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Old 09-08-2007, 06:10 PM   #1
EdB
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Under dash noise

I purchased a 94 1500 a couple of weeks ago to use as a daily driver.
It has a few issues that I am working out.

Next on the list is a thumping noise coming from under the dash. It sounds
like a motor turning a gear that is not meshing. It does this when the key
is first turned to start and when it is turned off. It will do this about
15 to 20 times and then stop. It is like it either meshes or times out.
It will do it sometimes while driving.

I only have a Haynes manual and the schematic shows three different
motors associated with the AC and heating system. I plan to go after it
tomorrow.

Has anyone had this problem? What did you find? Any shortcuts?

Any help will be appreciated.

EdB
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Old 09-08-2007, 09:07 PM   #2
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Re: Under dash noise

Hey Ed, my cousin had that problem and he never did get it fixed. But I don't know how determined he was. I do know it's fairly common.
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Old 09-08-2007, 09:40 PM   #3
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Re: Under dash noise

Thanks Krue.

I guess I will find out tomorrow.

BTW, How are things going up there in the hill country?

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Old 09-09-2007, 12:55 AM   #4
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Re: Under dash noise

Pretty good. How 'bout y'all in the low country?
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Old 09-09-2007, 08:41 AM   #5
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Re: Under dash noise

Quote:
Originally Posted by krue View Post
Pretty good. How 'bout y'all in the low country?
Alright, let's keep the post on topic

Sides I don't want these noises driving Ed crazy.

I've been lookin too Ed and haven't found anything new as yet. Hopefully our experts will chime in.
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Old 09-09-2007, 12:53 PM   #6
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Re: Under dash noise

Listened to Click and Clack on Saturday....a woman called in with a question about a noise that happened when she switched on some heat/ac activity. She said the noise shook the front of her car so she would slow down whenever she was going to hit the switch. As the call progressed, C&C were asking questions, and assumed ( as I am sure many, many listerners did) that the front end of the vehicle actually shook at that time. Further comments determined that the shaking was making her "feel" as if the front end was shaking.

I said all fo that to say this: perceived noises are often louder and more remarkable than they are in actuality. We hear them and our mind goes into overdrive and imagines all sorts of possibilities.

In her case (an '88 Cherokee), they surmised that it was a blending door in the heat/AC system that was the source of the noise. I'd check the vacuum connections on the AC system and the mounting of the fan motor/blower assy. It could be that something has gotten into the blower area and the squirrel cage is contacting it as it turns. That could give out that sound you hear. As you drive along, whatever it is can bounce around and either contact the fan, or not. Try driving with the fan switch off, along with assuring that the defrost isn't on or the AC, either. If you know that they are off, do you still have the noise? If nothing can be assured, pull the fuse that powers the blower motor and see if you still have the noise. I'm gonna guess that something has fallen into the defroster vent and has worked its way down to the blower fan and that is the source of your noise.
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Old 09-09-2007, 02:35 PM   #7
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Re: Under dash noise

This is something that only happens at startup and shutdown, in my cousins case anyway. It only lasted a few seconds (10-20 at the most). Truck didn't have to be moving for it to happen, usually starts before you have time to put it in gear.
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Old 09-09-2007, 06:24 PM   #8
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Re: Under dash noise

ED's is the same thing.
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Old 09-09-2007, 08:51 PM   #9
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Talking Re: Under dash. Found the Problem.

The problem is in the recirculation door actuator: A tooth is missing
from one of the gears and it was trying to mesh and could not.

This device attaches behind the blower motor. I tried to remove just
the blower motor housing but could not quite get it out. I had to
disconnect everything and move the whole heater and air conditioner
housing foward to get some room. The plug to the motor is very
difficult to reach. I also had to remove the computer support. It
took some Houdini type contortions to do this. There are four small
head (5mm) that hold the blower housing to the heater housing.
If I have to do this again, I will go ahead and loosen everything
and then remove the blower housing.

