C1185

What Does Code C1185 Mean?

DTC C1185 signifies an ABS Power Relay Output Circuit Failure. This code is set by the Electronic Brake Control Module (EBCM), also sometimes referred to as the Anti-lock Brake System (ABS) module or Vehicle Stability Control (VSC) module, when it detects an electrical anomaly within the circuit that controls the ABS power relay’s output. The ABS power relay is a crucial component responsible for supplying high-current battery voltage to the ABS hydraulic control unit, which includes the ABS pump motor and various solenoid valves. When the EBCM attempts to activate the ABS system or perform its self-check, it monitors the voltage, current, and resistance characteristics of this output circuit. If the EBCM detects an open circuit, a short to ground, a short to voltage, or an excessive voltage drop/resistance on the power relay’s output circuit, it interprets this as a fault and logs the C1185 code. This directly impacts the functionality of the ABS system, potentially disabling its ability to prevent wheel lock-up during emergency braking and often affecting integrated stability control systems that rely on ABS components.

Common Symptoms

• ABS Warning Light Illuminated: The most immediate and common symptom is the illumination of the Anti-lock Brake System warning light on the instrument cluster.
• Traction Control (TC) or Stability Control (SCS/VSC/ESP) Warning Light Illuminated: Since stability control systems often rely on the ABS, their warning lights may also illuminate.
• ABS System Non-Functional: The primary safety function of the ABS system, which is to modulate brake pressure to prevent wheel lock-up, will be disabled.
• Potential Firm or Spongy Brake Pedal: While less common, a complete power loss to the ABS unit can sometimes affect brake pedal feel, although the conventional braking system should remain operational.
• No Noticeable Change During Normal Driving: Under regular driving conditions, the vehicle may feel normal, but the critical safety feature of the ABS will be unavailable during hard braking events.

What Causes the Code C1185?

• Faulty ABS Power Relay: The relay itself can fail internally due to worn contacts, an open or shorted coil, or mechanical failure, preventing it from supplying power or creating an abnormal circuit condition.
• Open or Short Circuit in ABS Power Relay Output Wiring: Damage to the electrical wiring between the ABS power relay and the ABS hydraulic control unit (pump motor, solenoid valves) can lead to an open circuit (no power) or a short circuit (unintended path to ground or voltage). This can be caused by chafing, corrosion, or physical damage.
• Corrosion or Loose Connections: Poor electrical contact at the ABS power relay socket, the fuse box terminals, or the main connector to the ABS hydraulic control unit can create high resistance in the circuit, mimicking an open circuit or causing voltage drop.
• Blown Fuse: A fuse protecting the ABS power relay circuit may have blown due to an overcurrent condition, often indicative of a short circuit downstream in the ABS unit or wiring.
• Faulty ABS Hydraulic Control Unit (HCU) or Pump Motor: An internal short circuit or excessive current draw within the ABS pump motor or one of the solenoid valves can overload the relay or its circuit, leading the EBCM to detect an output circuit failure.
• Faulty Electronic Brake Control Module (EBCM): While less frequent, the EBCM itself could have an internal defect that prevents it from correctly monitoring the ABS power relay output circuit or from properly commanding the relay.

How to Diagnose and Troubleshoot

A systematic approach is crucial for diagnosing C1185, starting with basic checks and progressing to more detailed electrical testing:

