This document is a CAT C15 retarder troubleshooting procedure that provides step-by-step diagnostic instructions for compression brake solenoid circuit faults on C15 on-highway truck engines (MXS configuration). It covers diagnostic trouble codes 0121-05 and 0121-06 (Low/High Retarder current low and high), as well as 0122-05 and 0122-06 (Med/High Retarder current low and high), with detailed wiring checks, short and open circuit tests, and ECM connector pin identification.
Official CAT C15 retarder troubleshooting 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
- 0121-05 Low/High Retarder Current Low (14) — Conditions, System Response
- Retarder (Compression Brake) Solenoid Circuit — Test: System Operation Description
- Customer Specified Parameters Affecting Retarder Operation
- Retarder Brake Test via Caterpillar Electronic Technician (Cat ET)
- Schematic for C15 Engines (Illustration 1)
- P1 and P2 ECM Connector Pin Identification (Illustrations 2–3)
- Valve Cover Connectors for C15 Engines (Illustration 4)
- 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 5: Create an Open at the Valve Cover Connector
- 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
- 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 procedure applies to Caterpillar C15 on-highway truck engines in the MXS configuration (serial numbers MXS00001 and up). The retarder solenoids are factory installed by Caterpillar, while switches are installed by the vehicle OEM. Trucks equipped with SAE J1939 systems or Anti-Lock Brake Systems (ABS) may include additional relays in the retarder switch or clutch switch circuits. Vehicles with Eaton Top 2 transmission programming may also experience inhibited retarder operation during shifts. For official Caterpillar parts and service resources, visit Caterpillar Support.
Who needs this CAT C15 retarder troubleshooting manual
Fleet mechanics and independent workshop technicians diagnosing compression brake failures, check engine lamp illumination related to retarder DTCs, or intermittent retarder activation issues on C15-powered trucks will find every required test sequence, connector pinout, and wiring schematic in this document.
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