Let’s take the time to review some basics regarding electrical diagnosis. The technician performing either electrical or drivability diagnosis and repair should own a high quality digital-volt-ohm-meter (DMM). The DMM should have very high internal resistance in the neighborhood of 10 Mega-Ohms similar to the Fluke 87 meters used at the training center. This high resistance allows you to test the circuit without disturbing anything in the circuit; in other words the way it operates without the DMM being there.                                        

The first test I am going to talk about is available voltage; simply, is  voltage there, yes or no. 
The meter leads should be in place on the DMM at the V and Com receptacles (figure 1). Now take the black lead install an alligator clip and attach it to a known good ground. The black lead stays there on ground for the duration of the testing. You will now test for the presence of voltage with the red lead. Another method is to use the ground terminal at the connector the technician is testing, this allows you kill two birds with one stone if everything is okay. If voltage is not present switch to a known good ground before chasing what might be a ghost. Figures 2 and 3 show DC and AC voltage testing connections.
                                                                                                                                                                                     Figure 1       

  Figure 2                                        Figure 3                

Available voltage testing is also used to check for class 2 serial data before condemning a module. The one difference is that a Fluke 87 must be used to take advantage of the 1-millisecond capture rate of this DMM. The lead setup is the same and on the meter face range up to the 40-volt scale, then hit min/max followed by the peak button. Then read the results by toggling the min/max button to switch between low and high results.

Voltage drop testing requires the leads to go across a device and measure how much voltage is being used. The leads of the DMM stay in the V and COM receptacles during voltage drop tests. Remember, grounds, fuses, switches, and connections should have little if any voltage drop in a functioning circuit. This is critically important when working on low voltage circuits such as reference voltage circuits where there is only 5 volts to begin with; any inappropriate loss of voltage will greatly affect the circuit. Do not forget when testing for an intermittent problem use min/max to record while moving the harness or connector suspected of being the culprit.

Trust the meter and know what to expect from your readings  before making the test. Perform the test to see if the result is what was expected and bottom line is to believe your test. A good course for DMM usage for beginning tech, or for a veteran of many years  to use as a refresher,  is 18043.04H - Electronic Terminals and Connectors and is offered regularly at the Training Center. Check the schedule page to find out when it is available.  Good luck with your diagnosing, fix it right the first time and see you at the training center.

Check out Part 2 of the series by clicking here!