E39 Fuel Gauge proeblems

Fingers Mcginty

Howdy folks,
Need a little help on this one. I filled her up with petrol recently but the fuel gauge never went up to the full mark. As i drove the gauge kept dropping and evtually the empty amber light came on. I began doubting myself i had actually filled it up in the first place. The needle is now at the bottom with the light on all the time. I removed the back seat to the check the sender units. There is one on the passnger side with 2 wires and another one on the driver's side with 4 wires. 2 of the wires are a lot thicker ..fairly sure they are for the fuel pump. Any ideas how i could diagnose the problem (apart from using a dignostic unit ) ????

Cmar-Ireland

AFAIK this is a common problem on some bmw's. Something to do with capacitors on the pcb behind the guage cluster.
If that's what it is, there are write up's on the 'net for how to diy fix it on the E34, so I reckon there would be similar out there for the E39.

Fingers Mcginty

I think it might be the fuel sender unit behind the RHS seat in the back. I have measured the resistance and it's open circuit. I supect the wire may have come off the sender inside the fuel tank. Anyone ever take one out?:) Is it handy enough?

Fingers Mcginty

Just in case anybody ever has the same problem i might as well post the fix here for reference. There are 2 float sensors under the rear seat. One on RHS and one on LHS. The LHS has just 2 wires which connect to the float and are run in series to the RHS. The total resistance is summed and this equates to the capacity of fuel in the think i suspect. RHS has 4 wires...the 2 larger ones are for the fuel pump. I measured the resistance on the LHS and got a reading but nothing on the RHS. I removed the float and all looked ok. I checked the resistance at the actual strip on the float and got a reading but none at the connector. This indicated a break in the wire. When i peeled back the insulation it was indeed open. I think the whole neighboorhood heard me cry "Hallelujiah"