly Fuel pump and gauge wiring connections 6 mounting bolts that secure the fuel pump assembly to the top of the fuel tank
4. Raise up the fuel pump assembly and detach the fuel tubes and connector.
   Fuel gauge assembly Fuel pump with the fuel chamber
5. Pull up the front of the fuel pump chamber and slide the chamber forward.
   Fuel pump from the chamber O-ring seal or gasket

To install:

1. Install or connect the following:
   Fuel pump to the fuel pump chamber and slide chamber rearward Fuel pump with the fuel pump chamber Fuel gauge assembly using a new O-ring Fuel tubes and connector. Use new hoses and clamps. 6 mounting bolts to the top of the fuel gauge unit and torque the bolts to 18-22 inch lbs. (2-2.5 Nm) Fuel pump and gauge wiring connections New inlet and outlet fuel lines to the fuel pump assembly Negative battery cable
2. Start the vehicle, check for leaks and repair if necessary. Inspection cover and the rear seat

Hope helps (remember comment and rates this).

fuel sending float stuck. or it floats and the fuel sending unit----a resistor set up, is shorted. it's in the gas tank, remove rear seat and you'll see the fuel pump gasket. The fuel pump and sending unit are as one as you pull the assembly out.

Hi: Your Fuel Level Sending Unit That Is Located in Your Fuel Tank Is Probally Bad. BUT before you go to taking the Sending Unit out Of your Tank Check to see if ALL Connections To And From Tank to Gauge are Conected Good. Also You could have a Fuse that Goes to the Sending Unit,in Your Fuse Block.Check And See if You Have A Fuse Labeled Fuel Sending Unit or Fuel Sensor. If you end up Replacing The Fuel Tank Sending Unit Run Your Gas Tank Down To Almost Out Of Gas Before You Start to Remove The Metal Straps Holding Your Tank Up. Cause The Sending Unit Is on Top of Your Tank ,And Your Fuel Pump Is In The Gas Tank As Well. Hope This Has Helped You Please Send Me a Vote on How my Solutions Have Helped You And Also You Can Send Me A Testamony Thank You From the WYOCWBOY

the level sending unit in the tank is attached to the fuel pump module it is a varible resistor it moves as fuel level changes this sending unit can be replaced but if your going to the trouble of replacing it you just as well replace the fuel pump it comes together.

pos lead is used to measure ground...that means that the power goes through the fuel level sending unit and the ground lowers the power out from the unit....I would replace the fuel level sending unit.

Hi, this is a sending unit issue. you can try adding some Sea Foam additive to the tank. this may release the sender. if the additive doesn't work, replace the fuel level sending unit.

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur