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View Full Version : 94 Heater/AC Fan 'Banging/Thumping' Noise?


joed
10-30-2008, 12:51
A coworker has a 94 GMC 6.5 that makes a peculiar thumping or banging noise near the fan/box assembly under the dash.

What's weird is that it only does it for about 10 secs after the key is turned on and then also does it for about 10 secs after the key is turned off.

After the banging episode ends (key on), all the ac/heater control functions work fine, all fan speeds work, and no strange noises - just during power up and down.

Maybe one of the control doors is trying to open or close, or the fan is running on its own against a closed door? Switching the fan to off before shut down makes no difference.

Any thoughts? Weird.

Thanks.

Joe.
diesel65
10-30-2008, 14:34
The truck has a bad recirculation door motor, with ignition off the motor is trying to find home position.

Quick fix is to unplug the motor, to replace the motor GM says you need to roll the dash back.

I have replaced those motors without rolling the dash by using a 1/4 inch air rachet and a 5.5mm snap-on flexsocket and guessing where one of the screwhead is because you can't see it.
joed
10-31-2008, 06:12
Thanks. I figured it was something like that.

Can you guide me to the motor/connector location for unplugging?
diesel65
10-31-2008, 13:44
Looking up under the dash on the passenger side and imagine the blower motor is a clock, it would roughly be in the 9:00 position, close to the firewall.
It is a small three wire connector, the motor is about the size of a pack of smokes.
joed
10-31-2008, 14:03
Okay, thanks. We'll give that a shot.
Robyn
11-01-2008, 06:58
My 94 has done the same thing.

I set the thing for recirc in the summer and then back to outside air in the winter.
I have not tried to ferret the thing out yet but as is mentioned it is doable with a little patience and some time.

The recirc door motor runs until it stalls then once the motor stalls, the current load causes the contoler to shut it off.

The other door motors have a specific home feature.

The recirc motor gear box has plastic parts and they wear out. the gears slip and then there is not enough current draw to cause the contoler to shut it off so it goes thunk thunk thunk thunk.

Best

Robyn