HVAC problem, loud knocking!

I have a '93 Chevy Silverado 2500 that has an embarrassing problem. Every time I turn off the ignition (or on), there is a loud knocking sound coming from inside the dashboard. It sounds like a little guy inside there banging away with a hammer on the dashboard, and is loud enough that people turn to look.

It also does the same thing when using the heater (or AC) and I press the "MAX" button. It does switch from external to recirc (or vice versa), and then the knocking starts, lasting about 15 seconds.

Any clues as to what is causing the problem?

Thanks,

Robin

I have a '93 Chevy Silverado 2500 that has an embarrassing problem. Every time I turn off the ignition (or on), there is a loud knocking sound coming from inside the dashboard. It sounds like a little guy inside there banging away with a hammer on the dashboard, and is loud enough that people turn to look.

It also does the same thing when using the heater (or AC) and I press the "MAX" button. It does switch from external to recirc (or vice versa), and then the knocking starts, lasting about 15 seconds.

Any clues as to what is causing the problem?

Thanks,

Robin

Sounds like something is hitting the cage that the blower motor turns. I would pull the blower motor to take a look and go from there.

//2000CAYukon

It's just a guess but I would say that it's one of the electric servo motors that opens and closes the fresh air door. Look about 6" above the gas pedal on the end of the heater duct. It's not the one you are looking for but looks just like it. Then take your glove box out and look in behind it. I have had it out before but I don't remember exactly where it's at.

I think there is 3 of these motors but I could be wrong.

The mechanic at work had taken the motor off with the door in one position and put the motor back on after the door had moved causing it to bind. I had to take the motor back off, connect the wires and run it all the way one direction. Then I made sure the door was all the way the same direction and replaced the motor. No more problems after that.

James

It's just a guess but I would say that it's one of the electric servo motors that opens and closes the fresh air door. Look about 6" above the gas pedal on the end of the heater duct. It's not the one you are looking for but looks just like it. Then take your glove box out and look in behind it. I have had it out before but I don't remember exactly where it's at.

I think there is 3 of these motors but I could be wrong.

The mechanic at work had taken the motor off with the door in one position and put the motor back on after the door had moved causing it to bind. I had to take the motor back off, connect the wires and run it all the way one direction. Then I made sure the door was all the way the same direction and replaced the motor. No more problems after that.

James

After rereading the first post and James post, it does sound like it could be one of the actuator motors.

Auto parts stores that carry AC Delco parts will stock these (or can get them) and will be cheaper than a dealer.

//2000CAYukon

Thanks for the help, I'll try replacing the actuator motor.

---Robin

I've seen this many times. Is the motor that controls the RECIRculation (Max) has a stripped gear. All you need to do for now is to unplug it. If you look at your blower motor from under the dash, looks towards the left of it. You will see a wire plugged in way up in there. Unplug that.

It's a very complicated job replacing that motor.

I've seen this many times. Is the motor that controls the RECIRculation (Max) has a stripped gear. All you need to do for now is to unplug it. If you look at your blower motor from under the dash, looks towards the left of it. You will see a wire plugged in way up in there. Unplug that.

It's a very complicated job replacing that motor.
On second thought, who needs MAX setting anyway :) Thanks for the advice. The motor does look very hard to get at.

Thanks, ---Robin

On my '94 which has the R-134a in the A/C, it works better on MAX. So at the beginning of the summer, I plugged it back in and hit MAX on the panel. When it got to the recirulation position, I pulled the plug..

I would love to fix it, my interior is nearly 99% mint condition.. But oh well...

Pat