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Scope & Scan: Handy times two — a DMM as bidirectional controls

Tuesday, April 1, 2008 - 00:00

We all wish scan tools had enough bidirectional control commands to cover every output device on a vehicle. But even factory tooling does not cover every output. So how do you handle a lack of output commands from your scan tool?

On components that are simply powered ON at 100 percent duty cycle, you certainly could use fused jumpers to connect the component of concern either to power, to ground or both. This test will let you witness the physical actuation or at least hear an audible telltale that the component moved. But depending on how you break into the circuit and/or connect to the component, you might not be getting all the information you could have gotten with a little forethought and creativity.

When you need to jumper a component to power or ground, it is best to use the actual vehicle harness to do the test. This way you are testing almost the entire system: the component, the harness connectors and the harness itself. It is also best to perform this test "dynamically," meaning under conditions of actual current flow rather than just using an ohmmeter.

Using a Digital Multi Meter (DMM) in Ammeter mode provides a convenient fused jumper with which to test the amount of current flowing through the solenoids, the harness and connectors. Testing dynamically with current will reveal the presence of voltage drops anywhere in the circuit. And if you are providing the ground for the circuit with your DMM as shown in Figure 1, you are at the same time checking the voltage feed to the circuit. Or, if you are providing the power feed side for the circuit using your DMM as shown in Figure 2, you are checking up to the ground point of the circuit.

If either of these tests reveals a low current flow condition, you then can use a traditional straight jumper wire to provide the power or ground as needed and then use your DMM to check for voltage drops and various power and ground side points in the harness.

Use appropriate test leads when back probing the Powertrain Control Module (PCM) connector as shown in Figure 3. Install the test lead into the PCM connector first, then connect your Ammeter to power or ground so any arcing caused by current will not damage the PCM connector by pitting it. It is better to pit your DMM leads.

How much current should the circuit draw? The service manual troubleshooting chart typically has resistance specifications for each solenoid. Using Ohm's Law, a solenoid with 20 ohms resistance will draw about 0.6 amps at 12.5 volts. Also, I have found that the old rule of thumb stating, "any component will pull half the amperage rating of the fuse protecting it," proves to be very accurate.

If you are in the habit of back probing a solenoid circuit to jump it to ground with the harness still hooked up to the PCM, a word of caution is in order. Some European vehicles place a 5-volt potential on the ground-side driver circuit from the PCM while the solenoid is off. If you jump that to ground, you might cause damage or at least blow a 30-amp fuse (experience talking here). Be sure to use your DMM to test for the presence of voltage on the ground wire first. If it has voltage, disconnect the PCM harness and use a separate jumper wire.

Obviously, this DMM testing regimen does not tell you anything about the ability of the PCM to actuate the solenoid circuit. In order to test this function without bidirectional controls on your scan tool, see the August 2006 Motor Age Garage column.

Before you install that new PCM

Take a few minutes to test the environment that expensive new PCM is going to have to spend its useful life in. You should be testing all output solenoid circuits for excess current draw. If the solenoid circuit resistance is too low, the circuit will draw more current than the PCM will allow.

With too high of a current draw, PCM driver circuits typically shut off, but not always. If the current draw is just a little high, over time the PCM's driver circuit will be damaged due to excessive heat generation. I have even seen solenoids melted beyond visual recognition that damaged the PCM driver circuit before the PCM's over-current protection circuit shut down. The same fate can befall a PCM that is subjected to a direct short circuit to ground or a partial "current shunt" type short in the vehicle harness that feeds a solenoid, so it is important to test both the solenoid and harness before installing the PCM.

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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