FORGIVE ME....another AC question.

[Dre319]

I haven’t posted really at all in here but I follow along quite often and all you guys in here very informative.
I have a 98 GMC K1500.

So my issue/question is, when I’m driving on the highway, I can actually here my AC fluctuate between the dash vent, floor and window vent when I have the setting on dash vent only. Now while driving in the streets, this does not happen. It’ll stay wherever I set it.

I’m getting ready to replace the dash with a new one being that it’s that time. Anything else AC related I should replace while I have the dash off?

Thank you
David

 

[94burbk1500]

I'm just taking a shot int he dark here, but that could be a vacuum problem. If I'm not mistaken, the blend doors are vacuum operated, a leak or excessive vacuum at certain RPMs could cause some strange behavior.

 

[kv5137]

Change all three blend doors and make sure there are no obstructions


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[east302]

Agreed, it may be an actuator issue. The one by the gas pedal (mode actuator) controls the floor-panel-defrost position. The other two are for temperature and recirc, but those are probably fine and a pain to get to.

Check the mode actuator, there is a silver position indicator dial with a notch in it that rotates based on the panel setting. See if it rotates smoothly as the panel is adjusted from floor to defrost. There are small lines on it. It goes from roughly 11:00 to a 1:00 position.

There are also two linkages connected to it. See if they are intact; the springs sometimes pop out.

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[Dre319]

Thank you gents. I will take a look at it.

 

[Garrett Fulton]

If you're going to the trouble of removing the dash, you might want to go ahead and replace the a/c evaporator and the heater core. On my '93 C2500, the recirc. door actuator had gone bad and was impossible to replace without pulling the dash and I also had a leaking a/c evaporator. That actuator has no position feedback on a '93 and is commanded to full travel when you turn off the ignition for 10 seconds using a time delay. (Can't remember if it goes to close or open). After time, the plastic gears get brittle and one will pop off and the actuator will run making a popping sound in the dash for those 10 seconds after the ign. it turned off. Also, the blower resistor is very hard to get to and is cheap and easy to change with the dash out. You will_not_want to remove the dash again. Job from Hell on a '93 and likely a '98. Just my .02.