by Jake Schell
Sometimes it’s necessary to slow down and be mindful to basic details needed for diagnosis.
A truck equipped with an automated transmission was brought into the shop with a transmission code associated with the clutch’s failure to fully release.
Using a scan tool, the technician attempted to run the clutch apply leak test, which yielded an inconclusive result. Assuming there was a problem with the clutch, the tech proceeded to remove the transmission to conduct clutch component inspections.
However, to the tech’s surprise, all clutch measurements were within specification, indicating that there were no mechanical faults. Now, frustrated and more than a little puzzled, the tech reinstalled the transmission and restarted the testing procedure since the code quickly reset.
Slowing down, taking a deep breath and starting the testing procedure from the beginning, the technician discovered a crucial step in the procedure that had been overlooked. It stated that before any other testing could be carried out, the air pressure to the transmission needed to be verified at more than 100 PSI.
his preliminary pressure test would ensure that the necessary supporting elements for proper operation were in place. Unfortunately, this step had not been performed initially. This resulted in the inconclusive scan tool test because there was insufficient air pressure to release the clutch. Additional testing revealed a faulty air governor, which was promptly replaced. The transmission then worked properly, and the code did not reset.
By skipping the preliminary test step of checking for air pressure, the shop lost an entire day’s worth of productivity. Many diagnostic tests contain preliminary testing to ensure the system in question has all the inputs needed for the system to work properly.
These preliminary steps may get skipped for various reasons:
EDITOR’S NOTE: Check out Jake Schell’s complete line-up of monthly columns in Fleet Equipment Magazine at https://www.fleetequipmentmag.com/author/jake-schell/.
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About the Author
Jake Schell
Jake Schell is the Associate Product Manager for Mitchell 1’s commercial vehicle product line. He has been with Mitchell 1 since 2001. Prior to coming to Mitchell 1, Jake spent 20 years as a technician, with Chevrolet Master certification in the transmission category as well as ASE certifications in both car and truck.
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