1. Initial Visual Inspection:
   • Begin by inspecting the vehicle’s fuse boxes (typically under the hood and sometimes inside the cabin) for any blown fuses related to the ABS system. Replace any blown fuses, but be aware that a blown fuse often indicates an underlying short circuit.
   • Visually inspect the ABS power relay and its socket for signs of corrosion, burning, or physical damage.
   • Carefully trace the wiring harness from the ABS power relay to the ABS hydraulic control unit and the EBCM. Look for any signs of chafing, cuts, breaks, or severe corrosion, especially in areas exposed to road debris or moisture.
2. Retrieve DTCs and Freeze Frame Data:
   • Connect an OBD-II scan tool and confirm that C1185 is the primary code. Check for any other related ABS or powertrain diagnostic trouble codes that could provide additional context.
   • Review freeze frame data, which captures sensor readings and operating conditions at the moment the code was set. This can offer clues about when the fault occurred (e.g., vehicle speed, engine RPM).
3. ABS Power Relay Testing:
   • Locate the ABS power relay (consult a wiring diagram for identification and pinout).
   • Using a Digital Multimeter (DMM), test the relay’s coil resistance across terminals 85 and 86. Compare the reading to the manufacturer’s specifications (typically 60-120 ohms). An open circuit (OL) indicates a faulty coil.
   • Apply 12V to the relay’s coil terminals (85 and 86, observing polarity if indicated). Listen for an audible click, indicating the relay is energizing. While energized, use the DMM to check for continuity across the switch contacts (terminals 30 and 87). There should be near zero resistance. If the relay doesn’t click or doesn’t show continuity, replace it.
   • Alternatively, if a known good, identical relay is available (e.g., from the horn or another non-critical circuit), temporarily swap it with the ABS power relay to see if the code clears.
4. Circuit Voltage and Continuity Checks:
   • With the ignition OFF and the ABS power relay removed, identify the main battery voltage (B+) supply terminal (often terminal 30) in the relay socket. Use a DMM to verify battery voltage present at this terminal.
   • Identify the relay’s output terminal (often terminal 87) in the relay socket, which feeds power to the ABS hydraulic unit.
   • Disconnect the main electrical connector at the ABS hydraulic control unit.
   • Perform a continuity test with the DMM between the relay output terminal (87) and the corresponding power input pin at the ABS hydraulic unit connector. Resistance should be very low (near 0 ohms). An open circuit (OL) indicates a break in the wiring.
   • Check for a short to ground on this output circuit. Place the DMM positive lead on the relay output terminal (87) and the negative lead to a known good chassis ground. Resistance should be infinite (OL). A low resistance indicates a short to ground.
   • If accessible, check for a short to voltage on the output circuit by retesting the above step with the ignition ON (but relay still removed) and observing for any voltage reading.
   • Verify the EBCM’s ability to command the relay. With the relay removed and ignition ON, use a DMM or test light on the relay control terminal (often 86) while activating the ABS via a bi-directional scan tool (if available) or observing during key-on self-tests. The EBCM should provide a ground or voltage signal to activate the relay.
5. ABS Pump Motor/Solenoid Resistance Check:
   • If the relay and wiring to the ABS unit test good, disconnect the main connector at the EBCM/ABS hydraulic unit.
   • Consult the vehicle’s service manual for specific resistance values for the ABS pump motor and solenoid valves. Use a DMM to measure resistance across the relevant pins in the ABS unit’s connector. Out-of-spec readings or very low resistance (short) can indicate an internal fault within the ABS unit causing an excessive current draw.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are commonly recommended:

1. Replace Faulty ABS Power Relay: If diagnostic tests confirm the relay is faulty (open coil, failed switch contacts), replace it with an OEM-specified or equivalent quality relay. This is often the most straightforward and cost-effective solution.
2. Repair or Replace Damaged Wiring: If an open circuit, short to ground, or short to voltage is identified in the wiring harness, perform a professional repair. This involves cutting out the damaged section, splicing in new wire of the correct gauge, using solder and heat-shrink tubing for durable connections, and ensuring proper routing to prevent future damage. If damage is extensive or involves multiple wires, replacing the entire harness section may be more practical.
3. Clean or Replace Corroded Connections: If corrosion or poor connections are found at the relay socket, fuse box, or ABS module connector, clean the terminals thoroughly using electrical contact cleaner and a small brush. Apply dielectric grease to protect against future corrosion. Severely corroded or damaged connectors should be replaced.
4. Replace Blown Fuses and Identify Root Cause: If a blown fuse was discovered, replace it with a fuse of the correct amperage. Crucially, the underlying cause for the blown fuse (e.g., a short circuit in the ABS pump motor) must be identified and repaired to prevent immediate re-occurrence.
5. Replace ABS Hydraulic Control Unit (HCU) or Pump Motor Assembly: If all wiring and relay tests pass, and the diagnostic points to an internal short or excessive current draw within the ABS pump motor or solenoid valves, the entire ABS hydraulic control unit and pump motor assembly may need replacement. This is a more complex and expensive repair, typically requiring the hydraulic system to be bled afterwards, and potentially programming of the new module.
6. Replace Electronic Brake Control Module (EBCM): As a last resort, if all other components and wiring have been verified as good, and the EBCM fails to properly command or monitor the ABS relay circuit, the EBCM itself may be faulty. EBCM replacement often requires specialized programming and calibration using manufacturer-specific diagnostic tools.

After any repair, clear the DTCs from the EBCM using an OBD-II scanner. Perform a thorough road test to verify that the ABS system functions correctly, the warning lights remain off, and the C1185 code does not return. In some cases, a complete ABS system bleed or sensor recalibration may be necessary after replacing hydraulic components.