Hi Richard, since I didn't renew my membership I can no longer post on the POS avionics forum. I thought you and others might be interested in the final solution to coupling my Garmin GPS 400 to the Altimatic IIIB. The problem was indeed in the wiring. They had wired the GPS 400 correctly to my 360A HSI, but they failed to notice that the #1 nav had at one time been wired directly to the radio coupler, and those wires had never been removed. As a result VOR and ILS coupling worked perfectly, but even though the HSI showed the proper GPS info, the radio coupler was getting 2 different inputs when GPS was selected on the annunciator panel. As you predicted Richard, this incorrect wiring fried the analog portion of the GPS 400 board once again. Thanks again for your help.