Fuel gauge problems

Reubo

Howdy y’all I’ve recently got my 740 on the road and I’ve noticed the fuel gauge doesn’t work. A Mechanic friend of mine said I need to test the wires connecting to the fuel pump which I’ve taken a photo of. When I unplugged these wires Image description
(with the ignition on) the fuel tank went from completely full (which is inaccurate) to slowly lowering. I’m wondering if these are the right wires to be testing, any advice would be great, thanks.

carnut222

Reubo Based on my recollection, that photo you posted is the auxilliary tank that sits in the boot above the floor. Those 2 wires are related to the fuel level sender in the aux tank, which kinda works in series or parallel (?) with the main tank sending unit. Not sure how to fix your problem - probably need to consult the wiring diagrams. Could be a bad sender in the aux or main tank, or broken wire somewhere?

Major Ledfoot

That's the 87 740 turbo you have, yes?

Does it have the twin fuel tank system (tank behind the rear seat)?

That pic you've posted doesn't look like it has enough connections to the main fuel tank (which is under the car) - there's no connection for the tank lift pump.

Reubo

Major Ledfoot yes that’s the one.
I did not know it had an auxiliary tank! This tank is in the boot, I did pull up the back seat but there wasn’t an access panel to a tank that I could see, do I need to dismantle further?
Cheers

dmc

my guess is that is the top tank.

ramrod

The access panel is in the boot floor.

Look for a wiring loom bundle leading to a square plate, remove it and you’ll see the opening of the tank

A special tool is needed to open the lock ring and the o-ring gasket should be replaced

carnut222

ramrod He needs to remove the auxilliary tank in order to get at the main tank…aux tank sits on top of the floor in boot and covers where the access plate would be on a car without the aux tank.

ramrod

I see, I’ve never actually been fortunate enough to have to work on that system to remember the details but you learn something new every day

carnut222

ramrod Yeah it's a real pain as there's a large and small hose connecting the aux tank to the fuel filler hose and main tank, and those hoses both pass through the opening in the floor that would be the access panel hole on a car without the aux tank. Quite difficult to disconnect the aux tank from the hoses as it only lifts up so far and hard to access the hose clamps. It's pretty common for the large T hose in the fuel filler to dry and split, causing fuel to leak out if the tank is filled. Ask me how I know! 🙂

dmc

As a rough test, and I do mean rough, with the car running bridge those two wires you took the photo of and watch the fuel gauge. If the gauge moves continue checking the sender in the top tank. If not then it is either the sender in the bottom tank, or the gauge, or the wiring to the dash, or a fuse…

Removing the fuel sender in the bottom tank is a massive pain, as mentioned above. If you do go there get a new seal, the old ones are a nightmare to get back in. Best put a new sender pump in there while you are there.

Found this online:

Image description

OldnDecrepit

There may also be a setting on the back of the fuel gauge to switch between the dual tank model and the single tank mode. There is on my 1990 740GLE.