You might find this factory TSB interesting and/or helpful:
Bulletin No.: 05-08-42-001
Date: March 07, 2005
INFORMATION
Subject:
Diagnostic Information on Headlamps Flicker or Momentarily Dim While Driving
Models:
2004-2005 Buick Rainier
2002-2005 Chevrolet TrailBlazer
2002-2005 GMC Envoy
2003-2004 Oldsmobile Bravada
with 4.2L Engine (VIN S - RPO LL8)
A concern regarding headlamps flicker or become dim while driving
may be easily mis-diagnosed. It is very important to capture as much
information from the customer as possible. Below is a guideline to
assist with diagnostics.
Verify the customers specific concern.
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What time of day does this occur (is it light or dark outside) ?
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What driving conditions (vehicle speed, engine speed, throttle position, etc...)?
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What was the outside ambient temperature when the condition occurred?
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What accessories was the driver using when this condition occurred (A/C, heated seats, rear defogger, etc...)?
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If the concern is at dusk, could it be the automatic headlamps changing from DRL to Headlamp mode and possibly back to DRL again?
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Are there any other relevant operating conditions?
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Diagnostic Information
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Service Bulletin - 02-06-03-008A - Info - Low Voltage Display on IP
Gauge, Lights Dim at Stop Lights, Battery Discharged, No Start, Slow
Cranking, Dim Lights at Idle, Low Generator Output.
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Service Bulletin - 02-06-03-006B - No Start, No Crank, Battery,
Batteries, Generator, Gages, SES, SIR, Brake, Security, Theft, ABS,
Hot, DIC Messages, Lamp, Light, Battery and Generator Diagnostic
Testing and Cadillac Roadside Service.
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Service Information - Exterior Lighting Systems Description and Operation.
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Service Information - Battery Inspection/Test (Non-HP2).
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Service Information - Charging System Test.
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Service Information - Inspect for aftermarket devices that could
affect the operation of the system. Refer to Checking Aftermarket
Accessories in Wiring Systems.
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Important:
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Keep in mind high current draw items may cause a noticeable dimming
of lights and a fluctuation in the voltmeter may be noted and is a
normal operating characteristic of the vehicle.
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For 2004-2005 TrailBlazer, Envoy, Rainier and Bravada with the LL8
4.2L engine only, the Secondary Air Injection (AIR) pump activation is
thought to be the main contributor to this high current draw condition
and is a normal operating characteristic of the vehicle. Refer to
Secondary Air Injection System Description.
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For customers whose driving habits tend to run the AIR pump
diagnostic test that leads to concerns with the headlight flickering,
if the vehicle is allowed to idle for 20 seconds prior to driving, this
should allow the passive test to pass and prevent from running the
active test that may dim the lights. The passive test should/will
usually pass 99 percent of the time, and by allowing the 20 second idle
period, it should almost eliminate the active test. Be sure to check
for codes and correct related concerns if this condition repeats.
Additional high current draw items are rear window defogger, HVAC
blower motor, rear air suspension pump along with other components.
For easier diagnostics, it may be possible to control or monitor these components with a scan tool.
It is important to inspect all related connections, grounds and
battery terminals for integrity . A loose or corroded connection may
exaggerate the concern, and may cause other symptoms. Refer to Testing
for Intermittent Conditions and Poor Connections.
Corrections/repairs should only be made for specific concerns
identified and not related to characteristic of high current draw
components.
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