Hooking up the Serial Interface

Before we actually solder onto this thing, it is probably a good idea to take the GPS module out and make sure you know which one it is. I have seen postings on the web that the newer OnStar systems are using the Oncore M12. The basic idea for connecting to those systems should be the same, but the pin-out on the M12 is different from the GT. The pin-outs for all the OnCore GPS models are available from Motorola's Website. Their User's Manual has diagrams and model numbers that will help you identify your unit.

I decided that rather than connecting directly to the Oncore unit, it would be easier to make the solder connections to the back of the OnStar VIU Board where the socket for the Oncore unit is soldered. This is easy to identify because there are brackets in the four corners where the Oncore bolts on. In the image below, I marked these brackets in blue and circled the header connection in red. (Click images for a larger version)

The table below shows the pin assignments, with red font to indicate the ones we need.
(Refer to the Chapter 3 of the Oncore user's guide for pin diagram.)

I wasn't really sure which ground connection to use, since the Oncore had separate ones for Signal and Power. I checked across the two pins with an ohmmeter and found them to be connected internally anyway. If you use the RLC-1 TTL Converter, the connection should work as shown in the last column above. The yellow (RTS) and blue(CTS) wires from the RLC1 should be connected together (if applicable).

Now that the connections are done, it's time to put it all back together. In order to fit the interface wire out through the case, I clipped a small piece of the overlapping metal on the cover, and folded it out of the way.

Now put it back in your car, plug it in, and hook it up to your PC!