Great forum here. A lot of knowledgeable people. Im going to try and challenge that. I have a 96 Blazer 4 wheel drive. Im having a couple of problems and they might be related. I had a vacuum line break about a year ago that connects at a 3 point connector. One line goes to engine vacuum I believe. The next oes to the vacuum canister mounted on the hood and the third connects to a plastic line that goes through the firewall the driver side foot area. I repaired the hose for my son and all seemed well. Lately hes been complaining there is no heat. I feel the hoses and both are hot so I decided to check vacuum at that area that was damaged before. There is vacuum when disconnecting the plastic line that leads through the firewall. I tried sucking on the line going through the firewall but it would not draw vacuum. I know I should get a vacuum pump and I will. Should this line be going to a vacuum actuator that works the door that controls the switching between hot and cold and or upper vents and lower vents? Ive been reading about the actuator behind the glove compartment but dont know if it applies to the 96 with manual heater controls. I know the fan is going bad i can hear it and all speeds are available at the fan control. I know that has nothing to do with this. Do you feel I should check vacuum at the line going through the firewall first and where is it connected?

Sorry for being so long winded. It just seems all the threads Ive read dont address if this applies to the 96 or if its after 98.

Thanks for the help.
Mike

 

Temperature control is done by an electronic actuator, no vacuum iinvolved. It's commonly referred to as the "blend door actuator". It operates a door within the ductwork, that blends outside air with air passing through the heater core. It is located behind the glove box, and can be accessed through the glove box opening. It's a tight fit, especially if you have "bg hands" but it can be done. The actuator is about 4" square and almost an inch thick, attached with 3 vertical screws going down into the heater case.

The vacuum lines control air distribution: floor, dash, bi level, defrost, etc. The black plastic line connects to the rear of the mode switch in the dash. If air distribution is good, (comes out where you select it) I wouldn't worry about it. The system defaults to defrost, meaning if there is a loss of vacuum to the system due to a leak, air flow will exit the defroster ducts. If you select something other than defrost, and if you climb a long hill, if there is a vacuum leak in the system, it will default to defrost when vacuum in the system is depleted.

The system holds a substantial amount of vacuum, and sucking on the vacuum line would eventually work, provided there are no leaks, but you would definitely be blue in the face long before you created enough vacuum to operate the system A hand vacuum pump, or an AC evacuator is your friend.

If both heater hoses are equally hot to the touch, I would suspect a faulty blend door actuator, electrical circuit for it, or a binding blend door in the heater case. The entire heater case must be removed to service the blend door, which involves removing the entire instrument panel first. Big job. If you've never done it, plan on 8+ hours of work, and you'll need an assistant with some of the work. Not difficult, but time consuming, and more electrical connectors than Tinkerbell has grains of fairy dust! You will thank yourself for taking LOTS of pictures during disassembly

 

The post quoted below shows the vacuum diagram as well as a schematic layout of the HVAC system:
As Captain stated, the temperature control door is electronically controlled.

 

Guys I appreciate your time and knowledge. I picked up a hand vacuum pump today and I will check things out tomorrow. If I use the vacuum pump I should be able to test the line through the firewall. Now if I dont find the air moving its either an actuator the distribution block or a vacuum line correct? To see the actuators I have to pull all the bottom panels and the center counsel? I should be able to see them move right?
Thanks again, Mike

 

If you do not move any of the controls and apply a vacuum to the system, it should hold at that vacuum level. If it doesn't, there is a problem and you will need to move inside to diagnose further, testing each line from the back of the HVAC controls to the other end. The diagrams I posted will help if it is a vacuum problem.

Grab some golf tees as they work really well for plugging off lines.

 

swartlkk Can you give me a quick guide to reading the chart? Im guessing port 6 manifold vacuum. No matter which position the switches are in port 6 will always have vacuum. I'm not sure what vent applies to. Does it mean there will be air at ?? Never mind if you can explain it that would be great.
Thanks

 

The row across the top tells the mode switch position. Each column tells if the port has vacuum or is vented to atmosphere, (no vacuum). For example: In the DEFROST position, (far right column) port #1 & port #6 should have vacuum. The others should be vented to atmosphere. When checking for leaks, each position on the mode switch must be checked to make sure all positions hold vacuum when they should. Each position on the mode switch should hold vacuum. If you have one circuit that does not hold vacuum, the chart will tell you which lines and actuators could be leaking.

 

Ok That ones easy. Ok so for example AC in the Max position Vac at #1 #2 #5 #6 ?

 

You got it! The black plastic line under the hood goes to the mode switch. Connect the vacuum pump to that line, and apply ~18", check for leak. Should hold vacuum.

 

Will do I will let you know how things go. Thanks again for the help.