I'm investigating Suburbans, the 92 to 99 body style. I test drove on the other day that seemed in good shape but was puzzled by sounds coming from the firewall area under the passenger side of the dash, down by the black contraption that I assume is the blower motor/evaporator box. Although the AC worked fine every now and then varying between several times a minute and every couple of minutes there was some kind of noise like a flap that would open or close with a substantial hissing and banging sound. Sorry, no other way to describe it. Not very loud just enough to be noticable over the sound of the vehicle at speed. I'm guessing that might have something to do with vacum actuation of the air flow in these things. Does anyone have any idea if that might be right or if it could be something else? I assume this is essentially the same system used in all full size GM pickups of that era? If so would replacement of this unit be a problem?

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

I don't think it's normal because our 97 'burb makes no such noise. Certainly not to the degree that it sounds abnormal. There's a rather long hiss when you turn off the engine with the AC compressor engaged. It can go on for half a minute. I'm told that's normal.

I have to wonder if it's a vacuum system, though. Our 6.5TD engine vacuum pump serves only the wastegate actuator. Brakes have hydraulic assist. Maybe there's an electric vacuum pump for climate control.

Sixto
87 300D

yes 88 up chevy pickups us an all electric hvac system.

So if that's the case then it's likely and electrical issue.?

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

Or the flap isn't moving freely.

Sixto
87 300D

This sounds like a well known problem with GM and Ford HAVC systems. Where they used to use vac servos to move the blend doors they now use stepper motors. They are easy to install (once you get to them, that is) and take up a lot less room than a servo.

Normally they will make a clicking or thunking sound and on GM's will do it even if the key is off.

There is no repair; the one that is bad just has to be replaced. I think they run about $70 each for parts.

The good news is that it will not get any worse. The bad news is that some of them are very difficult to get to.

IIRC........ the recirc door is probably the one misbehaving. R/R the glovebox, actuator is on top of the case behind it...........done!

Actuator. That was the word i was trying to think of earlier.

You might be able to go to Rock Auto's web site and get a picture of the one you need. The ones I have seen look like big electrical breakers without a switch.

Very interesting and useful info as usual folks, thanks. I have not bought the vehicle, I'm merely starting to look around at them as there are a plethora of them going dirt cheap in this part of the wolrd. I've never owned one nor driven one so I figured I'd start testing as many as I could to find out what's what. Might be able to pick one up for a few thousand.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S