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Communication breakdown with a PCM

Wednesday, July 1, 2009 - 00:00
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There is, however, one more test that should be performed before pulling the trigger on that non-communicating PCM, and that is checking the sensor reference voltage output.

Doing this reference voltage test requires measuring for both proper voltage level and current draw on the same wire at the same time as shown in Figure 1. We need both current and voltage to be measured to first determine whether the lack of 5V reference is due to the PCM not being able to output the proper voltage because of an internal defect. In this scenario, the voltage will be low and very little current will be flowing in that reference wire. On the other hand, if the proper voltage output from the PCM is being pulled low due to a shorted sensor or harness defect, the voltage still will be low, but too much current will be flowing in the wire. The current flow level in the wire is the key in determining which of these two defects is the true cause of the problem.

How much current draw should you expect? On the defect-free systems I have tested using an inductive current probe, they have all come out to less than 100mA (milliamps or 0.100A) of current flow. This is barely enough for the current probe to measure accurately, so not much current at all.

Figure 2 is showing the voltage and current from a 2001 Dodge Dakota with a no start complaint due to a non-communicating PCM. This vehicle had a shorted cam position sensor (CMP).

As soon as we turned the key on, the PCM was never able to provide its required 5V reference. The current on the wire jumped from 0A to 2.25A! During the next 13 seconds, the PCM's internal protection circuitry ramped the output current down to 400mA, which is still much too high compared to the less than 100mA on a normal vehicle. Even with 2.25A of current flowing, the PCM could not get the voltage level above 1V, let alone up to a normal 5V, due to the shorted CMP.

Once you have determined that the defect is outside the PCM, you will need to locate the cause of the shorted reference line. Start by having your scanner plugged in and then start unplugging sensors, one at a time. Cycle the key off for 30 seconds between checks to let the PCM reset.

Then turn the key back on and see if the PCM now communicates with the scanner. Repeat this Key off/Key on procedure for each disconnected sensor.

I cycle the key off rather than just leave the key on and watch the current level, because I do not like to leave the key on for extended periods with an excess current draw on the PCM. I have never seen a PCM damaged by a shorted sensor, but I do not want to see one either.

In Figure 3, you can see where I unplugged the offending CMP sensor, and the reference voltage immediately jumped up to a normal 5V. At the same time, the current dropped to almost nothing. The PCM now had communication and would start and run.

Of course, if none of the sensors themselves are shorted, you might have to locate a short in the harness leading to them.

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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