 | |||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||
|
Are You Using a DMM Correctly?Posted 2/14/1998By Vince Fischelli
I once met an old-timer auto tech who never used anything but an inexpensive test light to troubleshoot automotive electrical systems. He was reasonably successful at it with many years of experience under his belt. His success with a test light up to my meeting him blinded his eyes to the need and benefits of a digital multimeter (DMM). His boss wanted me to train him, as well as the other techs, to use a DMM so they could do more advanced troubleshooting such as voltage drop testing. All the techs responded well to the training, except the old-timer. He was a tough one to convince and I think he was just humoring me during the training session. After I left I felt he would still use a test light when no one was looking. I thought about the old saying: "It's hard to teach an old-timer new tricks." The fact is, a DMM is an extremely valuable troubleshooting tool. It contains a digital voltmeter for checking voltage (both DC and AC), a digital ammeter for checking current and a digital ohmmeter for checking resistance. Additional features might include a diode tester, which is effective under certain circumstances, a frequency counter, a temperature testing circuit, a dwell indicator, an rpm counter, etc. The digital voltmeter is an exceptionally beneficial troubleshooting tool built into the DMM. For example, a digital voltmeter can determine the state of charge of a battery better than a hydrometer and reveal battery problems that a hydrometer cannot see. A digital voltmeter can check a connection or cable voltage drop to prove it is bad even when the cable or connection looks good, or prove it is good when it looks bad. This could save a lot of time and prevent simple electrical problems that are causing major vehicle problems from being overlooked. A digital voltmeter can check some sensor inputs so you can see what the computer "sees" when it looks at the sensor. Simply put, a digital voltmeter can tell you things about a circuit that you cannot know without a digital voltmeter. What does a digital voltmeter do? A digital voltmeter indicates the difference in voltage and polarity between its probe tips. It's so simple that this simple fact is often overlooked. The concept is illustrated in Figure 1.
DMM B is connected to the circuit in reverse polarity, the red test lead is connected to the negative polarity and the black test lead is connected to the positive polarity. Under these test conditions, the DMM indicates -14.45 volts; which means the test leads are connected in reverse order to the circuit. No harm is done to a DMM when measuring voltage in reverse polarity. To obtain the correct reading, reverse the test lead probe tips at the circuit measurement points and the DMM indicates 14.45 volts. The absence of a + sign in front of the voltage reading means the voltage polarity being measured is positive at the red test lead and negative at the black test lead. What does DMM C indicate with the voltages shown at the probe tips? First determine the polarity of the reading. The polarity of the measured voltage is positive and no sign is present in the reading. Next determine the actual reading; that is, what are the numbers in the readout? The readout says 0.10 volts because there is 0.10 volt difference between the probe tips. It does not mean the actual voltage being measured in the circuit is 0.10 volt, which is what most techs would assume. Then, what is the actual voltage being measured in the circuit? The red test lead of the DMM is touching 0.40 volts; that is the actual voltage at the measurement point in the circuit. Only 0.10 is indicated because the black test lead is erroneously connected to 0.30 volt instead of 0.00 volt, the reading of a good ground connection. If the black test lead is connected to 0.00 volt, the voltage at a perfect ground connection, the readout would correctly indicate 0.40 volt in the circuit when the read test lead touches 0.40 volt. The difference between 0.40 volt and 0.10 volt is the difference between finding a bad ground connection voltage drop (0.40) and a good ground connection voltage drop (0.10). This is why some techs are unsure of themselves checking ground voltage drops. They don't have a good ground for the DMM black test lead.
The solution is to always ground the DMM's black test lead at the most negative point in the circuit. If the engine is not running, the battery negative terminal represents 0.00 volt, the most negative point. If the engine is running, the generator (alternator) is providing the voltage and the alternator housing is the most negative point. If the voltage drop between the generator housing and battery negative is less than 0.10 volt, then the battery terminal can also be used for DMM ground when the engine is running. It is safer to ground at battery negative than to stretch the ground lead into the engine compartment while the belts and pulleys are moving.
|
||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | |||||||||||||||||||||||||||||||||
DMM A has +14.45 volts at both test lead probe tips. This can be confusing working on an electrical problem. Both test points are actually "hot" at +14.45 volts and would "arc and spark" if either test point were shorted to ground. Yet, the tech reads 0.00 volts on the DMM (thinking the circuit is "cold") while the circuit is still "hot." How can this be? (Tell me it isn't so!) DMM A reads 0.00 volts because the difference between the two probe tips is 0.00 volts. It's enough to make a tech reconsider working on electrical problems. This situation can cause "arcs and sparks" during the repair if one of the test points accidentally connects a "hot" test point to ground. This problem occurs because one test lead of the DMM, the black test lead, is not connected to a good ground point.
