Like the topic says, I just swapped to the whole 4th gen dash, and the HVAC controls work great, except for when it's set to off. When it's off, I get hot air coming from the defrost vents, but only when I'm driving (i.e. the faster I drive, the more hot air comes out of the vents). Anybody have any idea what might cause this?

 

is it alot or very little? i've always had a little from my factory and thought it was normal?

 

I dont think off is supposed to be 'off' completely, at least on the 3rd gen setup when you select off, the fan goes off but air can still be forced in
if you have it set to cold though dont see why you would be getting hot air, unless it was hot outside... heh

 

Not sure if this helps, but I'm looking at the factory shop manual.

There's this coolant by-pass valve with an vacuum control with hoses in between the heater core, TPI throttle body and radiator.

I suppose that when there's vacuum on the valve body the hot coolant bypasses the heater core.

Maybe there is a vacuum leak or disconnected vacuum hose at the a/c control to this valve. Could be a faulty by-pass valve too.

Some HVAC units do not have a vacuum control by-pass system and rely on closing a door over the heater core to prevent air from passing through and heating the incoming air.

I've have this situation in swapping a TPI motor into a TBI Chevy truck with the electronic a/c system(late 1988 to mid 1990's Chevy trucks).

Unless I find some more info. on GM HVAC systems, I'm going the route of installing a manual heater core bypass valve and a pair of "T" fittings.

DO NOT block-off the heater core passage hose(s) but complete the circuit and make the coolant flow back from motor to radiator.
This allows the purging of any air bubbles in the coolant.
Micro bubbles form on sharp edges and "hot spots"inside passages of the block and heads. These bubbles collect being trapped in low-flow areas, sooner or later causing mysterious overheating symptoms.

Hope this helps. If anyone out there knows more about this bypass valve and the electronic A/C controll for TPI motors,
please share!!!

Thanks