I’ll start by saying that I am a novice, and any terms used incorrectly stems from my ignorance. I apologize in advance, and I hope I convey my problem.

I’m in a 98 ZJ Lerado with a 5.2 V8

My climate control doesn’t only blow on defrost (the typical vacuum leak symptom), I can also select the floorboards and blow crisp air there. Where my problem stems is when I switch it to blow forward or towards the driver/passenger, it will go back to defrost. I know the hardware, the climate control electronics, and the actuator that controls the direction of the heat/AC since it will occasionally work in the mornings (I assume the cooler temperatures play a role).

I have been dealing with my climate control for the past few summers. A blown-out check valve leads me on a wild goose case last summer to try and figure out why I was only blowing defrost, however this summer, my issue isn’t as easy as it seems. I only noticed it this problem as the weather got warmer. I put up with it for a while since my AC lines were compromised, and I didn’t have the time to replace and recharge. When I did, this problem reared its ugly head, and made those 90-100-degree days miserable; but hey, at least my feet were cool.

I spent an hour or two checking all of the vacuum lines under my hood, and with no help from Chilton, they all looked good, and I don’t think I missed any. I’ll include a crudely Microsoft painted version of my vacuum and EVAP system as I am not sure if the latter is relevant (I’ll also include the Chilton’s diagram of what my system should look like but is a little simpler). I’ll also include the diagram on my core support of the vacuum system on my rig that I only spotted minutes before posting this (since I put work in the first two images, I’ll include them for the sake of posterity).

I fear that it is a line within my dash. After speaking with a mechanic friend who helped replace a heater core in a similar ZJ, that dash is not a simple thing to work with.

Any information will help

.
Chilton's 1995-98 V8 engine Vacuum Diagram

 

Your problem is in the heater control and it's vacuum actuators, or the mode switch that controls them. On the back of the control panel, there is a controller (the mode switch) that sends vacuum to various control diaphragms on the air doors. You may have one that isn't functioning or the control switch is leaking. It may be just a hose that's disconnected or a diaphragm that's punctured.

The problem or leak is not in the engine compartment, but in the heater control if you are getting anything but defrost.

 

That’s exactly what my mechanic friend said when I described my problem to him a day or so ago. He pointed to my climate control panel and said it is most likely due to either the controller itself or the diaphragm relating to the cluster of vacuum lines leading from it. He theorized that since I was occasionally getting front air on cold mornings was because of contraction in the vacuum lines when it was cold, thus, temporarily fixing my problem.

I am very unfamiliar with how the vacuum is set up behind the climate control (with the diaphragm and everything), but I’ll look at it once this nice blanket of Pacific Northwest smoke dies down. If anything, I can poke around a junker rig to see what kind of project this will turn out to be.

Thank you

 

Are you familiar with vacuum advance controllers on distributors? They are just cans with a diaphragm inside that moves a rod when vacuum is applied. You have one of those on each heat control door except the temperature control. The mode control is just a rotary switch that directs vacuum to the various doors to open or close them. On the back of the mode control is a rubber plug with various different color hoses coming off. The black one is the vacuum source from the engine. If one of those vacuum pots has a leak in it, it won't work. If it's a tiny leak in a seal, it might close up when it's cold. If the hoses don't fit the fittings tightly, they might seal in the cold. Vacuum is very hard to work with because the slightest leak can screw up the entire system.

The blend door is the one that controls the temperature of the heat. It has an electrical stepper motor on it that moves in steps by pulses from the control ****. When you turn the ****, you can faintly feel the clicks. Each click moves the motor shaft a small distance. The blend door blends hot air from the core with outside cold air to change the amount of heat coming out. Older heaters used a cable to move the blend door.

 

I’ll be sure to check that. I’ve seen one of those diaphragms before, but it was not in a very convent location to check on or even swap out. I hope the other ones don’t require me to remove my dashboard since there is only so much I can see when I remove the radio, climate control, and glove box.

 

If you have a vacuum source, you can check each hose, one at a time, and listen for the sound of that door working. Not easy with the engine running. I've seen a vacuum cleaner used, placed outside the shop with the door mostly closed, and a long piece of fuel hose.

 

For posterity's sake, I was able to find and fix the problem (last year), and yes, you were correct.

On a whim, I replaced this component (which Google is telling me is called an "Air Inlet Door Actuator") with a junk yard donor, and that completely fixed the issue. On the surface, it looked like mine was fine, so I assumed the issue was further upstream.


Product Details | SimplePart (lindsaychryslerparts.com)


This component was located here and can be accessed by removing the pannel below the steering wheel.