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Understanding our diagnostic test equipment

Monday, February 1, 2010 - 01:00
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A voltmeter always should be connected to a circuit in parallel with that circuit, not in series. As such, a voltmeter becomes an actual parallel leg of the circuit being tested. According to Ohm's Law, this has the effect of drawing more current through the circuit once the meter is connected as a parallel branch. The meter then has the effect of dropping the total resistance of a circuit an insignificant amount. The current drawn by the meter and the voltage used to push this current through the meter is extremely small, typically less than a few millionths of an amp and less than 1 percent of the available circuit voltage.

However, a high quality, high impedance (resistance) does not significantly affect the function or readings of the circuit being tested through this current draw.

A voltmeter can directly measure only two aspects of a circuit. The first aspect is called an Available Voltage (V) test; the second aspect is called a Voltage Drop (Vd) test.

An available voltage test simply measures the amount of potential difference in electrostatic charge between two points of a circuit. The electrostatic difference measured merely is the amount of energy that can potentially do work within that circuit once the circuit is turned on. No actual current is flowing in the circuit at this point, though there is a tiny amount of current flow through the meter.

A Voltage Drop test is the amount of voltage potential "used up" as current moves through a resistance (or load) in a completed, or turned on, circuit.

When making either your available voltage or voltage drop measurements, the meter reading acquired is determined by where you place your meter leads when attaching the meter to the circuit. Not only is where you place your leads important, but so is how you connect them. Too many technicians misunderstand what a parallel meter connection means in some situations. An example of this common mistake follows.

With the test set-up in Figure 1, what would you expect the meter to read if the PCM is turning OFF the ground circuit through its drivers transistor? If you answered 0V, you would be wrong.

It will read system voltage as it does in the left panel of Figure 2. In this figure, we are using the scan tool to command the PCM to turn off the ground circuit. Why then do we read what appears to be available voltage in an open PCM circuit? Because the circuit is not actually open.

A transistor, unlike a mechanical switch or relay contact, never is completely physically open. A transistor only blocks sufficient current from passing through the device in order to keep from operating.

However, some leakage current, as seen on the right panel of Figure 2, always leaks through the transistor. This leakage current is in the millionths of an amp range. Remember what a voltmeter does: It creates a current path of a few millionths of an amp in order to do its measuring. With the meter incorrectly hooked up in series with a solid state device as it is in Figure 1, your readings would cause you to conclude that the driver transistor in the PCM is stuck in the ON position. Big mistake.

Look what happens in Figure 3 when we command the transistor ON. We still read system voltage as shown on the left panel of Figure 3. We now have a completed series, high current, path to ground through the meter and PCM transistor to ground. Notice the amperage level on the right panel of Figure 3. It is 1.8 amps, plenty of current to drive a solenoid to do its job. This connection method was obviously not a proper voltage test set-up.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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