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Diagnosing ECU Faults/Failures

Protecting the most expensive fuse on the vehicle is an important job.
Thursday, July 26, 2012 - 10:42
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The most expensive fuses on the vehicle are any one of the several dozen computers found on a modern automobile. Many times techs come across dead or “no longer functions as designed” control modules, and what do they do? You guessed it. They call the parts store or dealer and order a replacement. Now what happens when they replace the Electronic Control Unit (ECU) and the problem doesn’t go away? That new part must be defective, right? When that second control module fails to fix the problem, it’s no longer the part’s fault. It’s the technician’s.

Today’s electronic modules are built pretty bulletproof. While they still can fail by themselves, more often there is a reason for their demise. The challenge facing techs is not only to verify that the ECU is faulty, but also to identify what killed it in the first place. In my previous example, there likely was nothing wrong with the replacement part. However, whatever killed the original module remained hidden and took out the new module as soon as the key was turned on.

It’s time to test and not guess. In this article, I am going to show you how to test the components that can burn out a computer with tools that you most likely already have. I’ll also share a few tips on making sure that the ECU you’re preparing to replace is, indeed, dead.

Too Much Can be a Bad Thing
The first step when testing any electrical problem is to verify that the component is getting power and ground. The same is true when determining whether an ECU has failed. Check the appropriate schematic and perform a standard voltage drop test to ensure that all power and ground circuits to the module are intact.

The cause of a control module failure often is excessive current passing through one of its drivers. Excess current can be a result of a failed or shorted component, controlled by that driver. You might be asking, “How do I test for shorted components?” No problem. In the rest of the article I am going to provide you with real world examples. We will start with a GM tester to give you a better understanding of the test and procedures that are covered in this article

GM was the first OE to have on its dash a Check Engine Light that alerted the driver and caused the dealer a bunch of headaches. The tool GM developed had you remove the ECM from the wiring harness and plug in the appropriate test board. These boards covered all the GM vehicles from 1980 to 1996. The tools came with plastic laminated cards that provided the information on what the numbers on the tool’s dial were going to be used for.

No. 1 on the dial, for example, might be for fuel injectors. To use the tester, you first flip the switch to On and then flip the Powers and Grounds button. The next step is to select the energize/pulse switch. If you’re testing a fuel injector, you press the red button to see what the current draw for the fuel injector is. The current draw information is displayed on the top screen. If the component draws more than 1.2 amps, the red over current light illuminates. Now you found what burned out the computer.

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