jim in oregon

Last year I had my A/C system upgraded to the R134A..lso replaced the heater fan a/c switch in the console..Blower works fine and so does the heater.However..even tho the compressor is working fine and everything is ice cold on the engine side of the A/C..the system isn't closing the vents properly to transition from A/C and allow cold air to circulate into the car's cabin.Any suggestion about where to start?jim

bpassmore

Sounds like you may have disconnected the source vacuum hose, or the vacuum manifold could be leaking. The white stripe hose to the heater control may also be something to look at. The vacuum hoses are small, so almost any vacuum leak will kill the vacuum control of the control diaphragms.

jim in oregon

Thanks..I guess..The small vacuum line coming from the control critter in line in the heater hose iIs yellow on my 78.It runs up and thru the firewall and is not crimped or damaged..With the car running and fan on high with A/C switched to max..that hose isn't sucking best I can tell so something between there and the heater fan ac switch on the console isn't sending the vac signal.The round inline control diaphragm on the heater hose may be faulty..but with no vacuum on the one line meant to change the flow of hot air and allow the A/C to pump cold air into the cabin I don't think that's the root cause.The CLYmers manual and the AIM manual are sadly lacking in this area..jim

bpassmore

The heater control valve gets vacuum in the max position, if there is a vacuum leak in that valve it will prevent control or slow the movement in all systems. Plug that hose,and see if anything changes.

jim in oregon

Thanks..
So..if I am understanding your direction..disconnect & plug the vac hose from the inline heater hose control valve with the a/c in max setting and blower on full and see if anything changes?And if nothing changes..the vac solenoid control valve that's inline in the heater hose may be the culprit???? IF the heater works full blast..the control valve isn't openig-shutting as it's supposed in response to the control changes/Jim

'75

Jim, on your 78 there should be a vacuum valve behind the glove box area that is controlled by the temperature cable. When the control **** is moved all the way to cold, the vacuum valve behind the glove box should send vacuum to the heater hose valve and cause it to shut off the hot coolant. That same temperature control cable moves a blend door inside the hvac box to direct more or less air through the heater core to change the air temp.

jim in oregon

Thank you much..Will take a look there also;Probably something relatively simple..Provided one know how things go together & function and what to inspect..adjust/repair/replace..Best regards, jim

bpassmore

Blower motor on low, high speed sometimes will not let the mode door open when vacuum is low. (this is just a quick check)

jim in oregon

Going to do a complete check on the vac lines from console component/switch( new and properly installed).AmaZING HOW MANY vacuum lines come off the heater/climate control switch-apparatus and how Few exit the firewall to go anywhere..The one from the heater control valve is the primary one I can readily see..
Not sure Chevy's Astroventilation system is great..but reckon it works after a fashion if I canferret out the problems..

Think the vacuum line that attaches to the vacuum connection behind the glove box is the culprit..
Have a new heater control valve just in case but it's not getting the vac that changes it's mode. Neither on lo or hi..No discernable vacuum at on the incoming vac line when controls are in proper mode at all that I can feel with wetted tongue anyhow..

That line runs thru bulkhead and I assume from there goes to the valve behind the glove box..Think I'll look behind the glove box first..and if I must remove the glove box..Think that's a more logical place to start rather than pulling the troublesome carpeted side panels off the console..Any other thots are certainly welcome.Jim