Do you need the transponder, or the encoder. All transponder units are Mode A, and all have Mode C capability if they are receiving signals from a separate, installed Encoding altimeter.
If your transponder works, and you simply don't have an encoder installed in the airplane, then the fix is to buy an encoding altimeter. You will however need to have a certified avionics shop install the unit and preform the tests required after a new installation.
http://www.aircraftspruce.com/menus/av/encoder.htmlEither the ACK unit or the Ameri-King AK-350 30K ft model would be good choices. These are both pizoelectric units with no moving parts and are quite reliable and when old, out of warranty and do fail, are cheap enough to scrap and replace with new.
These units weigh a few ounces and both come with harnesses already made up, all that is necessary is to terminate the transponder end (you avionics shop will need to obtain the correct pins for the type of transponder you have) and they easily can be mounted on the top or front of the glove box, as long as it does not interfere with the control Y-yoke under the panel.
Installation is relatively painless but expect it to take a couple of hours or more, drilling screw holes mounting it, routing and terminating wires, tying wire bundles, installing an additional T fitting and routing the static line, leak testing the system, connecting to the circuit breaker (usually the transponder breaker, they draw only a few miliamps) and doing a data correspondance test with a transponder ramp test box and a barfield pitot/static box, and completing the paperwork. It looks simple, but alot is involved in doing it correctly.
Charles
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