Diagnose This! – October 2008

Each month in Diagnose This! we present a real-life service scenario with clues as to what the ultimate diagnosis was. If you think you know the answer, be the first to e-mail dsmith@rrpub.com with the correct response for free Truck Parts & Service merchandise. The answer will be revealed in a future issue of the magazine.

Vehicle: 2006 Freightliner M2

Key components: Caterpillar C7 engine, Allison WTECIII transmission

Scenario: While making local deliveries, this vehicle suddenly has a flashing “Check Engine” light.


  • No driveability issues exist.
  • The truck was scanned with CAT ET and no active codes were found.
  • An inactive vehicle speed sensor fault (code 84-1) logged 110 engine hours earlier. After the code is cleared, it doesn’t return. However, the lamp continues flashing.
  • A logged event code exists for loss of a J1939 device (code 231-12) which occurred six hours earlier.
  • All J1939 modules on the data bus are responding.

The lamp remains flashing. Why?

Past Puzzlers
Also this month, we reveal answers for the past few months of Diagnose This!

Problem: A 2000 Autocar ACL64B has a Gus Pech drill apparatus and rear throttle controls. The throttle control uses an SAE J1922 interface and the rear control is a vernier-style potentiometer. The problem is an intermittent runaway engine and/or an idle-only condition. The J1922 throttle interface has limited self-diagnostic ability via an LED display on the control unit.

Solution: A failed remote electronic throttle was the culprit.

Winner: There were no correct submissions.

Problem: Customer complained of a knocking sound that suddenly occurred while his older Ford L8000 was in traffic on the interstate. The knocking sound happens only when the clutch is up. Technicians determined the knocking sound was coming from the transmission, which was removed. After removing the cover off the transmission, the source of the noise became apparent.

Solution: The main shaft has broken gear teeth due to operator error. The operator inadvertently made a range change and put the vehicle into a much lower gear than intended. A shock load resulted when the clutch was re-engaged, breaking the teeth.

Winners: John Shimansky (Power Train, Lodi, N.J.) and Harold Carvey (Weller Truck Parts, Farmington Hills, Mich.).

Problem: A 2002 Peterbilt Model 330 developed a sudden stalling problem accompanied by the “Check Engine” light illuminating. A scan of the electronic control module (ECM) showed fault code 164-11 – injection actuation pressure fault – occurring 76 times during the past two engine hours. Technicians changed the engine oil and the vehicle was retested. The same fault remained.

Solution: The cause was a weak high-pressure oil pump which operates the HEUI injectors. The weak pump lacked the ability to provide full pressure under load, and the code resulted from the ECM’s inability to achieve the desired pressure set-point upon command.

Winner: There were no correct submissions.

Damon Russell has been involved in the truck repair industry all his life. He is ASE Master certified in all 6 categories, holding 46 ASE certificates in all. He has been actively working to improve the trade by working with his local trade school’s advisory boards, judging SkillsUSA competitions, participating in evaluating NATEF program standards and ASE test writing workshops. He is also an active member of iATN

Damon and his wife Kathy own and operate Fleet Guardian LLC in Bridgewater N.J., a small five bay garage serving the needs of local and national commercial fleets of light-, medium- and heavy-duty trucks since 1992.

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