Scope & Scan - Service Repair

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A final look at advanced circuit diagnostics

Sunday, November 1, 2009 - 00:00
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For part three of the series on advanced circuit diagnostics, we are going to apply theory to an actual defect caught using a two-channel scope. The test set-up shown in Figure 1 shows channel 1 is a voltage probe attached to the PCM groundside controlled harness wire of a transmission pressure control solenoid. Channel 2 is a current probe placed around the same groundside harness wire.

This pressure control solenoid is a pulse width modulated (PWM) circuit. The circuit is being pulsed on and off constantly at varying frequencies in order to adjust the transmission line pressure. Because we are using channel 1 to probe the PCM controlled groundside of this circuit, we expect to see system voltage and no current flow when the solenoid is pulsed off. When the solenoid is pulsed on, we should see nearly zero volts and current flowing.

If you studied the first two articles in this series, you should be able to diagnose which side of this circuit in Figure 2 is defective. Before reading on, spend a few minutes analyzing the capture. Make a diagnosis, and more importantly, record why you came to that conclusion.

We will now analyze each characteristic of the waveforms shown in the horizontal areas marked out by the black letters in Figure 2.

At point A, the circuit is off. We have 12 volts coming from the system voltage feed, through our solenoid windings and up to the groundside connection. No current is flowing to ground at this time.

At point B, the PCM grounds the circuit, which causes the voltage to drop across the solenoid and current to begin to flow in the circuit. Because a solenoid coil creates a magnetic field, which resists current flow momentarily, the current curves or ramps up slowly from points B to C. This is normal.

Between points C and D, the current topples downward, yet the circuit does not appear to turn off. If the circuit had been turned off by the PCM, then the voltage shown on channel 1 should have returned to system voltage between C and D; it did not.

Between points D and E, the current drops to zero amps on channel 2, and still the voltage stays low on channel 1. If the circuit turned off or if we lost the ground path because of a harness or connector defect, then the voltage should have gone up to system voltage on channel 1. It did not. Because the voltage is low on the ground control side of the solenoid and no current is flowing through the solenoid, we must conclude that we lost the power feed up to our voltage probe point on channel 1.

Between points E and F, the current stays low, yet at point F the voltage goes high. This is what we should see when the PCM turns off the solenoid by opening the ground circuit.

Between points F, G and H, we see the voltage drop low and high, yet there never is any current flow. If the voltage drops low on a groundside controlled circuit, yet there is no current flowing through this wire, this again is an indication that we are losing the power feed through the solenoid. These drops between F, G and H are irregular compared to the pulse width square wave shape of the high and low points between point F and H, points L and N and points P and Q. This is further indication that this is not normal control of a pulse width modulated circuit.

Between points H and L, we see the voltage staying low and current flowing most, but not all of the time. We do have more current flow time than we did between points B and F, however. This is the reason we see the coil induced voltage spike at point L.

If we can get enough current for a long enough period of time to flow through an inductor (coil), we will store enough energy as a magnetic field to cause an inductive spike when the circuit turns off and the magnetic field collapses back on the coil.

I mention this inductive spike because we can use the lack of a spike to indicate either a lack of current flow through the coil or a defective coil that will not store enough energy in the magnetic field. In our case the lack of spike at points F and P are due to insufficient current flow. Indeed, this vehicle had a bad power feed connection to the pressure control solenoid that would only act up when shifted into gear.

Be sure you have the theory from parts one, two and three down pat. You can find all three of these stories at SearchAutoParts.com. No matter if you are analyzing current vehicle technology or systems yet to come, rest assured, you will use these skills for the rest of your career.

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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