Fuel Pump/Sending unit/jet pump questions...

[Casebrius]

So my fuel gauge quit awhile back and I've been dealing with it... 4/95 M3
I just ordered the sending unit for left side hoping that is the bad one. Does the jet pump on the left side transfer gas to the right side to be pumped to system? How often do these jet pumps go bad and are they hard to change? Is the jet pump powered by the right side fuel pump? According to the ETK, the jet pump in mine is NOT connected to the sending unit. I'm trying to understand the whole configuration. I've been doing well metering my gas by consumption, but It sputtered earlier yesterday when I should have had plenty of gas left. I added a few gallons and sputter went right away. To complicate, it leaks when I fill tank completely full, so hopefully new gaskets for pump and sender remedy that too. If not, I guess its the filler line leaking.
Thanks

[NeilM]

The jet pump is an unpowered device that's not likely to stop pumping unless one of its fuel hoses is blocked or comes off.

Both the actual fuel pump unit and the jet pump units incorporate fuel level floats and potentiometers. The two potentiometers are wired in series in order to provide a single output voltage signal proportional to combined fuel content, no matter where the fuel is sitting in the saddle tank. However since they're in series it follows that if either potentiometer fails open circuit, which would be the expected failure mode, then there's no output signal. Fortunately you can test them easily without opening the tank by disconnecting their plugs and applying an ohmmeter to each unit in turn. Bentley gives the wire colors.

Access is by pulling up the rear seat base at the front on both sides and removing it from the car. This exposes the fuel system wiring and connectors. There's a pair of metal covers which, once removed, give access to the pump area. The pump locking rings are a bit hard to get unscrewed — use a big screwdriver against the outer ribs and tap with a mallet until they start to move.

Neil

[Casebrius]

Thanks Neil.