Hummer H3. Manual - part 1290

open/high resistance. If the circuit tests normal, replace the ECM.

If greater than 5.2 volts, test the 5-volt reference circuit for a short to voltage. If the
circuit tests normal, replace the ECM.

4. Verify the scan tool A/C High Side Pressure Sensor parameter is less than 428 psi.

If greater than 428 psi, test the signal circuit terminal 3 for a short to voltage. If the
circuit tests normal, replace the ECM.

5. Install a 3-amp fused jumper wire between the signal circuit terminal 3 and the 5-volt

reference circuit terminal 2. Verify the scan tool A/C High Side Pressure Sensor parameter
is greater than 428 psi.

If less than 428 psi, test the signal circuit for short to ground or an open/high
resistance. If the circuit tests normal, replace the ECM.

6. If all circuits test normal, test or replace the A/C Pressure Sensor.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement (Left Hand Drive)
or Air Conditioning (A/C) Refrigerant Pressure Sensor Replacement (Right Hand
Drive)

Control Module References for ECM replacement, setup and programming

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 Descriptor

DTC DTC B3787

Blower Motor Relay Control Circuit Low

DTC DTC B37878

Blower Motor Relay Control Circuit High

2007 Hummer H3

2007 HVAC HVAC - Manual - H3

Diagnostic Fault Information

DTC P3787 or 3788

Circuit/System Description

When the Ignition switch is in the run position, the Body Control Module (BCM) will ground the
blower motor relay control circuit, which will switch the relay. With the relay contacts closed,
battery voltage is supplied to the blower motor.

Conditions for Running the DTC

The ignition voltage is between 11.0-18.0 volts.

The ignition switch is in the run position.

Conditions for Setting the DTCs

A short to ground on the blower motor relay control circuit.

A short to voltage on the blower motor relay control circuit.

An open circuit on the blower motor relay control circuit or relay.

An internally shorted or excessively low resistance blower motor relay coil.

Action Taken When the DTC Sets

The blower motor relay is commanded OFF.

The conditions for which the DTC was set will be stored in the Failure Records data only.
No information will be stored as Freeze Frame data.

Conditions for Clearing the DTC

A History DTC clears after 40 consecutive warm-up cycles.

Reference Information

Schematic Reference

Circuit

Short to

Ground

Open/High

Resistance

Short to

Voltage

Signal

Performance

Blower Relay Coil B+

P3787

Blower Relay Control

P3787

P3788

1 - Blower Relay Always On

2007 Hummer H3

2007 HVAC HVAC - Manual - H3

HVAC Schematics

Description and Operation

Air Delivery Description and Operation

Air Temperature Description and Operation

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

Scan Tool Reference

Scan Tool Output Controls

Scan Tool Data List

Scan Tool Data Definitions

Circuit/System Verification

1. Ignition ON with the engine OFF, command the A/C Relay ON and OFF with a scan tool.

2. Feel/listen for the A/C relay to click. Observe the scan tool A/C Relay Circuit Status

function or A/C relay solenoid turns on and off clicks.

Circuit/System Testing

The A/C clutch relay terminal numbers called out per the following testing were verified on an
actual vehicles underhood fuseblock and A/C clutch relay.

1. Ignition OFF, disconnect the A/C Relay.

2. Ignition ON, verify that a test lamp does not illuminate between the control circuit and

ground.

If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the ignition circuit, relay coil feed circuit and

ground.

If the test lamp does not illuminate, test the ignition circuit for a short to ground or a
open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test all

2007 Hummer H3

2007 HVAC HVAC - Manual - H3

components connected to the ignition circuit and replace as necessary.

4. Connect a test lamp between the relay coil B+ circuit terminal and the relay coil control

circuit terminal and ground.

5. Command the A/C Relay output function 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 ground. If the
circuit tests normal, replace the ECM.

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 ECM.

6. If all circuits test normal, replace the A/C Clutch Relay.

Component Testing

Relay Testing

1. Ignition OFF, disconnect the A/C Relay.

2. Test for 137 ohms across the relay coil terminals.

If the resistance is not close to the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

30 and 86

30 and 8

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 of resistance between
terminals 30 and 87.

If greater than 2 ohms, replace the relay.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for ECM replacement, setup and programming

Circuit/System Verification.

2007 Hummer H3

2007 HVAC HVAC - Manual - H3

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