What is Caterpillar Fault Code SPN1735?

SPN 1735 indicates a problem with the Diesel Exhaust Fluid (DEF) Pump Motor Circuit, specifically detecting an abnormal electrical condition in the pump's control system. This Suspect Parameter Number monitors the electrical integrity and operational status of the DEF pump motor that delivers fluid to the Selective Catalytic Reduction (SCR) aftertreatment system.

This fault code directly affects your Caterpillar excavator's emissions control system. The DEF pump is critical for injecting the proper amount of diesel exhaust fluid into the exhaust stream, where it breaks down harmful nitrogen oxides (NOx). When SPN 1735 appears, the Engine Control Module (ECM) has detected voltage irregularities, open circuits, short circuits, or mechanical failures within the DEF pump motor circuit. For used excavators, this is particularly important because DEF system components are prone to crystallization, corrosion, and electrical degradation over time.

Common Symptoms

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the dashboard
• Engine derate or power reduction after a predetermined time period (typically warning before derate occurs)
• DEF system warning messages displayed on the monitor indicating SCR system malfunction
• Possible speed limiting to 5 mph after continued operation with active fault (inducement strategy)
• Poor fuel economy and increased exhaust emissions due to non-functioning SCR system

Potential Causes

The most common technical causes for SPN 1735 on used Caterpillar excavators include:

• DEF pump motor failure due to internal winding shorts, bearing seizure, or crystallized DEF contamination
• Wiring harness damage from abrasion against frame components, particularly near the DEF tank mounting area where harnesses experience vibration
• Corroded or loose electrical connectors at the DEF pump, especially on machines exposed to road salt or coastal environments
• ECM power supply issues including blown fuses, damaged relay contacts, or poor ground connections
• DEF quality problems causing pump crystallization and increased electrical draw leading to circuit protection activation
• Failed DEF pump control module or internal driver circuit within the ECM itself

How to Troubleshoot and Fix Code SPN 1735

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) or equivalent diagnostic software to read active and logged fault codes. Check for multiple simultaneous codes that might indicate broader electrical issues. Record freeze frame data showing voltage levels and operating conditions when the fault occurred.

Step 2: Perform Visual Inspection Thoroughly inspect the DEF pump wiring harness from the pump to the ECM connector. On used excavators, pay special attention to harness routing near the DEF tank and frame rails where wire insulation commonly wears through. Check all electrical connectors for corrosion (white/green deposits), bent pins, or moisture intrusion. Inspect the DEF pump itself for crystallized fluid buildup around seals and connections.

Step 3: Electrical Testing Using a digital multimeter, disconnect the DEF pump connector and measure resistance across the pump motor terminals (typically 2-6 ohms depending on model). Check for voltage supply at the pump connector with ignition on (should show battery voltage). Verify ground circuit integrity with less than 0.1 ohms resistance to chassis ground.

Step 4: Pump Motor Function Test If electrical values are correct, perform a DEF pump prime test using Cat ET software to activate the pump directly. Listen for normal pump operation (smooth humming sound). Erratic operation, grinding noises, or no operation indicates internal pump failure. For used machines, pump motors often fail due to DEF crystallization from extended periods of non-operation.

Step 5: Repair and Verification Replace damaged wiring harnesses, clean corroded connectors with electrical contact cleaner, or replace the DEF pump assembly as needed. After repairs, clear fault codes, perform a DEF system prime cycle, and road test the machine under load to verify proper SCR operation and confirm no code reoccurrence.

---

Disclaimer: This guide provides general diagnostic information for SPN 1735 on Caterpillar excavators. Always consult the specific service manual for your machine model and serial number. If you're uncomfortable performing electrical diagnostics, consult a certified Caterpillar technician to prevent further damage to emissions control components.