HVAC Control Unit Vacuum
Thread Starter
Heel & Toe
Joined: Aug 2005
Posts: 16
Likes: 0
HVAC Control Unit Vacuum
My 85 HVAC contol head emits a distinct though faint "pssssssst" when I shut the car down. It sounds like the control unit or component in that area is bleeding off the vacuum. I am not sure if this is normal and would like to get some comments on this. The check valve seems to check OK and is new. I have also attempted to pull a vacuum on the main HVAC vacuum feed line but the system does not hold a vacuum in any position. Anyway, I am just looking for some comments so thanks in advance.
Have a similar problem too. What is that check valve for anyways?
Le Mans Master
Joined: Jul 2001
Posts: 8,183
Likes: 3
From: Santa Maria, CA
It might be that a line has become loose or fell off of it's connection, or cracked. At the age our cars are now, that vac line tubing material has become brittle and possibly cracked.
Not sure where to locate the exact same style tubing, but have noticed regular rubber vac line being used to replace it.
The line begins on the back of the upper intake plenum and tees off from there. I have a pic of the line without the elbow, check valve, etc:
It is inside the main harness loom and goes thru the bulkhead:
It then feeds the climate control. Here is a pic of an 89 version of that unit(mounted in a black box on the interior firewall, next to the steering column, behind the brake pedal)
From there, it routes vac to various locations. Maybe one of those is the source of your hisss.
SunCr has a really excellent knowledge, hopefully he will chime in here and help us out.
Not sure where to locate the exact same style tubing, but have noticed regular rubber vac line being used to replace it.
The line begins on the back of the upper intake plenum and tees off from there. I have a pic of the line without the elbow, check valve, etc:
It is inside the main harness loom and goes thru the bulkhead:
It then feeds the climate control. Here is a pic of an 89 version of that unit(mounted in a black box on the interior firewall, next to the steering column, behind the brake pedal)
From there, it routes vac to various locations. Maybe one of those is the source of your hisss.
SunCr has a really excellent knowledge, hopefully he will chime in here and help us out.
Thread Starter
Heel & Toe
Joined: Aug 2005
Posts: 16
Likes: 0
Thanks for the photos and notes- I will be looking at the system over the next couple of weeks as time allows.
A note on the check valve, I beleive it is there to limit loss of vacuum and change sto HVAC settings when the engine vacuum is in a temporary low vacuum condition such as sudden accelerations.
A note on the check valve, I beleive it is there to limit loss of vacuum and change sto HVAC settings when the engine vacuum is in a temporary low vacuum condition such as sudden accelerations.
