88-94 Hvac and heater box: are they all the same?

[loosh]

I have an 88 Cheyenne that came with no ac. Figuring out ac is next on the list of things to spend money on. My question is this: do I need to get a whole new hvac box or can I use the one I have? I kinda figure it’s not as simple as slapping some ducts onto it but I just wasn’t sure if it was the same piece of equipment with less functionality, or an entirely different thing.

You must be registered for see images attach

Pic semi related. There’s them ol heater controls I’m gonna replace with some nice old ac controls. And my old am radio next to new fancy radio

 

[Dropped88]

Yes and no. They all bolt in the same but the early recirc actuator is difference and the blower resistor hole changed In 94 cause the 4 speed resistor is bigger.

But neither would be a problem for you if you get it all from same donor

 

[someotherguy]

You'll need the whole box as your existing setup won't have an evaporator core and there are probably other major differences in a non-A/C box as well.

As Dropped88 mentioned '94 changed as that was first year for the 4 speed blower fan. I'd look at a '94 donor truck if you can, and get everything. Box, control unit, associated dash harness, and the underhood parts too. '94 was first year for R134a also, so any parts you get new or used for a '94 model won't require adapters for R134a.

Richard

 

[loosh]

You'll need the whole box as your existing setup won't have an evaporator core and there are probably other major differences in a non-A/C box as well.

As Dropped88 mentioned '94 changed as that was first year for the 4 speed blower fan. I'd look at a '94 donor truck if you can, and get everything. Box, control unit, associated dash harness, and the underhood parts too. '94 was first year for R134a also, so any parts you get new or used for a '94 model won't require adapters for R134a.

Richard

Found 94 box and vents and controls! Now just gotta worry about all the wiring, although it looks like the box has a lot of harnesses attached to it still so I might be in luck.

You must be registered for see images attach

 

[someotherguy]

I'm also gonna throw this out there, look at your firewall from the engine compartment side and be sure you have holes there for the evaporator core lines. I had a customer years ago that wanted me to add A/C to his later model GMT400 that was an AT&T fleet truck; there were no holes there so they would have to be drilled. If that's the case you'll want to use the donor truck to make a very accurate template so you get those positioned correctly.

If memory serves correctly you have 8 fasteners to remove to get the box out of the truck. Most of them are black screws with a 10mm head and large integral washer; these go right into the plastic of the box. A couple of them are nuts like where the accumulator bracket attaches and also I believe the one under the heater hose connections where the ground strap from the engine fastens. The one that will evade you if you don't know about it is a bolt from the inside of the cab, the only one, and you can actually see the end of it sticking out right between the top evaporator core line and the fuel pump relay. A long extension and 10mm socket will reach it.

Red arrows pointing to the two evaporator line holes in the firewall, red circles around the fastener locations, with exception of the blue circle around the one bolt accessed from inside the cab. Hope this helps.

You must be registered for see images attach

EDIT - LOL, while I was marking up that pic, you were posting that you already got the box.

Richard

 

[Dropped88]

Looks like that box has the hvac wiring harness on it minus control head

 

[Erik the Awful]

EDIT - LOL, while I was marking up that pic, you were posting that you already got the box.

I appreciate you posting that pic. I saved it to Roscoe's build folder so I'll have it for reference.

 

[loosh]

I'm also gonna throw this out there, look at your firewall from the engine compartment side and be sure you have holes there for the evaporator core lines. I had a customer years ago that wanted me to add A/C to his later model GMT400 that was an AT&T fleet truck; there were no holes there so they would have to be drilled. If that's the case you'll want to use the donor truck to make a very accurate template so you get those positioned correctly.

If memory serves correctly you have 8 fasteners to remove to get the box out of the truck. Most of them are black screws with a 10mm head and large integral washer; these go right into the plastic of the box. A couple of them are nuts like where the accumulator bracket attaches and also I believe the one under the heater hose connections where the ground strap from the engine fastens. The one that will evade you if you don't know about it is a bolt from the inside of the cab, the only one, and you can actually see the end of it sticking out right between the top evaporator core line and the fuel pump relay. A long extension and 10mm socket will reach it.

Red arrows pointing to the two evaporator line holes in the firewall, red circles around the fastener locations, with exception of the blue circle around the one bolt accessed from inside the cab. Hope this helps.

You must be registered for see images attach

EDIT - LOL, while I was marking up that pic, you were posting that you already got the box.

Richard

I agree, this picture is VERY useful and I thank you for it