Diagnosing ECU Faults/Failures

Jan. 1, 2020
The most expensive fuses on the vehicle are any one of the several dozen computers found on a modern automobile. What happens when they replace the Electronic Control Unit (ECU) and the problem doesn’t go away?
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 d
o 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.

I bet you are wondering how that’s going to help you on a BMW or any other vehicle. Don’t worry; I have procedures that will allow you to check any computer system. Now we are ready to move on and do just that.

Checking for Excessive Current Draw Actuators can be your worst nightmare when it comes to computers that no longer function as designed. When an actuator draws too much current (amps), it will burn the driver open, damaging the PCM. Never just change out a computer with a new one without checking for excessive current draw from the components the ECU operates. This includes solenoids, valves, fuel injectors and ignition coils. Any component that draws current through the ECU is suspect until tested.

It’s not as hard as you think. Just follow these simple steps. First, consult a wiring diagram for the vehicle’s computer and related wiring harness connectors. Most service information systems will also have pinout diagrams telling you exactly which connector cavity goes to what component.

Next, get your Digital Multimeter (DMM) out and set it up to test for current using the highest range your meter offers. Because most digital multimeters can measure only up to 10 amps or so, it always is a good idea to use a circuit breaker or fuse inline with one of the test leads as an added precaution. It’s cheaper than a new meter.

Then test each component by using your meter as the driver. If the module uses a driver that completes the path to ground, for example, put the meter’s negative lead on the battery ground post and momentarily touch the meter’s positive lead to the proper terminal of the module’s harness connector. (If you get no current reading, make sure there is power to the component. You might have to turn the key on.) If the driver supplies power, simply move the meter’s negative lead over to the positive battery post.

CAUTION! Apply the current briefly (only a second or two) to avoid causing a problem that didn’t exist before you got there. Leaving the direct connection too long can burn out the component you’re testing.

Finally, record the reading. A current draw in excess of 1.2 amps is an indication of a faulty circuit or component.Testing with an Amp Clamp You can use a meter and an amp clamp to test component circuit draw. You’ll still need to activate the circuit, using either a fused jumper wire or your scan tool’s bi-directional controls. If you own a
PowerProbe, you can use it to apply power or ground as needed, and you can place your amp clamp around its tip to minimize your invasion into the wiring harness.

Sometimes you’ll be testing a circuit where the ECU driver already failed, or the failure is in the primary power or ground circuit within the ECU. If you get no current reading at all using one of the methods already discussed, you might want to test the circuit with the harness disconnected. In that case, you’ll need to supply both power and ground to the component you’re testing.

Perform this type of test with the ignition key off. This is a good way to verify the entire circuit and its connections. If the component still doesn’t work, you’ll need to perform a voltage drop test on the circuit before condemning the computer.

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.

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.

Boosting Your Shop's Bottom Line with an Extended Height Paint Booths

Discover how the investment in an extended-height paint booth is a game-changer for most collision shops with this Free Guide.