gtr_gtti

I have bought a used cluster for my r129 SL280. When testing, all worked well. So I went on to unplug everything, swapped the cracked clear cover for my existing good one, and fitted it back. However, the fuel gauge suddenly decides not to work anymore.
I pulled it out again, plug in my old cluster, the fuel gauge works. So I went on to swap the fuel gauge itself in the newly purchased cluster with my old cluster. And it still doesn't work! Still shows empty with the warning light permanently on. I already have a full tank inside.
Ran the gauge diagnostics with the clock button centre, switched to the 25% to 100% reading mode, the gauge moves! So does this mean my main board is toast?
Please help!