Originally Posted by Lowell Goudge 
Check for dirty contacts on the teleconverter and lens. clean them with a gum eraser (not an abrasive one)
ALso the A mode pin is resessed on the camera this pin gets shorted to ground (mount) to turn A mode on.
check on the TC with the lens attached AND the A mode set on the lens, that this pin does stick out, and with an ohm meter, that with A mode set there is a very low resistance between this pin and mount.
Note, this pin is the third one from, the lens locking pin
If not it is time to troubleshoot/
I've been watching this thread, as I am in the same position as the OP. I have checked and found with my TC that there is an open circuit on the pin concerned. The lens itself provides a short to the mount when switched to A mode. Further checks show that both ends of that connection have an open circuit between them. Is this going to be a nasty fiddly job, or can I expect to open the converter and find a simple solution? As best as I can tell, all the connections are open circuit. I assume there should be a through connection on each contact. Is it likely to be a flexible ribbon cable, plugged in at each end or are the connections soldered? I don't fancy removing the mounts, for fear that a load of small components are going to fall out.
Any advice you can give would be much appreciated.
Regards, Keith