A steel processing plant reported that a Hyster S80FT internal combustion forklift would start and run for two minutes, then suddenly die and refuse to restart. The fault code displayed on the dashboard was "E-33," which indicated a throttle position sensor error. However, replacing the throttle sensor had no effect.
The technician connected the Hyster DART diagnostic laptop to the machine's GM 4.3L engine control module (ECM). While the laptop initially connected, the communication dropped out the moment the engine started to sputter. This loss of communication pointed to a wiring fault rather than a sensor failure.
The technician focused on the main wiring harness connecting the dashboard to the engine ECM. A visual inspection revealed a hidden splice connector tucked behind the dash panel. The connector was heavily corroded with green oxidation-likely caused by the humid, steel-dust environment. The corrosion had bridged the CAN High and CAN Low wires, causing data collisions on the network. When the engine reached a certain temperature, the resistance in the corroded connector changed, effectively shorting out the communication network and killing the engine.
After repairing the wiring, the engine still ran rough. A live data stream showed the Manifold Absolute Pressure (MAP) sensor reading a steady 102 kPa (atmospheric pressure) even when the engine was under load. The vacuum hose supplying the MAP sensor was cracked and loose, falling off when the engine vibrated. Without the MAP sensor input, the ECM could not calculate the correct air/fuel mixture, causing the rough running.
The repair involved cutting out the corroded CAN bus splice connector and soldering in new heat-shrink connectors. The cracked vacuum line for the MAP sensor was replaced. The ECM was reset to clear the fault codes. The forklift started immediately and ran smoothly through a rigorous load test, proving that the "throttle sensor" fault was actually a symptom of a dying communication network.
