C4 Heater/AC stuck coming out of defrost, thoughts?
#1
C4 Heater/AC stuck coming out of defrost, thoughts?
Heat and a/c fan ramps up when engine temp rises or compressor cycles. The air only comes from the defrost. Anyone with ideas to trouble shoot?
#3
Burning Brakes
Sounds like a loose vacuum connector. This happened on my c68 (manual) control and it was the vacuum cluster directly behind the control head. Resealed the vacuum lines and now the gates move freely.
#4
Race Director
The HVAC air flow is controlled by vacuum. If the vacuum is not getting to where it ought to go the air will only come out of the heater and defroster ducts (the default -- it's actually mandated by federal law). The most common cause of no vacuum in the HVAC system is the vacuum check valve next to the distributor on the driver's side (about $4 at any auto parts store):
C60 is the manual HVAC system with the sliding lever controls.
C68 is the automatic HVAC system with the digital temperature display.
C60 is the manual HVAC system with the sliding lever controls.
C68 is the automatic HVAC system with the digital temperature display.
#5
Le Mans Master
#7
Race Director
What year is the car?
The HVAC system was completely redesigned in 1990, so it makes a big difference.
That said, you can run HVAC system diagnostics on the earlier cars:
http://forums.corvetteforum.com/c4-t...stic-mode.html
or the later cars:
The HVAC system was completely redesigned in 1990, so it makes a big difference.
That said, you can run HVAC system diagnostics on the earlier cars:
http://forums.corvetteforum.com/c4-t...stic-mode.html
or the later cars:
#8
It's a 96, it sounds like a vacuum problem as the dash control seems to operate correctly
What year is the car?
The HVAC system was completely redesigned in 1990, so it makes a big difference.
That said, you can run HVAC system diagnostics on the earlier cars:
http://forums.corvetteforum.com/c4-t...stic-mode.html
or the later cars:
The HVAC system was completely redesigned in 1990, so it makes a big difference.
That said, you can run HVAC system diagnostics on the earlier cars:
http://forums.corvetteforum.com/c4-t...stic-mode.html
or the later cars:
#9
Burning Brakes
In my 86 sometimes the fan doesn't come on... sometimes the controller doesn't seem to send a command, then for some reason it WILL work just fine. The ambient and heater temp reading works. Sometimes I fiddle with it and it seems to react. So rather than a vacuum problem, I suspect something in the switch panel itself. Good thing it defaults to defrost, as that's where it's usually needed first/most.
#10
Race Director
https://www.corvetteforum.com/forums...l-problem.html
I think you can now see the importance of listing all the details you can (even if you don't think they are important).
Not all C4's are the same, as there were many changes throughout the very long production run.
Some of the things to keep in mind (Year, Coupe or Convertible, Auto or Manual transmission, auto or manual a/c system, etc)
Last edited by RollaMo-LT4; 01-23-2015 at 11:41 AM.
#12
Race Director
I had that problem until I removed the connector on the blower control module and found that it had melted from the heat of a bad connection. I found that wiggling the connector would make the fan go on and off. The power comes in on the red wire and it gets REALLY hot. I put a new connector shell on it and cleaned up the melted plastic on the connector pins.
Here's the blower control module connectors:
The black stuff on the pins is melted plastic:
Here's the blower control module connectors:
The black stuff on the pins is melted plastic:
#13
Race Director
#14
Burning Brakes
I had that problem until I removed the connector on the blower control module and found that it had melted from the heat of a bad connection. I found that wiggling the connector would make the fan go on and off. The power comes in on the red wire and it gets REALLY hot. I put a new connector shell on it and cleaned up the melted plastic on the connector pins.