Sub tank fault on 04 TD42

[Phat Patroller]

Ok patrollers I have encountered a problem.....

went through rather large bog hole, and about 10 mins later down the track the sub tank gauge had dropped to empty, and the sub tank light is on solid. Confirming that the tank had not been punctured (filled it up at the next servo)

I have read the repair manual and have tried the "sub fuel tank diagnosis system" but have been unable to to get it in diagnosis mode to find problem.

Things I have checked in sub tank unit:

Pump sender unit has resistance (roughly 5 ohms)
Pins on both plugs have good earths

Anyone with some experience in this matter please come to my rescue!!!!!!!!!!!

[Yendor]

Remove the sender from the tank and see if the float has fallen off.

[Phat Patroller]

Ok I shall give that a go tomoz, have you had that happen before?

[Yendor]

Yeah it's happened a few times.

[MudRunnerTD]

Have you tried disconnecting the Earth lead? i have had the subtonic light come on a few times and have disconnected the earth for 30minutes and cleared the fault. The gauge has never dropped to empty though? How hard is it to remove the sender Rodney? Drop the tank first?

[Phat Patroller]

I have tried to clear the fault, it keeps coming back within 30 sec of starting vehicle.

Yh I thought it was a bit different to some of the other problems on here no one mentioned a sub gauge showing empty.

And I already have access to the sender from inside the truck under the rear right hand seat. I'll check out the float today hopefully if it has come off, the thing is easy to find in a full tank :-/

[Yendor]

Have you tried disconnecting the Earth lead? i have had the subtonic light come on a few times and have disconnected the earth for 30minutes and cleared the fault. The gauge has never dropped to empty though? How hard is it to remove the sender Rodney? Drop the tank first?

Hey mate,

The key to this problem is the fuel gauge not reading at all.

There's an inspection cover in the floor for the sub tank just behind the drivers seat. Just lift up the carpet remove the inspection cover and you have access to the sender unit in top of the fuel tank. You don't need to remove standard fuel tanks to access the senders.

[Yendor]

I have tried to clear the fault, it keeps coming back within 30 sec of starting vehicle.

Yh I thought it was a bit different to some of the other problems on here no one mentioned a sub gauge showing empty.

And I already have access to the sender from inside the truck under the rear right hand seat. I'll check out the float today hopefully if it has come off, the thing is easy to find in a full tank :-/

You will more then likely have to disconnect the battery to clear the code once you have fixed the problem.

[Phat Patroller]

Just checked the float, no problem there. I confirmed by raising and lowering float in tank and checking for varying resistance.

I tend to agree it's the gauge not reading, but it's not at the tank.

Any other ideas?

[Yendor]

Next step is to confirm continuity in the wire between the sender unit and fuel gauge.

Get under the rear of the vehicle and check the wire harness as it comes out near the back of the sub fuel tank and runs across the chassis cross brace, just in front of the rear diff. Look for any signs of damage to this harness.

With ignition on measure the voltage in the fuel sender wire at the sender unit (sender disconnected). Try touching that wire onto earth and see if the gauge moves.

edit: Don't use the earth wire in the harness. Test using the body.

[the evil twin]

If you try Rodneys test and there is no power or it works on Chassis earth but not on the loom earth...

The Sub tank gauge wiring and also the main tank "not full" switch for the Sub Tank Computer all go thru plug B125.

The rest of the main tank wiring all goes thru B123 so it would make sense for a wiring issue (water or damage) in the loom at/from B125 to trip the fail lamp and also for the gauge to cease but for the main to keep working OK.

I can't for the life of me remember exactly where that plug is but from what I see in the Manual it is is pretty low down on the chasis/firewall.

[Yendor]

That would be the connectors on the LHS chassis rail directly under the passengers front seat.

[the evil twin]

Hiya Rodney, knew you would be on the money... for the bonus point, what colour is the middle one (just jokin').

Been a long while since I went looking for them.

Just raised my suspicions when the OP mentioned "bog holes" and has confirmed that his sender is in one piece

[Yendor]

I still think it's an open circuit. But seeing has he's checked the fuel sender unit and earths maybe it is a badly corroded plug.

I will be able to post the wiring diagram up later once I get home. Then I can tell exactly which wire it is in those connectors and the colour of the middle one for you :)

But if he has a voltage reading or the gauge moves when the wire is earthed then all this is pointless anyway.

[the evil twin]

Agree... open cct will send gauge low and set the fault in the Sub Tank 'poota

Those "B" plugs will be handy only if it is a fault in the loom.
Every time but one the faults I have found have been in the Main or Sub Senders (float fell off, worn or open wipers, broken wire etc) or the plugs at the Sender

[Phat Patroller]

I have found and rectified the issue. It was indeed a broken wire in the sub tank level circuit, but the bog hole threw me off I thought I'd be looking for water ingress or corroded plugs too, but it turns out that was a red herring...

Where the loom ducks back up into the cab, through grommet under passengers seat, I must have caught a branch and it pulled the loom out cutting the sub fuel gauge sender wire and damaging some others.

Solder, heat shrink, tape and some split tubing later and I'm all good again.

Thank you to everyone for your quick resoponses and ideas to put me on the right track.....champions the lot of ya!

[Cuppa]

This thread is a great example of the usefulness of forums. Excellent stuff folks. And Phat .... thanks for feeding back the good results.

[Phat Patroller]

I agree and is exactly the reason I choose to become a member :-)

[peterbr1]

I cant quite remember but I think you may need and extra earth wire some where near the subtank to the body of the car

[4bye4]

I cant quite remember but I think you may need and extra earth wire some where near the subtank to the body of the car

You can never have enough earths on a GU. Yes bad earth can cause subtank pump or guage issues.

[MudRunnerTD]

Great thread thanks Rodney and ET! Great addition to the brains trust.