TDR-94 ATC Transponders
are found on numerous aircraft types. They are extremely reliable units and are rarely pulled for failure. One particular job that does need to be performed on the units is a code strapping reconfiguration whenever the aircraft registration (in the U.S., the “N” number) changes. Strapping can be done at the rack plugs or at an external terminal block. Older aircraft tend to accomplish this at the plugs. Newer aircraft, such as this particular GIV have an external strapping junction.
The transponders on a GIV are located under the radome, aircraft left, on the top shelf.
Rack plugs. Strapping is accomplished on the P-1 plug, pins 33 through 56. This generation of rack plugs from Collins uses a flat “fork” terminal that is quite different than the normal type plug that uses round pins. Of course, working with these pins requires a special crimper and removal tools. These are “push out” terminals and they are removed from the rear of the rack plug. The numbering on these plugs can also be tricky. Viewing from the front side: Pin 1 is upper right. Successive pins are read right to left for each row. Pin 5 would be directly below pin 1.
This aircraft has two junction blocks on the outboard side of the racks. Normal thinking would tend to lead to the strapping being done at the blocks. After the wire bundles were opened up, the terminal block wiring led to numerous (disconnected) capped off pins.
Tracing wires revealed the coding is accomplished at two removable jumper plugs. This is quite a handy option. The jumpers can be removed and recoded in a shop environment instead of messing with rack plug pins or onboard terminal blocks. These two were most likely assembled at Gulfstream and probably are attainable from them, at a cost.
Coding for these plugs took a little chart. The pins on the plug did not match the pins on the rack. They were “32” off. Pin 33 on the rack matched pin 1 of the plug
(I found that pin 25 was connected to the jumper array. I did not continuity check this to a corresponding rack plug pin, but it must be an actual “ground” provided by the TDR-94 when it is in the rack.)