A perplexing issue arose with a 2001 Dodge Laramie: it refuses to start unless an OBD2 scanner is connected to its OBD2 port. This problem surfaced after a recent PCM replacement, initially performed to address intermittent power loss. The truck’s journey to this point involved alternator issues and a subsequent PCM replacement, leading to this unusual dependency on the OBD2 port for starting.
Initially, the Dodge Laramie presented with intermittent power loss. Diagnostic testing revealed a low alternator voltage, registering at a mere 10 volts. Following an alternator replacement, the charging system seemed to stabilize, and the truck performed well during test drives. However, upon the customer retrieving the vehicle, the power loss and stalling recurred en route home.
Upon its return, further investigation uncovered fluctuating voltage issues, oscillating between 14+ volts and below 5 volts. Suspecting CAN Bus issues, modules were systematically disconnected as per Identifix troubleshooting guidance. Disconnecting the Powertrain Control Module (PCM) normalized the voltage, pointing towards a faulty PCM.
A rebuilt PCM from ARES Technologies was procured and installed. This is where the new challenge began: the 2001 Dodge Laramie would crank but not start. Intriguingly, connecting a scanner to the OBD2 port instantly allowed the engine to start and run smoothly. Disconnecting the scanner immediately resulted in the engine stalling. This OBD2 port dependency was consistent.
Grounding points, specifically the PCM housing to firewall strap and the engine ground strap to the frame, were meticulously inspected and confirmed to have good connections. Even supplemental grounding attempts at these points yielded no change – the truck remained reliant on the OBD2 port connection for starting.
Suspecting a faulty rebuilt PCM, the original PCM was reinstalled to isolate the issue. Surprisingly, the problem persisted with the original PCM as well. The truck now exhibited the same behavior with both the old and the new PCM, a symptom not present before the initial PCM replacement. Crucially, communication with the PCM via the CAN Bus is currently absent.
This situation presents a significant diagnostic challenge. The 2001 Dodge Laramie’s reliance on the OBD2 port for starting, post-PCM replacement, is an anomaly that requires further investigation into potential wiring issues, PCM compatibility, or deeper CAN Bus communication faults related to the OBD2 port circuit. Further diagnostic steps are needed to resolve this no-start condition and restore the vehicle to normal operation, independent of OBD2 port connectivity.
