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Yesterday I flipped on the master switch and my entire instrument panel was not functioning. Also, the transponder was not functioning. The only things that were working were the coms and the annunciator panel. I check all the fuses and everything was fine. I hooked the battery up to a battery tender and waited 5 minutes and then everything came back. However, I went back today and tried again and everything was dark again, even though the battery tender remained hooked up since the previous day.

I felt like our battery was near the end of its life when we went in for annual 4 months ago but they told me otherwise. Probably because they were too lazy to do the work since the job is pretty involved. Could this be a battery issue or something different?

Thanks.

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I suspect you have a wiring problem. All those are powered by the "Inst. 1" Fuse circuit. If the fuse is good (check it with a multimeter) first place I'd check is to make sure there a good connection into the spade connector to the fuse block. There is several other troubleshooting steps that you could go through but I don't have time to walk through them all. The wiring diagrams are in the maintenance manual a competent mechanic should be able ring wires in a basic form using these diagrams. Replacing the battery is huge pain in the ass but since you have power to the radio and other things its not likely it especially since your battery light isn't on which means the battery is charged.