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Using voltage and current in Advanced Circuit Diagnostics

Tuesday, September 1, 2009 - 00:00
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Part two of this study will explain live three-channel scope captures of all the concepts outlined in these two charts. Part three of this series will demonstrate the effectiveness of understanding these charts, which use both voltage and current, employing real world scope captures from actual defect vehicles.

The techniques shown in this article require the use of, at minimum, a dual channel oscilloscope or graphing meter. I say a dual channel because we will be looking at both circuit voltage and current simultaneously. Looking at both voltage and current flow in a circuit can identify the exact nature of the circuit defect in considerably less time and with more accuracy than multiple voltage or current measurements alone, especially on intermittent or non-steady state defects.

During this article and in the two provided charts, I will reference using three channels of a four-channel scope. I do this to more quickly demonstrate how each half — the power side and the ground side — of a circuit behave differently relative to the type of circuit defects you may encounter. If you have a four-channel scope, then great. Testing will be quicker in that you can use three channels to test both the power feed side, ground feed sides and total circuit current flow of the entire circuit at once.

If you do not have a four-channel scope, you will now know what readings to expect when you move your channel 1 voltage probe of your dual channel scope from the power feed side of the intended circuit load to the ground side.

Take some time to study Figure 1. This simple circuit represents the most common type of circuit in today's automobiles — a power feed circuit, represented here between points A and B; a component or intended load between points B and C; and a solid state controlled ground circuit between points C, D and E.

I have chosen the battery negative post (F) as my scope ground reference point. The lowest potential test point in the electrical circuit is the battery negative post when the key is on and the engine is off (KOEO). With the engine running (KOER), the lowest point should be the frame of the alternator.

I have chosen the power feed side connection (B) of the component as my feed side test point for channel 1 of my scope. I typically like to include the entire feed side circuit to the component in question in my testing. Then, if I determine there is either an open connection defect or voltage drop due to unwanted circuit resistance in this side of the circuit, I will begin to work backward from B to A to find the location of the defect.

I have chosen the ground side connection (C) of the component as my ground side test point for channel 2 of my scope. This way I am able to probe the voltage drop across the component as represented by the difference in voltage levels between probe points B, the channel 1 scope voltage reading and probe point C the channel two scope voltage reading. I will also determine the ability of the ground circuit to be completed by the PCM using points C, D and E.

Channel 3 is connected to an inductive current probe in order to measure total circuit current flow.

Use the provided graphic in Figure 1 to study the chart in Figure 2 in order to learn how the type of defect and its location will determine what your scope reads with this probe setup. Once these are understood and not simply memorized, you will be able to predict what effect different probe points will have on your readings of the particular types of circuit defects you might encounter. 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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