Caterpillar 385C MHPU Fault Code 39:1657-8: Complete Diagnostic Guide

What is Caterpillar 385C MHPU Fault Code 39:1657-8?

Fault Code 39:1657-8 indicates an "Abnormal Frequency" or "Abnormal Update Rate" issue with the Engine Control Module (ECM) communication network, specifically related to CAN (Controller Area Network) messaging from the hydraulic control system. This code appears when the ECM detects irregular data transmission patterns from hydraulic pressure sensors or control modules that don't align with Caterpillar's programmed communication protocols.

This fault is critical for the 385C MHPU (Material Handler/Purpose Unit) because the machine relies on precise coordination between the engine management system and hydraulic control circuits to maintain optimal performance. When communication timing falls outside acceptable parameters, the ECM cannot properly regulate fuel delivery, hydraulic flow, or implement coordinated machine functions, potentially causing operational inefficiencies or protective derating.

Common Symptoms

• Intermittent engine derating or sudden loss of hydraulic power during operation
• Yellow warning light illuminated on the instrument cluster with diagnostic code displayed
• Erratic hydraulic response or delayed implement movement, particularly during combined functions
• Engine RPM fluctuations when hydraulic demands change rapidly
• Communication error messages appearing on the monitor display, sometimes accompanied by other related network codes

Potential Causes

The most common technical causes for Code 39:1657-8 on used Caterpillar 385C MHPU excavators include:

• Corroded or damaged CAN bus connectors at the ECM, hydraulic controller, or junction points—especially common in material handling applications with exposure to metal debris and moisture
• Harness chafing along the main boom where wiring bundles flex repeatedly, causing intermittent short circuits or open connections
• Failing hydraulic pressure sensors (particularly the main control pressure sensor) sending irregular signal patterns due to internal component degradation
• ECM software corruption or outdated calibration files incompatible with hydraulic system updates
• Poor ground connections at chassis grounding points, creating voltage fluctuations that disrupt CAN communication
• Water intrusion in sealed connectors, particularly common in older machines with compromised environmental seals

How to Troubleshoot and Fix Code 39:1657-8

Step 1: Connect Caterpillar Electronic Technician (Cat ET) Software Use Cat ET diagnostic software with the appropriate communication adapter to retrieve active and logged codes. Check the frequency counter and occurrence timestamps to determine if the fault is continuous or intermittent. Review snapshot data to identify which specific module is reporting communication errors.

Step 2: Inspect CAN Bus Harness and Connectors Physically trace the CAN bus wiring from the ECM through the chassis harness to the hydraulic control module. On used excavators, pay special attention to wear points where harnesses contact the frame near the swing bearing and along the boom. Check all Deutsch connectors for:

• Corrosion on pins (clean with electrical contact cleaner)
• Bent or backed-out terminals
• Proper locking mechanisms engaged
• Moisture inside connector bodies

Step 3: Test CAN Bus Resistance and Voltage Using a digital multimeter, measure resistance across the CAN High and CAN Low terminals at the ECM connector (with harness disconnected). You should read approximately 60 ohms, indicating proper termination resistors. With the key on and harness connected, measure voltage between CAN High and CAN Low—expect approximately 2.5V differential at idle state.

Step 4: Inspect Hydraulic Pressure Sensors Check the main hydraulic control pressure sensor connector for oil contamination or corrosion. Test sensor output voltage while operating hydraulics—voltage should change smoothly without erratic spikes. Replace sensor if readings are unstable or out of specification (typically 0.5-4.5V range).

Step 5: Verify Ground Connections Inspect all chassis ground points related to the ECM and hydraulic controller. On used machines, grounding studs often corrode beneath bolt heads. Remove, wire-brush clean, apply dielectric grease, and reinstall with proper torque specifications.

Step 6: Update ECM Software If all physical inspections pass, use Cat ET to check for available ECM flash file updates. Caterpillar occasionally releases calibration updates addressing communication timing issues discovered in field applications.

Disclaimer: This guide provides general troubleshooting procedures for experienced diesel mechanics. Always consult the official Caterpillar service manual for your specific machine serial number, and consider professional diagnostic assistance for complex electrical issues. Improper repairs can cause additional system damage or safety hazards.