fuel gauge/tach and mpg problem

[SedanSandwich]

After replacing the batteries on the S.I. board the fuel Gauge started to work again but jumps around and the low fuel gauge came up when the needle was around 3/4. the mpg needle just stays at 40mpg at all times and the tach can be jumpy at times. for the fuel gauge should I check and clean the sender in the trunk 1st or should I get a clean S.I. board. I also noticed that the S.I. board in my cluster is from the old clusters using the 2 varta ni-cad batteries even though the production date is in 89. would it cause a problem since it's not using the board with the lithium batteries? sorry for the long winded wall of text. thanks for any help in advance. not sure what to start with.

[bmwm42]

Get a new si board alfo make sure the nut behind the gauges are tight

[jlevie]

The SI board has nothing directly to do with the fuel gauge. A bad SI board (or bad batteries) can affect the tach, econometer, and temperature gauge, but not the fuel gauge. Bad contacts where the fuel gauge mounts to the PCB in the cluster, a bad level sensor, bad cluster ground, bad solder joints on the PSB, or a wiring problem are all suspects at this point.

[SedanSandwich]

Thanks for the info. I will pull out the cluster and check the PCB... I know that the fuel gauge does not have the nut on the back of it.