What is Caterpillar 735 Fault Code E328(3)?

Fault Code E328(3) indicates a Transmission Output Speed Sensor Circuit Intermittent Signal on the Caterpillar 735 Articulated Truck. This code specifically identifies an unstable or erratic electrical signal from the output speed sensor to the Transmission Electronic Control Module (TECM), which monitors the rotational speed of the transmission output shaft.

This sensor is critical for the TECM to calculate ground speed, determine optimal shift points, and manage torque converter lockup strategies. When the signal becomes intermittent, the transmission control system cannot accurately assess drivetrain performance, potentially causing harsh shifting, incorrect gear selection, or activation of transmission protection modes. For used Caterpillar 735 machines, this code often surfaces due to accumulated wear on wiring harnesses and connectors exposed to debris, moisture, and vibration over thousands of operating hours.

Common Symptoms

• Transmission Warning Lamp illuminated on the operator display with code E328(3) stored in fault memory
• Erratic or delayed shifting between gears, particularly noticeable during acceleration or deceleration
• Speedometer fluctuations or complete dropout of speed readings on the instrument cluster
• Transmission derate mode activation, limiting vehicle speed to protect drivetrain components
• Intermittent loss of torque converter lockup, resulting in reduced fuel efficiency and increased transmission temperatures

Potential Causes

The most common technical causes for E328(3) on used Caterpillar 735 machines include:

• Damaged or corroded wiring harness at the transmission output speed sensor connector, especially where the harness routes near the driveline (a known wear point on older units)
• Faulty output speed sensor with internal winding degradation or bearing wear causing inconsistent signal generation
• Loose or corroded connector pins at the sensor or TECM connection points due to moisture intrusion
• Damaged sensor tone ring (reluctor wheel) on the transmission output shaft with missing teeth or metallic debris accumulation
• Intermittent ground connection in the sensor circuit caused by frame corrosion or loose chassis ground points
• TECM internal fault (less common) affecting the signal processing circuitry for the speed sensor input

How to Troubleshoot and Fix Code E328(3)

Step 1: Visual Inspection and Connector Check Begin by locating the transmission output speed sensor on the rear section of the transmission housing. Disconnect the sensor connector and inspect for corrosion, bent pins, moisture, or physical damage. On used excavators, pay special attention to harness routing where it may contact the driveshaft or frame—look for worn insulation or chafing. Clean connectors with electrical contact cleaner and apply dielectric grease before reconnection.

Step 2: Sensor Resistance and Signal Testing Using a digital multimeter (DMM), measure the sensor's resistance across its terminals (typically 800-1400 ohms for inductive sensors, but verify with Caterpillar ET diagnostic software for exact specifications). Check the wiring harness continuity from sensor to TECM for opens or shorts to ground. With the engine running and transmission engaged, use Caterpillar ET to monitor live sensor signal frequency—it should increase proportionally with output shaft speed without dropouts.

Step 3: Tone Ring and Mounting Inspection If electrical checks pass, inspect the sensor air gap (should be 0.5-1.5mm typically) and examine the tone ring for damage, debris buildup, or missing teeth. Metallic particles from transmission wear can accumulate on the tone ring in used machines, causing erratic signals. Clean thoroughly with a non-metallic brush and verify proper sensor mounting torque.

Step 4: Component Replacement and Verification If intermittent issues persist, replace the output speed sensor with a genuine Caterpillar part, as aftermarket sensors often have quality issues. After replacement, clear fault codes using Cat ET, perform a test drive through all gear ranges, and monitor for code recurrence. Check for proper TECM software calibration updates that address known sensor circuit issues.

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Disclaimer: This guide provides general troubleshooting information for Caterpillar 735 fault code E328(3). Always consult the official Caterpillar service manual and consider professional diagnosis by a certified technician, especially for complex electrical diagnostics or transmission repairs on used heavy equipment.