This repair requires you to work with the diesel fuel system.  You should not proceed with this repair unless you are comfortable and understand the dangers associated with working on fuel systems.  You should also have readily available, shop rags to soak up any spilled diesel fuel and safety glasses should be worn to protect your eyes while accomplishing this repair.  While this repair is simple to complete, you should always remember safety first!  

If you are reading this repair section it is probably because you suspect that there is a problem with your fuel system.  In the case of the C4500 we were working on, the fuel gauge suddenly stopped working.  Another symptom of this failure was that upon filling the fuel tanks, it became obvious that the rear tank was not transferring into the front tank as it was designed to do.  The front tank was the only tank that required filling.  This repair assumes that you have a factory dual tank system and the transfer pump is rail mounted.

The C4500 came from the factory with a 40 gallon dual tank system.  The front tank has a 25 gallon capacity and the rear tank has a 15 gallon capacity.  We verified with the dealer by providing the VIN number of our vehicle, the correct part number for the transfer pump, and two replacement fuel fitting seals that would fit our specific vehicle and that it was a frame mount pump.  As noted in the above picture it is located above the rear differential on the drivers side frame rail.  It has easy access without requiring lifting or removing any access panels.  The pump itself only has the two fuel line fittings on either side of the pump and an electrical connector plug that is easily removed from the harness.  



The first step is to carefully disconnect the pump's harness wire from the truck's wiring harness.  For the beginner, this disconnect is not as obvious as some of the other connectors.  You must carefully insert a small, flathead screwdriver to depress the plastic gray detent piece as noted in the lower part of the connector in the above picture and carefully slide out the gray locking retainer clip.  Once removed, the connector can be separated, by using a screwdriver to carefully pry/lift up on the black plastic retainer clip inside the upper middle end of the connector, grasping the lower part where the grey plastic clip was removed and carefully pulling apart the connectors. This may sound overly complicated, but it will become obvious once you examine the connector.




After the wiring harness is unplugged, the pump can be disconnected from the fuel lines by using a 20mm open end wrench to stabilize the pump fitting. Carefully loosen the fuel line with a 17mm wrench.  This process is repeated for both ends of the pump.  In our repair, the pump had completely failed and there was only a couple of drops of diesel.  You may not be as lucky, so be prepared.  Once the pump is disconnected it is easily removed by sliding it out from the plastic bracket it is mounted in.



Replace the small O-ring fuel line seals on either end of the fuel line tips, slide in the replacement fuel transfer pump and reverse the process.  Once all the fittings and connectors are reconnected and any leakage has been cleaned up, you are ready to start the engine and examine for leaks.  If all has gone well, there will be no leaks, but you should inspect the connections again after you have driven the vehicle and brought it up to temperature.  You should be able to perform this repair in about 30 minutes.

Upon completion of our repair the fuel gauge still did not function.  This was due to a stored DTC/fault code that must be cleared from the vehicle.  This can be accomplished by your friendly local dealer by way of the OBDII connector or you can clear it yourself if you have access to one.  You may also be able to take it to your local parts store and they can connect the OBDII scan tool to your vehicle and allow you to clear the code yourself.  Fortunately, we have an OBDII scan tool so we were able to clear the code ourselves and the fuel gauge and pump are now working smoothly.  The use of theOBDII scan tool will be the topic of a future repair article.


Recap
1.  Secure vehicle on a level surface.
2.  Disconnect transfer pump harness
3.  Loosen both fuel line fittings and place them out of the way for pump removal.
4.  Remove pump and re-install in reverse order.
5.  Clear DTC code from vehicle.
6.  Check for leaks.