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Restoring Engine-Computer Communications

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For this month’s Real World case, we will attempt to provide a plan of attack for communication issues on General Motors products with the Class 2 Protocol.

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0-Opening2004_chevrolet_tahoe

 

 

 

Our diagnostic journey begins with a 2004 Chevrolet Tahoe.

_Fig-1

Figure 1

 

This vehicle was tested at the EPA test facility in our area and it was determined that there is no communication with the PCM.

 

Our subject vehicle is taken to a local repair facility to be evaluated. The first step there is to confirm the no communication issue.

 

The technician uses a Tech 2 scan tool to access data and the scan tool is able to communicate with the Tahoe without a problem.

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So, he calls the EPA test facility to report that the vehicle does not have a communication problem.

 

The state responds in a very pleasant manner and advises the tech that the vehicle must communicate with their equipment in order to pass the emissions test.

_Fig-2

Figure 2

 

It’s interesting to note at this time that the state communicates on a generic level.

 

The tech takes out his generic scan tool and finds that the vehicle does not communicate with it. The problem has been confirmed.

 

The first step in our ­diagnosis is to review schematics for the communication lines. Figure 1 denotes the circuitry.

 

The PCM is denoted in Figure 2.

 

Now we have a complete picture of what our testing will entail.

 

A comb device will be removed out of the splice pack to take all of the modules on the schematic offline momentarily.

See Figure 3.

 

_Fig-3

Figure 3

The modules will then be placed back online one by one.

 

A lab scope is placed on the data line to view the quality of the signal.

A jumper wire will be used to place each module back online one by one to view the serial data quality for each one. The serial data line is a 0 to 7 volt pulse, which is pulse width modulated.

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Figure 4 is an example of a known-good pattern for your review.

 

The jumper wire was used to bring each module back online one by one, a pattern showing 0-7 volts was seen by all of the modules except one.

 

If your guess was the PCM, you are incorrect!

 

An example of the bad pattern is shown in Figure 5 for your review and analysis.

 

_Fig-4

Figure 4

This pattern showed a range of 0 to 5.8 volts, this was below the threshold needed in order for proper communication to take place on this vehicle.

 

The bad pattern occurred on the circuitry for the SDM or ­sensing and diagnostic module (airbag module). The airbag module circuitry was loading the circuit to this value, causing the PCM not to communicate with the generic scan tool.

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The pin for the airbag ­module was taken off the bus and generic communication was restored.

 

The vehicle then passed the ­emissions test, and the repair shop was advised to repair the airbag circuit.

This Pulling Codes case is now closed.

Note: If you’re interested in reviewing websites that provide examples of known-good waveform libraries on a variety vehicles and vehicle systems, you can ­contact me at [email protected] and I will provide you additional information.

 – By Carlton Banks, contributor, Tech Shop magazine

_Fig-5

Figure 5

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