GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 HUMMER 🢒 2008 🢒 H3 🢒 HVAC 🢒 HVAC - Manual 🢒 DTC B3787 or B3788

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 Descriptors

DTC B3787: Blower Motor Relay Control Circuit Low

DTC B3788: Blower Motor Relay Control Circuit High

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

HVAC Relay Control

B3787

B3788

B3788

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Circuit/System Description

When the ignition switch is in the run position, the body control module (BCM) will ground the HVAC relay control circuit, which will switch the relay. With the relay contacts closed, battery voltage is supplied to the HVAC module.

Conditions for Running the DTC
    • The ignition voltage is between 9-18 volts.
    • The ignition switch is in the run position.

Conditions for Setting the DTC
    • A short to ground on the HVAC relay control circuit.
    • A short to voltage on the HVAC relay control circuit.
    • An open circuit on the HVAC relay control circuit or relay.

Action Taken When the DTC Sets

No action is taken.

Conditions for Clearing the DTC

The history DTC will clear after 40 fault-free ignition cycles.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

    •  Air Temperature Description and Operation
    •  Air Delivery Description and Operation

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 Testing
  1. Ignition OFF, disconnect the HVAC relay.
  2. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 86 and ground.
    3. ⇒If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
  3. Install a test lamp between the B+ circuit terminal 86 and the control circuit terminal 85.
  4. Command the blower motor ON and OFF with the scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
    5. ⇒If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.
    ⇒If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.
  5. If all circuits test normal, test or replace the HVAC relay.

Component Testing
  1. Ignition OFF, disconnect the HVAC relay.
  2. Test for 60-180 ohms between terminals 85 and 86.
    3. ⇒If the resistance is not within the specified range, replace the relay.
  3. Test for infinite resistance between the following terminals:
    4. • 30 and 86
    • 30 and 87
    • 30 and 85
    • 85 and 87
    ⇒If not the specified value, replace the relay.
  4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.
    5. ⇒If greater than the specified range, replace the relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for BCM replacement, setup, and programming