Question about Chevrolet TrailBlazer

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My 2004 6cfl. Trailblazer rear window defroster is not working. The wires appear to be making a good contact. The outside mirrors defrost ok using the same switch as the test window defroster. Is there s separate fuse for the year window defroster? Is there a module that runs the rear window defroster? Where would that be located? What else could be the cause of the rear window unit not working? Think you for your help.

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There is a module that controls the rear window defogger grid . LGM/ EGM Liftgate or endgate module .Receive's message on class 2 serial data network from the HVAC control module to turn it on when button pressed . Fuse # 3 30 amp located in the rear fuse box . Do you know where the rear fuse box is ?
Rear Window Defogger Description and Operation
Rear Window Defogger System Components
The rear window defogger system consist of the following components:
• HVAC control module
• Liftgate module (LGM)/endgate module (EGM)
• Rear window defogger grid
Rear Window Defogger Operation (With LGM)
Battery positive voltage is supplied through the LGM 2 fuse to the LGM for the rear window defogger system. When the rear window defogger switch is depressed, the HVAC control module illuminates the rear window defogger indicator, and sends a class 2 message to the LGM to enable the rear window defogger. The LGM enables the rear window defogger by allowing battery positive voltage to the rear window defogger.
When the rear window defogger switch is pressed for the first time, the defogger cycle lasts 10 minutes. Further operation results in 10 minute defogger cycles.
Rear Window Defogger Operation (With EGM)
Battery positive voltage is supplied through the EGM 2 fuse to the EGM for the rear window defogger system. When the rear window defogger switch is depressed, the HVAC control module illuminates the rear window defogger indicator, and sends a class 2 message to the EGM to enable the rear window defogger. The EGM enables the rear window defogger by allowing battery positive voltage to the rear window defogger.
The following conditions must be met for rear window defogger to operate:
• The end gate glass position is not below the index position.
• The window express down movement is not requested.
• The window down movement is not requested.
If the end-gate glass is not fully closed, the DIC displays "Tailgate Glass Down" when the rear defogger operation is attempted.
When the rear window defogger switch is pressed for the first time, the defogger cycle lasts 10 minutes. Further operation results in 10 minute defogger cycles.

Fuse Block - Rear
Beneath the left rear seat

Posted on Jul 13, 2017


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Vehicles equipped with the optional heated mirror
system have an electric heating grid located behind
the mirror glass of each outside rear view mirror.
The heated mirrors are controlled by the rear window
defogger switch. Electrical current is directed to
the heating grid inside the mirror only when the rear
window defogger switch is in the On position.
The heating grid behind each outside mirror glass
cannot be repaired and, if faulty or damaged, the
entire power mirror unit must be replaced.

The rear window defogger switch is integral to the
rear window switch pod, which includes the rear
wiper and washer switches. The rear window switch
pod is installed in the instrument panel lower bezel,
which is located near the center of the lower instrument
panel, below the heater and air conditioner controls.
The rear window switch pod also contains the rear
window defogger logic and timer circuitry, an amber
defogger indicator lamp, the rear window defogger
relay, and two switch illumination lamps. The indicator
and illumination lamps in the switch pod use
incandescent bulbs, which can be serviced.

The momentary-type rear window defogger switch
provides a hard wired ground signal to the rear window
defogger logic and timer circuitry, each time it is
depressed. The rear window defogger timer and logic
circuitry responds by energizing or de-energizing the
rear window defogger relay and the amber defogger
indicator lamp, which lights to indicate when the
defogger system is turned On. Energizing the rear
window defogger relay provides electrical current to
the rear window defogger grid.

The rear window switch pod cannot be repaired. If
any function of the switch pod except lighting is
faulty or damaged, the entire switch pod must be

Turn the ignition switch to the On position. Set
the defogger switch in the On position. The rear window
defogger operation can be checked by feeling the
rear window or outside rear view mirror glass. A distinct
difference in temperature between the grid lines
and the adjacent clear glass or the mirror glass can
be detected within three to four minutes of operation.

Using a 12-volt DC voltmeter, contact the rear
glass heating grid terminal A (right side) with the
negative lead, and terminal B (left side) with the positive
lead (Fig. 1). The voltmeter should read battery

The above checks will confirm system operation.
Illumination of the defogger switch indicator lamp
means that there is electrical current available at the
output of the rear window defogger logic and timer
circuitry, but does not confirm that the electrical current
is reaching the rear glass heating grid lines.

If the defogger system does not operate, the problem
should be isolated in the following manner:

(1) Confirm that the ignition switch is in the On
(2) Ensure that the rear glass heating grid feed
and ground wires are connected to the glass. Confirm
that the ground wire has continuity to ground.
(3) Check the fuses in the Power Distribution Center
(PDC) and in the junction block. The fuses must
be tight in their receptacles and all electrical connections
must be secure.

When the above steps have been completed and the
rear glass heating grid is still inoperative, one or
more of the following is faulty:
Rear window switch pod
Rear window grid lines (all grid lines would
have to be broken or one of the feed wires disconnected
for the entire system to be inoperative).
If setting the defogger switch to the On position
produces a severe voltmeter deflection, check for a
short circuit between the rear window switch pod
defogger relay output and the rear glass heating grid.

Posted on Aug 31, 2010

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