Windows and Glass

DTC 34 Rear Window Defogger Open CKT:

DTC 34 Rear Window Defogger Open CKT (Part 1 Of 2):

DTC 34 Rear Window Defogger Open CKT (Part 2 Of 2):

CIRCUIT DESCRIPTION
The rear defogger switch is a momentary switch that returns to the rest position after it is pressed to the ON or OFF positions. When it is pressed to the ON position, it supplies voltage to the internal timer relay coil. With voltage supplied, and ground provided, the internal timer relay energizes, and closes the relay contacts. With the contacts closed, voltage is supplied to the radio antenna amplifier which routes the voltage to the rear defogger grid. Voltage is also supplied to the heated outside rear view mirrors through the HTD MIRROR fuse. With voltage supplied and ground provided to the defogger grid and outside mirrors, the grid and outside mirrors begins to warm and the defogger switch Light Emitting Diode (LED) turns ON.

Approximately 15 minutes after the defogger switch is pressed to the ON position, the internal timer relay shuts off. This open the relay contacts, and terminates the voltage source to the defogger grid and outside mirrors. The rear window and outside mirror glass begins to cool.

The rear defogger operation can also be shut off at anytime before the 15 minute time period expires, by pressing the rear defogger switch to the OFF position.

In addition to the rear defogger and outside mirrors receiving voltage, the heater and A/C control also receives a voltage signal (feedback) at terminal 6 (C1). Whenever a rear defog request is made, a voltage output signal is sent to the rear defogger relay, the heater and A/C control checks this signal (feedback) before and after the signal is sent. The heater and A/C control looks for the feedback signal to change from a LO state to a HI (B+) state. This indicates that the signal was sent and the relay energized completing the circuit to the heater and A/C control.

CONDITIONS FOR SETTING THE DTC
^ Ignition switch in the ON position and the rear defog function is selected.
^ The heater and A/C control did not receive the feedback signal (open circuit).

ACTION TAKEN WHEN THE DTC SETS
The heater and A/C control stores a DTC 34 in memory.

CONDITIONS FOR CLEARING THE DTC
^ The condition for the fault is no longer present.
^ Using a scan tool.
^ A history DTC will clear after 20 consecutive ignition cycles if the condition for the fault is no longer present.

DIAGNOSTIC AIDS
^ If the DTC is a history or an intermittent. Try to perform the tests shown while "wiggling" wiring harnesses and connectors, this often can cause the malfunction to appear.
^ When moving related harnesses and harness connectors, visually/physically inspect wiring and connectors for the following:
- Wiring broken inside the insulation.
- Poor mating of the connector halves or a terminal not fully seated in the connector body (backed out).
- Improperly formed or damaged terminals. All connector terminals in the related circuits should be carefully reformed or replaced to insure proper contact tension.
- Poor terminal to wire connection. Inspect for poor crimps, crimping over wire insulation rather than contact tension.
^ Check the power and ground circuits before replacing the heater and A/C control.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. The scan tool can verify than the heater and A/C controller did or did not receive the signal.
4. The first step is to check the fuse. After determining the heater and A/C control did not receive the signal. The next test is to check for voltage indicating if the circuit is complete.
6. Determines if the relay contacts closed. The voltage can not be measured at the terminal with the relay still in place. Measuring the voltage at the backside allows easy access and does not harm the terminal.
9. By using terminal 6 (ground) for the DMM a check of three circuits can be performed. This also checks if the relay is receiving voltage.
11. Most likely the ground was open at terminal 6 and no voltage was measured. If the ground proves good there is an open circuit in one of the voltage supplies.