I did find trash that had made its way down to the heater core.
My shop vac will make short work out of this

I hope GM still has the motor and gearbox. Now I will have to get
this all back together.

I am attaching some pictures that did not come out that great.
It is the top large gear that is missing the tooth. The small gear
that it meshes with is very worn. The two gears would fix the
device. It will probably have to be bought as a unit.

Thanks for the input.

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Old 09-09-2007, 09:15 PM   #10
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Re: Under dash noise

Glad you found it! I'll tell my cuz what the noise is all about.
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Old 09-09-2007, 09:26 PM   #11
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Re: Under dash noise

Good deal. Sounds to me like Click and Clack's advice to the lady on Saturday may have been helpful to you. So, there's a vacuum line hooked up to this door that enables it to actuate and that it happens on startup/shutdown, explains it. Thanks for letting us all know.

Stupid me....should have looked more closely at the picture. The electric motor is plainly visible. No vacuum connection showing. So it's powered on start-up and at shut-down it is not, allowing for the movement at both times. Sorry to have sounded stupid, but it's my right to be wrong and I have to exercise that right, or I'll lose it.
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Old 09-09-2007, 09:28 PM   #12
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Re: Under dash noise

Dang you did some work today Ed. I should have come up and helped supervise!

Glad you found the problem, hope they have the part for you.
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Old 09-11-2007, 10:15 AM   #13
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Re: Under dash noise

this must be a common problem because this same thing happened to my 1994 silverado, had it replaced at a dealer at a cost of 275.
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Old 09-11-2007, 09:52 PM   #14
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Re: Under dash noise

Thanks for the input. I got it all back together last night. It is wonderful
not having to listen to that noise. Like a lot of things, I think that I
could do it in half the time now. I am not volunteering though.

The next issue is changing the rear engine seal. Have any of you done
this?

EdB
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Old 09-11-2007, 10:03 PM   #15
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Re: Under dash noise

Glad to hear it Ed, so crank up the sounds and cruise now!
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Old 09-12-2007, 12:53 PM   #16
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Re: Under dash noise

Ed,

I have a 1993 C/K1500 and located the click to the same unit but have not tried to get at it as of yet. I am curious about a couple of things. First, what was the cost of the part that you replaced? Next, what all must I remove to get the part changed out quickly? Finally, how much time do you think I'm looking at since I haven't done this before?

Thanks,

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Old 09-13-2007, 01:01 AM   #17
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Re: Under dash noise

Two Fingers,

The actuator cost $91 at the local Chevy dealer. My biggest shock
was that they had it on hand and I did not have to wait for it to
be ordered.

If your truck does not have ac, it will be easier than if it does.
This is because I had to pump down the ac and unscrew
both fittings that go through the firewall to the expansion
coil. I also removed the heater hoses to free the main
box. The actuator attaches to the blower housing which fits
very thghtly in place. There are four screws that hold it in.
I was able to get this part out with everything loosened
on the main box except the ac. I could not get it back in
with the ac in place. It would have gone smoother if I had
done this to start with.

You do not have to take the main box out but it needs to
be free enough to give you a little space to work with.

You also need to free the bottom of the dash to give a little
room to work with the main box. There are two attachments
(bolt and nut) on each side and I had to loosen the two bolts
that hold the steering column in place.

You will also need to remove the glove box and the panel
to the left of it. There is one long bolt on the top of the main
box that is the key to getting everything lined up for the other
bolts that screw in from the firewall.

You will need a 5mm open and box end wrench to remove the
blower unit. The rest are 7mm, 10mm and, I believe, a 13mm.

You can unplug the harness behind the glove box and this will
give you relief in moving the unit out. The plug at the actuator
is in a tight place. I was not able to unplug it until I almost
had the blower unit out.

Give yourself a day to do this. Keep track of the screws.
The Haynes manual cover a lot of this.

