Woodse

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?

jv9999

Manual or auto climate control?

BigDaddy5

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.

Cliff Harris

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.

Cruisinfanatic

Mine did that. Found mice had chewed through vac lines in back of controller. Take out radio to access

Woodse

Automatic control

Cliff Harris

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:

Woodse

It's a 96, it sounds like a vacuum problem as the dash control seems to operate correctly

86C4Z51

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.

RollaMo-LT4

See this thread, as this seems to be a common problem with the '96 model year.
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)

1993C4LT1

If the vacuum line isn't connected, it will be under the passenger side fuel rail cover.

Cliff Harris

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:

Cliff Harris

The vacuum check valve for your car is under the passenger side fuel rail cover. They seem to be especially prone to cracking on the later cars, which will stop your HVAC system from working correctly:

86C4Z51

Hey thanks for that, Cliff, very much appreciated. I will take a look at it. I suspect the controller in the dash is aged/dusty, but when a system has a fault, it's best to look at the easiest stuff to get to first.