This summer I had two episodes when it seemed my Onan was starving for fuel. After changing the fuel filter I found this excellent diagnostic guide and concluded the fuel pump was bad
http://www.rvwithtito.com/articles/generator-does-not-start-fuel-pump-testSince I have a PC in my garage I decided to write up the process in detail
Onan 4000 Fuel Pump Replacement: Step-by-Step with pix
Photos are here:
https://photos.app.goo.gl/eyAjZKhU52tsJ9X7AThe procedure below is for a 1995 Onan 4000 on a Class C RV. The design has been around a long time so other years should be similar.
This is one of those jobs that looks impossible but is actually straightforward. The key is how Onan engineers mount each component in the extremely limited space.
Each component, including the fuel pump, has a tab on the far side that goes into a slot and a T30 screw on the accessible side. This allows you to remove and then remount four components with just five T30 screws. Once you understand this pattern the puzzle is easily solved.
You will need a T30 bit with a 3-inch or so extension, a ratchet, a flat screwdriver for the hose clamp, a small flat screwdriver to help with the tabs on electrical connectors, needle-nose pliers, Vise Grips to clamp the fuel line, and 9/16 normal and 1/2 deep sockets.
A strong LED headlight will be very helpful in peering into the generator where all the steps below are going to happen.
A multimeter, a catch pan, and yellow Teflon tape are good too.
Burning up your RV is no fun, so here is how to assure the power is removed where there may be cross-connects between chassis and house or multiple batteries or whatever.
Use a multimeter to confirm 12 VDC between the near terminal of the generator solenoid, and ground. Now disconnect the RV (house) battery positive lead, making sure the cable or cables cannot move and reconnect themselves. Disconnect any shoreline or solar power. Now check the solenoid again. It should now show no power at the generator.
Remove plate covering front of generator and fuel supply line: 9/16-inch socket
Put a pan under the front of the generator. Clamp off the fuel supply line with Vise Grips and remove the fuel line from the fuel filter.
You can remove the fuel filter with a 1/2-inch deep socket now or wait until later when the pump comes out.
Remove outer nut on solenoid post, releasing large hot cable and small white ring terminal
Remove T30 holding the ignition module. This has a large capacitor on top.
Disconnect plug in bulkhead by prying tabs OUTWARD to the left and right
Remove ignition module and set to side
Remove lower plug in bulkhead, again by prying open tabs on sides. A small flat blade screwdriver may be able to reach behind and pry the hidden tab.
Mark one pair of the spade terminals on solenoid so they go back correctly
Now stuff the solenoid up by the air filter so it is out of the way. You donโt have to remove the back side cable.
Next remove the outside T30 screws which hold the base plate which the solenoid sits on.
Wiggle the plate side to side until the hidden tab in the rear comes loose and plate pulls out
You can now access the front T30 that holds the fuel pump in place
Like everything else, the pump has a rear tab. Wiggle it around and the pump will come loose.
Pull the pump towards you and the connection to the carb fuel line will become accessible.
Before removing the fuel line to the carb, clean around the pump so that there is no dirt to get into the new pump
Use needle nose pliers to compress the hose clamp and pull the hose off
Disconnect the two power supply wires, noting what they connect to
The old pump is now out
REASSEMBLY
Itโs easiest to install the fuel filter now, before inserting the pump. I use yellow Teflon tape on the connection.
Pull the carb hose up and insert new pump into it, being careful not to get any dirt into the connection
Expand the hose clamp with pliers and move it into position on the hose
Connect the black lead to a circular plate where lots of wires go to ground
Connect the light-colored lead to the insulated connector that you previously noted
Now shove the far side tab of the pump into its slot
Use your T30 extension to start the screw on the near side of the pump, then the ratchet to tighten it up
Slide the base plate in so its far tab goes into its slot
Reinstall the two T30 screws to secure the plate
Push the ignition module into its slot and install the front T30
Pull the solenoid down, hook the far side to its tab, and install the front T30 so it is firmly in position
Reconnect the two spade connectors to the solenoid. One pair should be marked from before.
Put the white ring terminal on the solenoid post, then the big positive cable, and tighten the nut
Reconnect the two bulkhead connectors, making sure both tabs snap securely
Look over the generator to make sure everything went back into place
Reconnect the fuel supply line and the cover plate on the front of the generator
Reconnect the positive battery lead
Again make sure nothing has been forgotten. Confirm the new pump works by firing up the generator. Check for any fuel leaks.
