Chevrolet Silverado / GMC Sierra. Service manual — part 2797

If there is an aftermarket trailer brake controller, the ITBC must be disconnected. Do not power
both ITBC and aftermarket controllers to control the trailer brakes at the same time.

Reference Information

Schematic Reference

Trailer Brake Control Schematics

Description and Operation

Trailer Brake Controls Description and Operation

Connector End View Reference

Component Connector End Views

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

Scan Tool Reference

Control Module References for Scan Tool Information

Circuit/System Verification

1. Ignition ON, observe the scan tool brake controls Trailer Brake Relay Status parameter.

The reading should be Enabled or Disabled.

2. Ignition ON, observe the scan tool brake controls TBCM Command Duty Cycle parameter.

The reading should be 0 - 100 percent.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the solid state relay module.

IMPORTANT: All electrical components and accessories must be turned OFF

and allowed to power down.

2008 Chevrolet Silverado 1500

2008 BRAKES Trailer Brake Controls - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

2. Test for less than 5 ohms of resistance between the ground circuit terminal B and ground.

If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, test for 9 - 11 volts between the 10-volt control circuit terminal E and ground.

If less than the specified range, test the 10-volt control circuit for a short to ground or
an open/high resistance. If the circuit tests normal, replace the TBCM.

If greater than the specified range, test the 10-volt control circuit for a short to voltage.
If the circuit tests normal, replace the TBCM.

4. Ignition OFF, reconnect the harness connector at the solid state relay module.

5. Connect a test lamp between the control circuit terminal C of the trailer connector and the

ground circuit terminal B.

6. Command the trailer brake relay ON and OFF with a scan tool. The test lamp should turn

ON and OFF when changing between the commanded states.

If the test lamp is always ON, test the control circuit for a short to voltage. If the
circuit tests normal, replace the TBCM.

If the test lamp is always OFF, test the control circuit for a short to ground or an
open/high resistance. If the circuit tests normal, replace the TBCM.

7. If all circuits test normal, test or replace the trailer brake solid state relay (SSR).

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Trailer Brake Control Module Replacement

Control Module References for TBCM replacement, setup, and programming

DTC C1115

Diagnostic Instructions

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Review Strategy Based Diagnosis for an overview of the diagnostic approach.

Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC C1115 08

Manual Trailer Brake Apply Request Switch Signal Invalid-Manual Apply Switch Stuck

2008 Chevrolet Silverado 1500

2008 BRAKES Trailer Brake Controls - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Closed

DTC C1115 5A

Manual Trailer Brake Apply Request Switch Signal Plausibility Failure

Diagnostic Fault Information

Circuit/System Description

The manual apply switch rationality diagnostic monitors for circuit rationality faults on the
manual apply switch input. The manual apply switch consists of a primary circuit, which is used
as the basis for determining the trailer brake manual apply gain, and a redundant circuit that is
used to rationalize performance of the primary circuit.

Manual Trailer Brake Apply Request Switch Signal Invalid" diagnostic monitors for a manual
apply switch that is stuck in an applied position.

Manual Trailer Brake Apply Request Switch Signal --plausibility failure diagnostic monitors for
circuit rationality faults on the manual apply switch input. The manual apply switch consists of a
primary circuit, which is used as the basis for determining the trailer brake manual apply gain,
and a redundant circuit that is used to rationalize performance of the primary circuit. The manual
apply switch position fault is detected when the absolute value of the difference between the
primary manual apply switch input and redundant inputs exceeds a threshold for a certain amount
of time.

Conditions for Running the DTC

DTC C1116 or C1117 are not set as current DTC.

The system voltage is in the range of 9.5 - 17 volts.

The module is powered up and completes initialization.

Conditions for Setting the DTC

C1115 08

Manual trailer brake apply request switch signal invalid fault is detected when any one of the

Circuit

Short to

Ground

High

Resistance

Open

Short to

Voltage

Signal

Performance

TBCM Manual Apply Signal
Circuit

-

-

-

C1115 5A

C1115 08

2008 Chevrolet Silverado 1500

2008 BRAKES Trailer Brake Controls - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

following conditions is true:

The ignition is turned from Off to On, the Manual Apply Switch is not activated, then it is
detected that the Manual Apply Switch is activated for 5 minutes.

The ignition is Off, the Manual Apply is already activated, and then turn on the ignition, and
detected the Manual Apply Switch is continuing to be activated for 30 seconds.

C1115 5A

Manual Trailer Brake Apply Request Switch Signal plausibility failure:

The ignition is turned from Off to On, the Manual Apply Switch is not activated, then it is
detected that the Manual Apply Switch is activated for 5 minutes.

The fault is detected if the absolute value of the difference between the primary manual
apply switch input and redundant inputs exceeds a threshold for an extended amount of time.

Action Taken When the DTC Sets

The trailer brake control will be disabled for the remainder of the ignition cycle.

The Service Trailer Brake System will be displayed on the Driver Information Center
(DIC).

Condition for Clearing the MIL/DTC

When the condition for the DTC is no longer present.

The TBCM automatically clears the history DTC when a current DTC is not detected in 40
consecutive drive cycles.

Diagnostic Aids

If there are any DTCs set for GMLAN communication such as U0100, U0121, U0140, or U2099,
diagnose them first before to resolve DTCs described in this section.

When the manual trailer brake apply switch is shorted to ground, shorted to battery, or open
circuit, DTC C1116 Manual Apply Switch Primary input circuit fault may also be set as a current
DTC. If DTC C1116 is also set as current DTC, then diagnose DTC C1116 first before to
diagnosing DTC C1115 08 and C1115 5A. If there is an aftermarket trailer brake controller, the
ITBC must be disconnected. Do not power both ITBC and aftermarket controllers to control the
trailer brakes at the same time.

Reference Information

2008 Chevrolet Silverado 1500

2008 BRAKES Trailer Brake Controls - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado