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Hey, i'm new to this, so please bare with me. Recently I purchased a 1993 Nissan Altima GXE, and it has been running fantastic. Since I bought it though, i've noticed that about 75% of the time, my defrost is the only thing that blows any air. Any other setting I change it too (floor, head, both), it still blows through the defrost. Any suggestions? Thanks.
 

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Biru O' Kudasai
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It sounds like it is a problem with the mode door motor. The mode door motor is in the center of the dash and can be be seen on the drivers side of the center console in the heater box toward the front of the vehicle.
First you will need to pull the heater A/C control panel in the dash.
Next push the Vent button on the control panel and turn the ignition switch to the on position and then turn it off again.
Then check to ensure that continuity exists between both terminals 1 (Blue/Orange) or 2 (Blue/Yellow) and body ground.
Procede to the next mode, push the Bi-Level button and turn on the ignition switch and then off again. Test the continuity between both 2 (Blue/Yellow) or 3 (Blue/Red) and body ground.
Repeat the procedure for the Foot switch position with both 3 (Blue/Red) or 4 (Blue/White) and body ground.
Again with the Foot/Defrost switch position and both 4 (Blue/White) or 5 (Blue/Black) and body ground.
Finally pushing the Defrost switch position and testing the continuity for both 5 (Blue/Black) or 6 (Blue) and body ground.

If it tests good, then check the control linkage and pull the mode door motor out to check its operation.
Switch between the different panel modes and the motor should move smoothly.
If it does NOT operate correctly skip to the next procedure.
Check the linkage movement and hold it in Defrost mode (where the door deflects all the air up).
Then reinstall the mode door motor and reconnect the harness (if it was disconnected).
Turn the ignition switch on and push the Vent button.
Attach the mode door motor rod to the side link rod holder.
Press the Defrost switch to observe the link moves to the fully-open positon.

If any position is NOT good, then unplug the mode door harness at the motor to check the continuity between the terminal 101 (Black) on the outside and body ground.

If that is NOT good, the harness or the connector is the problem and needs to be repaired.

If it is good, then test the continuity between the harness on the control panel connector and the corresponding color on the mode door motor connector end.
Reconnect the harness at both ends.
If the test was good then go to the next test.
The next test is to check the output of the push control panel, by connecting a voltmeter with the positive lead on cotrol panel terminal 7 (Green) and the negative lead on terminal 8 (Green/Yellow).
Then turn the ignition switch on and observe for approx. 12v when switching back and forth between Vent and Defrost on the control panel.
The meter will show -12v when going from Vent to Defrost and +12v when going from Defrost to Vent.
Also watch the mode door linkage for rotation with a clockwise movement when switching from Vent to Defrost and vise versea.

If this test is NOT good then the control panel is bad.
If it is good then the mode door motor is bad.

This should be everything you need to diagnose the problem.

Troy
 

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Thanks for the quick reply "Altima SSS". I'll check the dealer and junkyard.

also, another question i had was should i be able to move the mode door motor by hand? I was trying to rotate using the two handles that fit into the mode door assembly.
 
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