Ed
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Old 09-13-2007, 01:03 AM   #18
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Re: Under dash noise

Bill,

I know it would have gone much smother if I would have had
some good supervision. Sometimes you have to just make do.

Ed
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Old 09-13-2007, 07:51 AM   #19
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Re: Under dash noise

Ed,

Thanks for the detailed information. I'm scared to death now.

:-D

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Old 04-29-2009, 06:23 PM   #20
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Re: Under dash noise

I had the same thing on my 94 K1500 with a/c. Instead of ripping the dash apart, I cut the lt green/black wire running to the blend door actuator. Of course the recirculation for MAX AC will no longer work, but that I can live with in my quite truck cab.

Here is a pick with the glove box out and the wire I cut. Its poking out of the harness with the fresh electrical tape.



Peace,
JC

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Old 04-29-2009, 07:36 PM   #21
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Re: Under dash noise

These door motors have a position sensor circuit in them. If this position portion of the motor fails the controller (in the AC head) will not know the correct position of the door, and could run it into the end of travel and then make the gears jump as it runs after it should have stopped. The system has a fail safe design that shuts the motor off after it sits and slips a while.


The recirc door motor can be changed without removing the airbox, but it is not easy.
Here is how I do the job:
I pull the carpet and carpet pad away from the firewall and fold it back by the seat to give a little better access.
I use a ľ inch drive air ratchet to remove the screws that attach the motor to the airbox.
I pull the motor down and then unplug it.
Plug in the new motor.
Put the screws in their holes and poke each of the screws through a piece of plastic to hold them in the motor. The plastic can not be so strong, or think, that it causes a problem, but must be substantial enough to hold the screws from falling out.
You may need to run the motor if it is not in the correct position to install it.
Get the motor in place and tighten each of the screws partially so you donít make a screw fall out as it get pushed by the motor moving to a fully seated position.
Once all screws are started, then you should tighten all the screws.

The only way I can do this is with my small 1/4" air ratchet. There is not enough room to swing a regular ratchet in that small area.
Care must be used to run the ratchet very slow and not over tighten the screws.
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Old 05-02-2009, 08:11 AM   #22
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Re: Under dash noise

Correction on information:

I was looking at a 1993 wiring schematic last nigh and noticed that the fresh air door does not use a position sensor in the motor for the door. This means the motor is run for a period of time by the control head. If the motor does not move as it should for any reason, when the control head reverses polarity on the wires to run it the other direction, it will not need to travel as far and will click for a while as the gears slip.

A thought that comes to my mind is, do the control heads get problems and amount of time if runs the fresh air door gets compromised/altered.

The mode door and temperature door motors both use the position sensor in the motor so that the control head knows how long to run the motors.
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Old 08-24-2014, 10:21 PM   #23
Larry.sisk
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Re: Under dash noise

Man that's a lot of work saving information ,thank you guys I have this job to do on my 94 . That's funny what you were saying about chevy having the part ,you know that there is a problem if they quickly produce an odd part like that, means they sell a lot of them. What is the correct name of the part to ask for?
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Old 08-25-2014, 10:14 AM   #24
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Re: Under dash noise

I found it at Rock Auto under blend door actuator.

EdB
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Old 12-13-2014, 03:03 PM   #25
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Re: Under dash noise

Just did this to my 92 Sport Truck. Spent Friday night and Saturday morning under the dash.

Removed glove box door and insert, control module and its support, blower motor/fan and its rubber cover, loosened the right side lower dash.

Loosened all screws from under dash and engine compartment holding the blower housing, tried to get it out but it was to little space between ac housing and kick panel.

Didn't have a 1/4 air ratchet but went over to a friend and borrowed one. It worked like a charm to use the method described by ChevyTech.

Many thanks to ChevyTech and others for sharing your experiences.

Have a question though.
What is actually operating/controlling the recirculation door? There is no button or anything to select recirculation or outside air like I have seen in other cars/trucks.

Thanks
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