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Diagnosing ECU Faults/Failures

Protecting the most expensive fuse on the vehicle is an important job.
Thursday, July 26, 2012 - 11:42
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As I mentioned before, the most expensive fuse on a vehicle is an electronic control module of any kind. Use caution when replacing ECUs, because they just don’t happen to burn out. Actuators or other components that have been damaged and are drawing excessive current easily can impair the sensitive circuits inside an ECU. Damage to PCMs commonly is caused by shorted solenoids. Use proper diagnostic procedures to test all of the solenoids before replacing a PCM, because the shorted one left behind will draw current and burn out the new one.

Also be sure to follow the published service procedures when disconnecting or replacing an ECU or related component to avoid accidental voltage spikes that can lead to future failure of the circuit board. Among the more common mistakes technicians make is unplugging ECUs while they are powered up and not discharging the static electricity from their bodies when handling sensitive electronic modules.

A Case in Point
Let’s apply a little of what we’ve learned so far. This example involves a 2007 Saturn ION that came into the shop with an evaporative emissions system problem. After scanning the PCM and coming up with the P0411, I looked up the information on possible causes of this Diagnostic Trouble Code (DTC). Some of the problems that would cause this DTC to be detected can be as simple as a disconnected air supply hose that goes to the engine, a blown fuse for the secondary air pump or a defective air pump relay. The worst possible scenario, however, would be that the PCM circuit that controls the pump was burnt out.

The first thing to do was to perform a visual inspection to make sure everything is connected and to look for any obvious problems. I noticed someone had been poking holes in the wires. Before jumping into removing wires and checking for voltage and ground, I used the Tech 2 bi-directional control to see if the air pump would work, but it did not. Remember always start with the easiest test first before moving on to a harder pinpoint test. Next, I tested the fuse and relay. Both passed with flying colors. The motor/solenoid was another story.

It didn’t take too long to isolate a failed driver in the PCM as the base cause of the P0411 code. But what caused the driver to fail? Was it age? Vibration? Or was there a second culprit yet to be discovered?

I decided to test the motor for amperage draw. According to the specifications listed in the service information, the solenoid should have 4 to 7 ohms and draw 2 to 3.5 amps. That’s higher than the 1.2 amps specification I gave you, isn’t it? This circuit uses a relay, just like a fuel pump circuit does, and often the relay contains a diode to protect the ECU driver from voltage spikes that normally result anytime current to a coil is shut off. While the relay looked like it had been recently replaced, I wasn’t sure it was the correct one for the application. That went on the list of suspects I’d check into later.

I like to use amperage to make sure that the solenoid does not exceed what the PCM circuit can handle. I used a scope and a current clamp to capture the current waveform. The solenoid was pulling about 6 amps! That is way over the limit and I suspect that that was the reason for the replacement relay.

The fix for this secondary air injection code was to repair and seal the wires, replace the air pump solenoid and PCM. This was one expensive repair that could have resulted in an expensive comeback if I did not test the AIR solenoid and had just replaced the PCM. Remember, that’s one expensive fuse.

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