You all already know how important it is for an ECM/PCM (or any other control module, for that matter) to have a good power supply and good grounds.  Typically, grounds fail at their point of connection, and testing these ground connections can be made a little easier using this tip.

Pins 4 and 5 of the diagnostic link connector (DLC) share the ground connections with the ECM.  Pin 16 shares the key off battery supply the ECM needs to keep KAM alive. 

After you've verified that pin 16 is supplying the correct voltage, you can test the integrity of the shared ground points by measuring the voltage drop at pin 4, then pin 5.  But first, there has to be current flowing.  This path can be loaded in one of two ways...first, you can plug in your scan tool and turn it on.  This will allow  you to measure the drop by backprobing the two pins from the back of the DLC.  However, in some cases, it is too hard to get to the back, so method number two can be used as an alternative.

Make up a test light with leads attached that are the proper fit for the DLC pins.  Then plug one into pin 16 (for the power side connection), and the other into pin 4 (for the ground side connection).  Use the bulb as your "load" and perform the voltage drop test at the bulb.  Last, move the ground attachment to pin 5 to check the chassis ground.

This does not test that section of wiring from the module to the splice point, only the shared portion of the ground path.  But since many failures are related to this point, it can shorten diag time by checking the most likely first.

You can also test ECM voltage drop with your scan tool.  Many enhanced data formats list PIDs (Parameter Identifers) for CASE GND and BAT VOLT.  The first is the voltage drop on the ECM ground as determined by the ECM, and of course, the other is the battery voltage level it sees.  Both should be darn near identical to your own measurements.

Hope this tip saves you time, makes you money, and helps you fix that next car faster!