check for power and ground at the DLC, dont jam the multimeter prongs into the terminals! once thats confirmed as good you can use the ACv seting to check the serial data line for a signal.
more than a few times with older vehicles ive found weak terminals that have intermittent/no connection....the DLC is quite common as it's used all the time and many people wiggle the connector up and down instead of left and right to install/remove the plug. all flat pin connectors should be wiggled parallel to the flat pins orientation.
it could be as simple as using a sewing needle to slip under the banana spring and re tighten it.
also after major work double check that all your engine/pcm block grounds are good. dont forget the unibody ground on top of the unibody frame rail below the battery or pcm dep on model/year.
the bellhousing stud should have a tan and a black wire to one ring terminal and usually another pair of black or black with a white tracer stacked with it and the main batt ground cable.
i have had schmegged pcm's cause no serial data. so an offboard/standalone setup would help diagnose that one....you will be able to scan/read the pcm, but the data pids will be default values/open circuit voltage.
if you leave the pcm in the car and only unplug the blue connector and insert the offboard blue conn, you will be able to see the data for the sensors/etc it gets from thhe car on the clear connector you left plugged in....this will let you see PCM voltage and double check it has no voltage drop vs the multimeter batery voltage reading.
also be aware the serial data wire runs through the harness above the radiator and more than a few times ive found wires rubbed through/shorted to ground from dogbone bracket/radiator services.
also check the pcm/etc fuses and loock for corrosion in the fuse block
checking the obd2 dlc connector - YouTube
