Air Conditioning problem 94 GMC

[IN46131]

I have aproblem with the air unit on my 94 GMC that I hope someone can help with. I bought the truck 5 years ago wrecked and rebuilt it. I charged the air on it at that time and it has worked fine for 5 years. Last summer one day I started getting a knocking noise from the passenger side area down by the heater housing. It knocks for about 30 sec when you turn the key on and quits. Then does the same thing when you turn the truck off. In conjunction with that the air quit working about 2 days after the noise started. I bought a cheepy air gauge setup with freon thinking it had lost charge but when I hooked it up it says the pressure is to high and its over charged. Whats up why all of a sudden would it think its over charged and stop working. Any help would be appreciated as summer is coming on ( eventually I hope ) and air would be cooool! pun intended.

Thanks

Rick

[pappyl]

Rick,

Gotta give more info man.

- Is your fan working? - sounds like it is jammed up...
- If your fan is working - air u getting airflow from the vents?
- you got a problem in your plenum - check your vent cable - may have come off - check your vaccum lines (open the doors)
- Is it cold?
- no - you gotta rip off your housing on the firewall and check the cooling rad (actually not sure of the term) - may be plugged - should be nice and cold. May also have a piece of insulation blocking flow
- Otherwise, your compressor may be screwed - your silver evaporator bottle should be frosting up when you run the air

[nukmanz]

My 92 Silverado does a similar thing. Turn ignition on---thumps about 10 seconds. Turn ignition off---same noise. Push recirc switch then wait 10 seconds---same noise. I can feel where the problem is by reaching under dash (passenger side, under glove box). There is a vent door thump that I can feel and hear. The door opens and closes like it should but when it reaches full open or full closed it thumps.

The darn thing still cools so I'm as happy as a dead hog in the sunshine. But, every time I park in public, I walk away as fast as I can, looking around like--- "Who's truck is that making that thumping noise?"

[pappyl]

My truck is a little older - '89. I've had the plenum out - that main door to the air conditioning evaporator is a cable - the newer ones are probably either vacuum driven or electric - but don't know for sure without ripping it out. Either way its a few hours work to pull the plenum and check the door, and springs.
If it happens before the motor turns on - electric for sure.

[Portlander]

My 94 Silverado just started doing the same thumping noise....the other day when I turned on my AC, I apparently had hit the "max" button as well. The letters REC showed up on panel next to AC...that is the only thing different that I did. I usually never use Max. Now, w/ all fans shut off, when I start it, it will do the thumping noise for about 20 seconds or so, then stop. Same thing happens when I shut it off. If you get under the dash on the passenger side, you can see the vent that opens and closes to recirculate air...you can feel something hitting it, but I can't see inside.

[Blutooth]

Originally Posted by Portlander

My 94 Silverado just started doing the same thumping noise....the other day when I turned on my AC, I apparently had hit the "max" button as well. The letters REC showed up on panel next to AC...that is the only thing different that I did. I usually never use Max. Now, w/ all fans shut off, when I start it, it will do the thumping noise for about 20 seconds or so, then stop. Same thing happens when I shut it off. If you get under the dash on the passenger side, you can see the vent that opens and closes to recirculate air...you can feel something hitting it, but I can't see inside.

I have a 93 Silverado and it does the same thing. Has anyone fixed this and how?
Thanks,
Ron

[black94]

Originally Posted by Portlander

My 94 Silverado just started doing the same thumping noise....the other day when I turned on my AC, I apparently had hit the "max" button as well. The letters REC showed up on panel next to AC...that is the only thing different that I did. I usually never use Max. Now, w/ all fans shut off, when I start it, it will do the thumping noise for about 20 seconds or so, then stop. Same thing happens when I shut it off. If you get under the dash on the passenger side, you can see the vent that opens and closes to recirculate air...you can feel something hitting it, but I can't see inside.

aaaaa

[mgandrew]

I'm not seeing much in the way of a direct "fix".

My 1994 GMC Surburban does it too. Is there a relay that could be sticking? Would such a stuck relay be causing my battery to drain?

Does anyone wish to buy an old Surburban? 244,000 Mi

Mark

[brcidd]

Your Recirc door actuator needs replaced- those trucks are programmed to open/shut the door when you key off/on- (so you can close the cab doors) and the air inside the cabin is vented to outside- believe it or not--the actuator stops by getting to the end of its travel- and the voltage spikes- and the controler shuts it off- but when it can't find the end of its travel it keeps running- because it's stripped- and then times out----very common failure mode

[crohnie]

How did you fix your plenum whack whack whack whack a/c door problem in your 1994 gmc truck

[crohnie]

How did you fix your a/c thumping problem in your truck?Or did it get fixed yet?Please dont say you took it to a repair man but if so what was the problem? How much did it cost?

[crohnie]

How did you fix your a/c thumping problem in your truck?Or did it get fixed yet?Please dont say you took it to a repair man but if so what was the problem? How much did it cost?my plenum door is electric according to your input
Bobby

[crohnie]

I have the exact same problem in my 94 gmc sierra z71 so have you fixed it yet?what was the problem and the cost or did you do it yourself?
Bobby

[cliffordbaker1]

I had this knocking in the right plenum,-sounded like coming out of the defrost right side and could be heard outside.

This is the actuator blending cold and hot air on the right side of the plenum,, just left of the blower.

It has 3 screws, inserted from the right side, threading toward the driver side, which with a tiny hand MIGHT be accessible, but no way for me.
There is a stiff cable bundle in the way and two of the screws are hidden in the back.

Dealership estimated at $350.

I purchased th actuator online (AC Delco 1571937 - RockAuto $64.97) and took to my mechanic. who replaced it by swinging down the dash $ 180.