Automotive HVAC Hackery - Help Needed!

We've owned our '06 RAV4 since new, and our oldest son is going for his license later this week.  This is the car he's been driving for the majority of his permit time.  One of the biggest issues is that the HVAC mode control broke a few years ago.  So air flows out of two places - the circular side vents on the side of the dash, and at the windshield.  It's never been ideal in the summer, but we've lived with it for 6-7 years this way.  Replacing the entire HVAC assembly requires removing the dash, steering column, etc and the assembly itself is something like $1700 from a dealer. I may bite the bullet, grab a unit from a junk yard and replace it eventually, but for right now I'm looking for a quick fix.

The AC blows cold, but the majority of the airflow is at the windshield. So we're missing out on cold AC that's wasted on the windshield, but we get a ton of condensation on the outside of windshield when it's humid out.  I drove it a few nights ago when it was humid out, and the condensation buildup on the outside of the windshield was distracting.  With a new driver, I want as few distractions as possible, and my wife says he can't drive the car until this issue is fixed.

Some options I'm exploring:

1) Duct taping off the entire front windshield vent.  Solves the condensation issue on the outside of the windshield, but doesn't put the cool air where I need it.

2) Taping off part of the front windshield vent, and creating a vent that runs from the base of the windshield into the cabin.  This solves the windshield condensation issue as well as the lack of airflow issue.

I build a prototype of option #2 yesterday out of the finest rapid prototyping materials known to man - cardboard and duct tape.  It worked better than I expected from a functionality perspective, but it leaves something to be desired from a looks perspective.  I could care less, but my teenage sons (and my wife) would prefer it looks less like a leftover box of frosted mini wheats and duct tape.  I think it adds character, but they're less than convinced. I assured them I'd use black duct tape in the final iteration 

I think I have the basic design down, but where I need help is figuring out a material to make the 1.0 version out of this.

Options as I see them:

1)  I could spray paint cardboard to match the dash, but it's still cardboard

2) I could use 1/4 plywood and build a similar box to what I made in cardboard, and paint it to match

3) What other options am I missing?  Fiberglass?  

Rough dimensions are 13" long, 5" wide, and 2" high.

Also open to design feedback, as I'm definitely not an engineer.  Would a narrower box in the middle be more effective? I'm thinking a trumpet end that directed some air at both front passengers might be more effective, something like this:

Here are some other pictures for context, the dash does have a slight curve to it from front to back that would make the 1/4" plywood option more difficult.

Is this your problem at 4:30?

Dollar store foam board and hot glue?   Corrugated plastic (like old political signs)?  A can of pseudo-matching black paint would help with the camouflage.

Slippery said:

Is this your problem at 4:30?

That was a great video, I had high hopes that was the issue, but something broke on the mechanism itself. I took the knob itself apart and the metal clip was in the wrong spot like in the video, but I was able to spin the mode control knob freely until it's hooked up to the cable. There's a giant gear in the side of rhe HVAC assembly that has a very limited range of motion, and a part that integrates with that gear broke off.

Is that part that broke off easily accessible? Maybe it can be 3d printed or somehow replicated. 

It might be easier to manually change the position of the cable to make the air blow through the dash vents until you get the parts to fix the car correctly.

Noddaz said:

It might be easier to manually change the position of the cable to make the air blow through the dash vents until you get the parts to fix the car correctly.

This. There should be a way to get the blend door to the position you want. 

An old outback had this issue, it was a vacuum operated flapper motor that was leaking.  Constant click click click if trying to move the door was worse than all the AC air going to the windshield.  Fixed without removing the dash but was challenging and required bending a wrench or 2 to get in there.  

Would making some sort of plug to go into the vent slot also deflect the air to other vents?

First, I appreciate all the feedback - Thank You! 

Slippery said:

Is that part that broke off easily accessible? Maybe it can be 3d printed or somehow replicated. 

I think this is a minor part, because the gear has a very limited range of motion right now, here's a quick video showing that:

I think replacing the entire HVAC box would be the way to go at this point, because I think something internal broke in it that's preventing the gear from moving through all the different modes

Noddaz said:

It might be easier to manually change the position of the cable to make the air blow through the dash vents until you get the parts to fix the car correctly.

I also looked into this, unfortunately the blend doors for the middle vents/windshield seem to be buried within the HVAC assembly itself, and the gear has a very limited range of motion.  I can access the blend door for the floor vents and have played around with propping them open.  Unfortunately, the airflow through them doesn't really help to cool down the car.  However, I haven't found a way to access the blend doors that work the middle vents on the dash, which are the ones I really care about in this case.

Porschenut said:

Would making some sort of plug to go into the vent slot also deflect the air to other vents?

The plug is basically what the contraption I made accomplished - the duct tape blocked off most of the dashboard vent, and then the cardboard box took the air from the remaining part of the windshield vent that wasn't blocked, and pushed it into the cabin.  It actually was very functional, which surprised me! 

This is what the HVAC unit looks like:

The gear you see in the video above is attached to the right hand side of the left part of the assembly, sandwiched in between the main assembly and the blower motor and outside air blend door, which is on the right.  All of this is buried deep under the dash.

I took the center vents out to see if I could manipulate the blend doors, but didn't have any luck.

This is the main HVAC mode gear, the piece that broke is in the center.  It engaged with the back side of that main gear (the side you can't see). I'm not sure what it controlled, it many have been the floor vent blend door.

First, once the sun hits that duct tape glue, it's never coming off. And it will collect all kinds of dirt and bugs forever. Not an ideal solution.

Short of actually fixing it properly (which, I'm sure, would be a nightmare), I'd probably start with one of these:

https://www.amazon.com/Evelots-Window-Stopper-37-Inch-Keep-Insect/dp/B072QC1J2X/ref=sr_1_3?crid=ESOO8WTZUSLC&keywords=draft+dog&qid=1689558334&sprefix=draft+dog%2Caps%2C102&sr=8-3

Also, a CT DMV inspector won't even let him start his driver's test if there's any kind of hackery going on.

They will actually fail you immediately if you have a headlight or tail light out.

Appreciate the words of caution around the duct tape, Woody! I'll have to come up with a better solution, although the one you proposed looks pretty good!

My son will take his test in the driving school car, the few extra bucks was worth the lack of stressing about the car being perfect. Heck, this one has had the TPMS light for 13 years straight before I bought all new sensors a few months ago. I'm shocked the bulb didn't burn out!

I think I'd just fix it. It might take a long time but with the amount of work it would take trying to not fix it, you might break even!

I assume this will be driven in the winter so don't forget that he will need to use the defroster when the temperature drops so easily switching back to the windshield vent is important 

Could maybe modify something like this..

I think the amount of time it would take to build a nice version of the redirect vent is probably a solid down payment on the real fix.

I'm leaning towards fixing it the right way. The biggest challenge will be sourcing the factory HVAC assembly, us-based eBay came up empty, along with Car-Part. It's called an "AC Evaporator" in the parts diagram, but Car-Part's search is painful. I'll probably just call a local junkyard I like and go from there.

No promises as it has been raining quite a bit around here, but there seems to be one that might have what you need and it is pretty taken apart. If I have time I will swing by and see how easy/difficult it is to remove. 

Does a 2005 have the same dash?

Hi Slippery - I appreciate the offer, I'm going to check in with a local yard that I've had good luck with in the past and see what they have in stock.  The toughest part is the description of the part, it's listed as teh 

The 2006-2012 are all the same, it's the model with the manual HVAC controls (no automatic climate controls).