This 17-page PDF is a C15 retarder troubleshooting DTCs guide for diagnostic code 0121-06 (Low/High Retarder current high). It covers the compression brake solenoid circuit test procedure for C15 On-Highway Truck engines (MXS configuration), detailing step-by-step electrical diagnostics using Caterpillar Electronic Technician (Cat ET) software.
Official C15 retarder troubleshooting DTCs document details
| Brand | Caterpillar (CAT) |
| Model coverage | C15 Truck Engine MXS (C15 On-Highway Truck MXS00001-UP) |
| System/Group | Engine Retarder (Compression Brake) Solenoid Circuit — SMCS 1129-038 |
| Publication no. | RENR5088-10 |
| Year/Revision | Published 01/02/2009, Updated 25/02/2009 |
| Pages | 17 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- 0121-06 Low/High Retarder current high (14) — Conditions Which Generate This Code, System Response
- Retarder (Compression Brake) Solenoid Circuit – Test — System Operation Description
- Retarder Brake Test via Cat ET — conditions and procedure
- Schematic for C15 engines (Illustration 1) — P1 and P2 ECM connector pinouts
- Valve cover connectors for C15 engines (Illustration 4) — terminal identification
- Test Step 1: Inspect Electrical Connectors and Wiring
- Test Step 3: Create a Short Circuit at the Valve Cover Connector
- Test Step 4: Create a Short Circuit at the ECM
- Test Step 6: Create an Open at the ECM
- Test Step 7: Create a Short Circuit at the Engine Retarder Solenoid
- Test Step 8: Create an Open Circuit at the Engine Retarder Solenoid
- Test Step 10: Check the Retarder Switch Circuit at the Firewall Bulkhead Connector
- Test Step 11: Check the Operating Parameters of the Engine Retarder
- Test Step 12: Check the Accelerator Pedal Position Sensor
- Test Step 13: Check the Clutch Switch
Applicable truck models
This document applies to the Caterpillar C15 On-Highway Truck Engine, specifically the MXS serial number prefix (MXS00001-UP). The procedure also references related DTCs 121-05, 122-05, and 122-06 covering both the Low/High and Med/High retarder solenoid circuits. Trucks equipped with SAE J1939 systems and Anti-Lock Brake Systems (ABS) that interact with retarder control are addressed. The guide notes compatibility with Eaton Top 2 transmission programming. For official Caterpillar parts and service resources, visit Caterpillar Support.
Who needs this C15 retarder troubleshooting DTCs manual
Diesel mechanics and workshop owners diagnosing retarder malfunction on C15-powered trucks need this document. It provides the complete ECM connector pinouts (P1 and P2), valve cover terminal mapping, wiring schematics, and the structured fault-isolation sequence using Cat ET’s Retarder Brake Test to distinguish between open and short circuit faults in the solenoid harness.
The full PDF is available in the DiagTechPro manuals library, free account required to download.
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