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Through the eyes of a scope

A simple tool can make complex diagnostics easier to solve.
Tuesday, May 10, 2016 - 07:00
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Vehicle diagnostics can be quite simple or very complex depending on the problem you're  faced with. Any tech knows that the first item to check when a customer says their vehicle pulls to the left is tire pressure — simple, right? When a customer states their car’s engine stalls once or twice a week, the problem and solution can be much more complicated. The focus of this article will be on complex problem diagnostics using multi-trace lab scopes. I have been asked to solve several tough, intermittent problems and the key to finding the source of the concern is data collection and analysis.

Figure 1
Figure 2

When diagnosing a problem such as a very intermittent engine stall the choices for a technician fall into two categories, replace the parts you guess may be responsible or connect equipment and operate the vehicle to capture the event and then analyze the captured data. If you decide to start replacing parts, there are several items to consider. You may perform some research to determine what common problems occur on the vehicle in question and replace these items, but if the problem remains the customer will probably not be happy and may ask for a refund or to have the old parts put back on. Guess work is not what most consumers want from the professional technicians they hire to repair their car. When confronted with an intermittent diagnostic problem, a solid plan is necessary along with the right test equipment.

A problematic Impala
I will use a 2003 Chevrolet Impala with a 3.4-liter V\6 to illustrate the point of this article. This Impala has a very intermittent stall and no start problem. The dealer has looked at the car and said nothing seems wrong, but they were unable to get the problem to occur so no diagnosis can be made. When you choose to take on a problem like this you must clearly inform the customer of the challenges and costs involved along with the time needed to resolve their issue. You certainly would not want to connect a scope and scan tool and then have the customer call and say they need the car now to pick up the kids. Make sure up front you have the time you need to find the problem and fix the car.

The next item you must know is system operation and layout so you can connect test equipment to gather the data you need. Connecting a multi-channel scope takes some thought so you have the best chance to capture the data you need when the problem occurs. This vehicle will be tested with a Pico 4000 series 8-channel scope. While not a common tool in most shops I need to make an important point here, the more information you have when making a decision allows you to make a better decision. When scan testing an engine for a fuel trim code, would you want to look at two2 data items or 1212 items? Can you make a diagnosis with engine RPM and oxygen sensor voltage alone, or do you need STFT, LTFT, Mass Airflow, load and other important data to make a correct diagnosis? The same holds true with scopes, the more channels you have, the more circuits you can test. The problem is you may only get one crack at capturing data if you can get the problem to occur, so the more data you have the greater the chance to make a correct diagnosis. An otherwise normal engine can stall or not start due to a lack of spark or fuel, so the circuits responsible for the control of these systems should be tested.

After careful review of the engine wiring diagram I decide to connect the scope to the following circuits. Channel A is connected to the battery feed to the ignition module. There are 3 rotation sensors on this engine, channel B is connected to the 7X CKP signal at the ignition module. Channel C is connected to PCM terminal 8, the 3X reference input from the ICM. Channel D is connected to PCM terminal 7, the cam sensor input. Channel E is connected to PCM pin 9, CKP 24X sensor input and channel F is connected to PCM pin 79 which is injector #6 control or ground. Channel G is connected to PCM pin 19, switched ignition feed to the PCM and channel H has a low amp current probe connected to the DFI fuse in the under hood fuse box. This fuse powers both the ignition coils primary circuit and the ICM. These signals should allow me to determine what was lost if the engine stalls or does not start. The scope is set up with channel A at the bottom and the remaining channels stacked on top of one another with channel H on top.

Figure 1 shows some of the scope connections at the PCM located inside the air cleaner housing.

Figure 2 shows a capture of the engine running, then turning off the key and the engine stops. I always capture a normal key off event to compare to an engine stall. After four days of extensive test driving the Impala never misses a beat and I’m somewhat annoyed at the growing time involved in this job. On the fifth morning, when attempting to pull the car out of the bay, a cranking no-start occurs.

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