What is Komatsu PC400-7 Fault Code E985?

Komatsu PC400-7 fault code E985 indicates a malfunction in the hydraulic pump controller communication circuit, specifically a timeout or loss of data transmission between the engine ECM (Engine Control Module) and the pump controller. This code triggers when the engine controller fails to receive proper feedback signals from the hydraulic pump's electronic control unit within the specified time interval.

The PC400-7 uses an advanced load-sensing hydraulic system where the pump controller continuously communicates engine load, pump displacement, and pressure data to the ECM to optimize fuel efficiency and hydraulic performance. When this communication breaks down, the machine cannot properly match hydraulic output to engine power, compromising both productivity and component protection. This fault is particularly critical because it can cause the excavator to enter a derate mode or prevent the hydraulic system from operating at full capacity.

Common Symptoms

• Hydraulic system operates at reduced power or feels sluggish during digging and lifting operations
• Monitor panel displays E985 code along with a hydraulic system warning light
• Engine may not respond properly to throttle inputs under heavy hydraulic loads
• Intermittent loss of auxiliary hydraulic functions (breaker circuits, thumb controls)
• Machine may enter automatic derate mode, limiting engine RPM to protect components

Potential Causes

The most common technical causes for E985 on used PC400-7 excavators include:

• Corroded or damaged wiring harness connectors between the ECM and pump controller (often found near the hydraulic pump mounting area where vibration and heat cycling occur)
• Failed pump controller module due to internal circuit board degradation or moisture intrusion
• Broken or chafed CAN bus communication wires, particularly at harness routing points near the swing bearing or engine mounts
• Poor ground connections at the pump controller mounting bracket or ECM chassis ground
• ECM software corruption or outdated firmware versions (common in machines with 8,000+ operating hours)
• Voltage supply issues to the pump controller caused by failing alternators or battery connection problems

How to Troubleshoot and Fix Code E985

Step 1: Initial Inspection and Data Verification Connect Komatsu diagnostic software (KOMTRAX or aftermarket PC-based tools) to retrieve active and historical fault codes. Check freeze frame data for engine RPM, hydraulic pressure, and voltage levels when the fault occurred. Perform a visual inspection of the pump controller harness from the ECM connector through to the hydraulic pump mounting area, looking specifically for abraded insulation, oil contamination, or connector corrosion.

Step 2: Electrical Circuit Testing Using a digital multimeter, verify battery voltage (should be 24-28V with engine running) at the pump controller power supply pin. Check ground circuit resistance between the controller case and battery negative—readings above 0.5 ohms indicate poor grounding. Test CAN-H and CAN-L communication wires for continuity and proper termination resistance (should measure approximately 60 ohms between CAN-H and CAN-L with controllers disconnected).

Step 3: Component-Level Diagnosis Disconnect the pump controller connector and inspect for bent pins, moisture, or green corrosion—a common issue on used excavators stored outdoors. Measure resistance on communication circuit pins according to Komatsu specifications (typically 100-150 ohms). If wiring tests pass, substitute a known-good pump controller if available, or send the existing unit for bench testing before purchasing replacements.

Step 4: Used Equipment Specific Checks For older PC400-7 machines, pay special attention to harness rub points where cables contact the main frame near the hydraulic tank and swing motor. Check for previous repair attempts using incorrect wire gauges or non-shielded cable. Inspect the ECM mounting bolts for tightness, as loose mounting can cause intermittent ground faults.

Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Komatsu service manual for your specific machine serial number and software version. If you lack proper diagnostic tools or technical training, contact a certified Komatsu dealer or qualified heavy equipment technician to avoid misdiagnosis and unnecessary component replacement.