Heat/AC/Mixer HVAC Tech Info (2 Viewers)

[Honger]

Some questions in my build thread about more details on the HVAC controls spurred me to do a tech thread of sorts.

The whole HVAC is comprised of three primary parts. The intake and blower motor are on the far right (for a LHD vehicle) and includes the diverter for fresh/recirculated air supply. It also includes your blower motor and blower motor resistor. It fastens to the firewall and mates up to the fresh air intake that opens into the area behind the hood. Here's a shot of mine after clean-up and new foam/seals prior to install.

There's an AC housing which sits in the middle of the whole setup. It sits to the left of the blower motor and fastens to the firewall as well as to the blower motor housing AND "mixer" housing. For non-AC units this is essentially just a pass through duct. For AC trucks this has your evaporator, pressure switch, and a drain. The evaporator has two lines that pass through the firewall to be plumbed to the balance of the AC equipment in the engine bay. Here's an end-view of mine after clean-up and new foam/seals/pressure switch.

Finally, there's a heater and "mixer" unit that sits in the center of your dash. It sits to the left of the AC housing and fastens to the firewall. This houses a heater coil (on trucks with heat) as well as multiple foam-covered flaps that divert air between different diffusers (defrost, floor, front vents, side vents). This unit has two lines that pass through the firewall to be plumbed to the appropriate coolant lines for heat. Here's the heater-featured unit I rebuilt for my truck recently.

 

[Honger]

My truck was an AC only vehicle. This meant my mixer housing wasn't full featured. The spot for the heater core was actually sealed up. Part number for LHD AC-only housing.

I bought a heater-featured unit from @joekatana and cleaned that up. Part number for LHD Heat-only housing (I'm deducing this is heat only as it didn't contain features to divert from the center vents).

Here's the torn down AC-only unit. You can see the front ducting removed that feeds the dash vents and the diverter flap for the center vents is visible on the housing.

This linkage is what was on my AC-only unit. There's no provision for floor air. This linkage arrangement offers either dash vents, defrost, or a mix.

This linkage is what was on the Heat-only unit. There's no provision in this assembly for diverting air from the center dash vents. Regardless of floor or defrost selection, all dash vents would receive air.

 

[Honger]

So how does it all work?

Behind the control switches/levers there are up to 4 cables that feed to various points... depending on heat/ac/etc.

In that photo, the upper most cable, running to the left of the assembly, is your heater valve cable. It passes through the firewall via a small spot in the heater coil grommet, and ultimately turns your heater valve on/off. Here it is passing through (though not yet connected to a heater valve).

The upper most cable, running to the right of the assembly, controls the heater-coil diverter flap inside the mixer housing. It runs around, and under, the mixer housing controlling a small cam arm. This either forces air through the heater coil, or allows it to bypass the heater coil, or a combination (if you set the top hot/cold lever in the middle of its position). Here's the attachment point on the bottom of the mixer housing.

Moving down to the middle lever, you have your fresh/recirculating air intake control. This cable runs back to the firewall, along the top, over to the far right, and down to the side of the blower motor housing. This selects whether you are getting fresh air from under the bezel piece at the base of the windshield, or recirculating air from the passenger footwell. Here's the attachment point on the blower motor housing.

Finally, the bottom lever controls the "mix" of the air. Dash vents, floor air, defrost, or some combination thereof. It has a very short cable that runs to the left of the mixer housing and controls the various cam arms that positions the various vents. It's coiled end snaps into a metal clamp on the middle cam arm.

 

[Honger]

A video walkthrough of the mixer functions...

 

[Honger]

A random sampling of a few more photos...

Some of the controls assembly.

The combined blower-speed/AC switch.

The internal defrost flap closed and open.

 

[FJBen]

Great stuff!

Now the bottom "mix" has 2 cables correct? I had to replace my control as my levers were broke but the connections are different. I haven't given it much thought but as I stated before, I get air to basically all vent basically all the time at some level. Pretty sure it's that other cable that I couldn't hook up with out some modifying.

 

[Honger]

The bottom "mix" lever only has 1 cable. On both of the control units I have (which I combined into one for my truck), the bottom lever only has a spot to anchor and connect one cable. The top lever appears to be the only lever that accommodates two cables. I suppose there could be a different control assembly... or RHD may be a bit different, not just a "mirror" of the LHD.

 

[Honger]

A final detail to add... there are apparently different kinds of knobs for the levers. Mine had skinny ones... the one that came with my heater had fat ones.

If anyone has some skinny ones they want to get rid of let me know... I'd like to go back to that look.

 

[coldtaco]

Nice write up

 

[RHINO]

great to see the parts in detail, thanks


pretty sure the skinny knobs are pretty generic, like even the HELP! section at auto parts store

 

[OGBeno]

Nice work, Joel.

 

[svsisu]

A final detail to add... there are apparently different kinds of knobs for the levers. Mine had skinny ones... the one that came with my heater had fat ones.

If anyone has some skinny ones they want to get rid of let me know... I'd like to go back to that look.

I'm pretty sure I have 2 new, skinny white ones. I'm out of town for a couple of weeks but when I get home I'll look. Happy to pass them on to you, or maybe trade for your "fat" ones if you no longer need them.

 

[FJBen]

great to see the parts in detail, thanks


pretty sure the skinny knobs are pretty generic, like even the HELP! section at auto parts store

Interesting, I believe you are right on the skinny knobs. I never noticed that before.

 

[bottombracket]

Mods, this should be a sticky.

Nice work!

I think the skinny buttons are still available.

 

[RHINO]

Mods, this should be a sticky.

I second that

 

[Willis]

The real question is, has anyone got it wired up in an elegant way?

Resister, switches and blower.

 

[TonyP]

Great write-up. This should help a lot of folks adding A/C, heat or refurbing their system.

If this section had a FAQ sticky, this should be in it.

 

[Honger]

The real question is, has anyone got it wired up in an elegant way?

Resister, switches and blower.

Great question.

I lucked out... all of the factory harness wiring for the full HVAC is related to the AC (pressure switch and temp sensor) and blower (motor and resistor), which I already had. I have an AC control module that sits on the underside of the dash behind the glovebox. The heater unit actually has no wiring whatsoever. So I didn't need to add wiring to make the heater work.

The FSM's floating around out there have the wiring diagram for this. But that's not much help if you don't have the AC control module I suppose.

 

[Honger]

Which reminds me of a further component for the AC system, the AC Controller. This unit is fastened to the underside of the metal dash above the glovebox. It has two knobs on it. One is for Temp and is covered by the label as I'm assuming they generally want you to keep it at the factory setting. The other is on one of the edges and is for engine idle speed and would be straightforward to adjust while in place

The only pic I took of it...

This is connected to the AC switch, the pressure sensor, the temp sensor in the AC box, and the ac idle-up vacuum switching valve. Perhaps other things as well...

 

[WarDamnEagle]

Very nice write-up Joel. I didn't get quite as deep as you did but then again I had heat and A/C and was just going in for a deep clean.