Anyone with Humminbird Experience...?

[Willy]

Hey all,
I have a used humminbird 535 fish finder I just got working (Thanks Dgax65!!!!).
I'm having a big problem with the transducer (transom mount):

It won't read a proper depth. The temp sender on it works fine, but it always says I'm in anywhere from 100 - 800 feet, with either a flat black bottom, or a bottom reading that looks like a bar graph. The depth usually sits around the 750 range, even in Mission Bay or my swimming pool.
The transducer worked well in the last kayak, mounted as a thru-hull.
I'm trying to use it on my new skiff, as a proper transom mount.

I'm pretty sure Mission Bay isn't 800 feet deep.


Anyone have any experience with this issue?
(Sorry for the awful picture)

Thanks all,

Willy