Hello all, I have an 89 Coupe that gives me good volume of cold air out the dash vents sometimes but at other times the air movement cuts way down to where there is not enough flow coming out to cool the car. I have the electronic control a/c / heat system. All this time the fan speed is set at 10 and the temp control at 60.

I think there is a air flow control "door" under the dash that is vaccum operated. The system acts like the door is not staying open. I have very cold A/C lines under the hood, but can not reliably keep a full force of air coming out the vents when I have it set on "high". Acts like a vacuum leak.

I have replaced the check valve for the vacuum hose at the intake manifold. and have have a good connection and vacuum from that to the small vacuum line going inside to the cabin.

Anyway to easily find out what's going on or how to block the door wide open if possible?

Thanks
Gary / Louisville

 

I think that door is electrically controlled by a switch and an arm right behind the ECM mounted on the heater-core box, maybe ya got a loose connection.

 

How do I get access to that area?, Is that area behind the passenger side dash head crash pad thing?

 

No, ya access it by removing the knee panel. I'm gonna post a few pics in "My Garage" for you to look at, give me a few minutes

 

Great! I appreciate your input..

 

The pics are up, I got them from AGENT86. You'll see in the first three pics ,the switch and the metal rod that activates the door.

 

How do I get to your "garage" pics?

 

Go to "Me" on the left "Caddyboy84" and click on 1988 Corvette

 

Got it, sorry not used to where things are on here. Nice lookin Vette!
So....I if remove the kick panel on the passenger side, I should be there. In the second pic, I think i see the control rod. I assume that should be moving if I run the fan control up and down 1 - 10, etc....

 

Thank you and yes, thats the rod that controls the door. It should move when you go from A/C to heat, be gentle with that stuff, don't break anything. See if everything is working there, then we'll take it from there. Like I sais, I believe that it's an electric switch but you'll see when you go down there. Agent86 gave me this info because I have to change my heater-core, which I still didn't get to yet.

 

Thanks again, I will check the connections, etc.

 

It may be the control unit not sending a signal to the switch as well. If you only want A/C, just bypass the core. A bypass might be easier with less problems, something to consider before you rip in to stuff.

 

caddyboy84 I've got an 85 that I disconnected the wiring from the cruisecontrol. Mine is a 4+3 is there any way that you could take a picture of the wiring for me. It's located by the brake pedal in an auto &by the clutch pedal on a manual. I don't remeber the colors of the wires going too the swtich. There is a vacume line on it and also two connections on the switch.TIA

 

I think that switch is the cruise disengauge switch that works off the brake pedal. I'm looking at my electrical manual and I don't see exactly what colors those wires are but if nobody here knows, I'll give you the colors tomorrow morning, I'm in my P-J's right now I think it's a 1 green wire switch that grounds through it's mount, but I forget right now so don't quote me. What is it your trying to do, reconnect everything?

 

Remove the two electrical plugs on top of the Evaporator housing that
plug into the Blower Control Module.

Verify the connector pins are in good condition and have no burn marks.

The HVAC Control Head provides a voltage to the Blower Control Module
(located inside the evaporator housing). The Blower Control Module then
outputs a voltage that drives the blower motor.

Voltage on the Brown wire from the HVAC Control Head varies from
2.5 - 7 volts DC depending on what the blower speed should be. The
output voltage of the Blower Control Module on the Purple wire
should vary from 4 to 12 volts DC to power the Blower Motor.

When the problem occurs, use a volt meter and measure the voltage
on both the Brown wire and the Purple wire.

For instance at full Blower speed the Brown wire should measure around
7 volts and the Purple wire should measure 12 volts.

All of the air from the Blower motor blows thru the Evaporator Coil.
The air is then diverted by the Blend door to force some, all or none
of the air thru the Heater core depending on the temperature called for.

The volume of the air should be the same so your problem is probably
not the Blend door.

The vacuum is used to control the vacuum actuator valves that determines
what vents the air flow comes out of.

You could have a bad Blower motor, Blower Control Module or the
evaporator is freezing into a block of ice. Are you using a refrigerant other than R12?

Here's the schematic for the Blower motor.

 

You can cause the blend door motor to move by setting the A/C to maximum and minimum temperature. It is controlled by the HVAC programmer right above the gas pedal. There are two 340 ohm resistors on the HVAC programmer that can overheat and cause the blend door motor not to work. See here (he got the resistor values wrong):

http://www.batee.com/corvette/acrepa...oard/index.htm

and my article here:

http://www.misterpeachy.com/VetteStu...rogrammer.html