[Phil750]

Have an 86 C-20 pickup with factory air. I have searched every forum on this site to see if I could come close to finding the answer. My problem is I cant figure out how the vacuum controls the door for air to come through the vents. Right now the door stays half open with air coming out of vents and floor. I have no defrost either. I thought it might be the actuator but there is no vacuum there either. I don’t know where the the vacuum even starts to begin trouble shooting.
I also read that Helms INC makes a good repair manual, any replies on that.http://www.hotrodders.com/forum/newthread.php
Thanks Phil

[joe_padavano]

Originally Posted by Phil750

I don’t know where the the vacuum even starts to begin trouble shooting.

Begin by buying a factory service manual for your truck. They are all over ebay. The HVAC system is well covered in the manual.

[75gmck25]

I don't think Helm made the manuals for GM in those days. Most that I've seen on Ebay for slightly older trucks have had only a GM logo on them.

The heater controls for vacuum are fairly easy to understand. There is a large rubber junction plugged into the back of the heater controls, and from there you have all the vacuum lines running to the controls. The lines connected to the controls are:
- Input from engine compartment vacuum, from the vacuum reservoir
- One line to each vacuum motor on the plenum- 3 motors I think
- One line running to the vacuum motor in the cowl and to the recirculate vacuum motor on the right side kick panel

- Start out with checking the vacuum reservoir under the hood. Look at the line running to the passenger compartment and check for breaks, then pull the line and check the canister fitting to see you have vacuum with the engine running.
- Now with the heater control at off, put a hand vacuum pump on the line running into the passenger compartment and see if it holds vacuum. If not, you probably have a break in the line or bad controls.
- To check the lines, remove the glove compartment so that you can see the top of the heater plenum. From there you should see the line coming in from the engine compartment (just above plenum), the line running to the cowl (up high), the line to the recirculate door (right side), and the line running to the vacuum door on top of the heater plenum. Check for broken lines.
- To check the other two motors you may be able to feel around behind the defroster plenum and trace the lines that run to the motors.

Bruce

[balder]

Originally Posted by Phil750

Have an 86 C-20 pickup with factory air. I have searched every forum on this site to see if I could come close to finding the answer. My problem is I cant figure out how the vacuum controls the door for air to come through the vents. Right now the door stays half open with air coming out of vents and floor. I have no defrost either. I thought it might be the actuator but there is no vacuum there either. I don’t know where the the vacuum even starts to begin trouble shooting.
I also read that Helms INC makes a good repair manual, any replies on that.http://www.hotrodders.com/forum/newthread.php#
Thanks Phil

Hi Phil,

When the engine runs at idle do ya hear a hissing under the hood or dash?
if ya dont------turn the motor off
and heres where to start--remove the air cleaner first and closely note where anything hooks up to the cleaner - then locate where the H/AC vacume line hooks up ---pull the vacume hose for the H/AC controls off and get down and suck on it-- then notice if the air keeps coming if it does have a person suck on it and while they do that find the leak by listening under the dash- it could be just a cracked vacume-hose--(most common) or a vacume control diaphram
if it gets real hard to suck - while sucking put your tounge over the hole on the end of the hose and hold it there for a few seconds--if the vacume slowly leaks away the hose will slowly not stick to your tounge-

With a ride that old- the problems are -
1--some clown under the hood removed the air cleaner and didnt put the vacume hoses back right or not at all-
2--some or all of the vacume hoses under the hood and under the dash are cracked or so old they dont seal at the connections.
3--all the above is assuming the ride is stock. and also the intake manifold has no leaks and is torqued down to spec. and the carb body is tight and not loose on the intake manifold.

im up tonight on another project - ill guide or suggest more after you reply,
vacume problems just need a progressive checking to find the problem(s)

Balder