You should do a search on '900 fuel gauge'. The 900's have had a number of fuel gauge problems which have been discussed here. The problem sometimes has been with the sender in the tank and sometimes with the wiring in the dash cluster.
I believe the 700/900 FAQs give a procedure for determining if the problem is in the sender or the dash. Once you have done that, you can proceed. If you haven't looked at the FAQs, go to 'select link' in the upper right corner of this page and select 700/900 FAQ from the pull down menu. A really great resource!
In my '93 945, the problem was in the dash cluster. When the gauge was reading low, I'd thump the dash above the cluster and often the gauge would start reading correctly. I pulled the cluster out and traced the fuel gauge circuit on the flexible printed circuit. I resoldered all the connections as I came to them . I finally found a solder connection which was cracked but you could only see the crack with a 10X magnifier. Once I resoldered that connection, the gauge has worked fine for over a year.
I think I remember other's have has a dirty or defective sender in the tank, a more difficult problem to work on but there were a number of good posts, some with pictures, on how to deal with this.
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