When you have inserted the RS-232 plug first make sure that the RS-232 is functioning according to the specification, see: How to create a RS-232 loopback plug.
Now plug your RJ-45 loop-back adapter into the Tech2 device and navigate to the RS-232 test. Depending on the software version there are various options to get to the RS-232 test menu.
While using the SAAB 44.000 software you need to follow this procedure;
While using the SAAB 148.000 software you need to follow this procedure;
Now the Tech2 should respond with the following: RS-232 Test: Passed
If the Tech2 responds with RS-232 Test: LoopBack Not Connected or ADDRESS ERROR - PROCESSOR HALT the chances are high that the MAX202 or the MAX232 is borked, which means replacing the SMD RS-232 communications chip (SOIC type), noted as U202 on the main PCB.