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 [email protected] with the correct response for free Truck Parts & Service merchandise. The answer will be revealed in a future issue of the magazine.
This month’s Diagnose This! is submitted by ASE master technician and owner of Fleet Guardian, Damon Russell.
Vehicle: 2000 Autocar ACL64B
Key components: Cummins N14 engine, Roadranger transmission
Scenario: Vehicle 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.
Clues:
- With the key off and engine not running the throttle interface flashes an error code for loss of J1922 data.
- With the engine running no codes are present on the J1922 interface or engine ECM.
- When the fault occurs and the engine only idles, scan data shows idle validation toggles correctly with cab throttle movement and scan data shows the correct engine RPM and accelerator position. Rear throttle controls do not appear to change scan data readings.
- When the fault occurs and the engine runs away, surging, rolling intermittent fast-idle or continuous fast-idle occurs. However, it never exceeds the governed RPM and the engine never stalls. Scan data coincides only with cab throttle pedal activity, but the fuel rate and engine RPM data matches actual engine activity.
