Automotive Repair Case Studies (Diagnostic Strategies of Modern Automotive Systems Book 9) by Concepcion Mandy

Author:Concepcion, Mandy
Language: eng
Format: epub
Publisher: www.autodiagnosticsandpublishing.com
Published: 2011-06-19T04:00:00+00:00

Fig 2 – CAM & CRK sensor relationship. The two signals are out of synchronization. This pointed to a possible jumped timing belt.

Fig 3 – Broken key in the timing belt sprocket.

The Distributor Reference signal coming from the ignition module had good amplitude and was not braking up. This signal is the synthesized CRK sensor signal coming from the ignition module. At the same time, we can see that after start-up the EST signal is also good. The By-Pass signal at the bottom of Fig 1 is the switchover by the ECM to computer controlled timing. In most GM ignition systems, after the engine reaches over 400 RPM the ECM takes ignition control from the ignition module. The ECM does this by switching the By-Pass line high. All of the ignition signals were in proper order. So the problem was definitely not an ignition-related fault. But what else could cause the backfire condition and the no start?

The next thing I did was to make a quick scanner PID diagnostic, while cranking the engine. All the PIDs seemed fine with one exception. The MAP sensor was not registering correctly. I had seen enough cranking MAP readings to know that a cranking engine should have at least 5 ’ Hg of vacuum, which would give me a reading of around 3.00 volts or so. This wasn’t the case.

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