Modern donor vehicle for 72 Chevelle AC

[resto_newbie]

Has anybody ever swapped a late model 80s-90s AC and heater from, say Crown Vic or Caprice, into a GM A-body vehicle (circa 72)? I feel like this should be possible. Seems like the vintage air units are really similar to the more modern factory units so there should be a factory equivalent to them for the A-body.

Ideas please! pictures?

Thanks

[mikes2nd]

should work fine, I don't know why people don't look for swaps more often.

Beware the bigger cars though and watch out for the controls. If its too modern it will be integrated into the body computer and stuff. I would think, late 80's early 90's cars (maybe a mid size or smaller), the AC should still be a separate self controlled unit. once you get mid 90's they are integrated pretty heavily and you will be digging through wiring harnesses and trying to hack a computer controller in there(no fun)...

[dontlifttoshift]

The oems didn't start using 134 until 1994. Anything before that will run on R12 and you will have to retrofit or find some R12. Neither of which is impossible, just putting it out there.

Another thing to consider is that most oem setups of that vintage relied heavily on engine vacuum to operate the doors in the unit and/or the heater control valve. That may or may not be an issue in your case.

Hopefully Greg Fast will chime in here......it seems he knows more about it than most people should and offered a lot of insight in this thread. https://www.pro-touring.com/showthre...efore-purchase

[Twentyover]

Ok, deal me in

I like that- more than most people should


First, need to clarify. When you refer to Caprice and Crown Vic units, what you're really saying is underdash units, where the major components are not in the engine compartment. Looking for a smoooooth firewall. Yes? With regards to 134a, the HVAC UNIT won't care. Maybe need to change TXV or orfice tube if the system was run on R12 before and you want to plug the hole in the ozone layer- usually not necessary, but good for system optimization. That said, most underdash systems I'd consider are native 134a. And I'm not sure I'd gofull size. Many full size units use the same blower/involutes and evap cores as intermediates, they're just bigger to accomodate the car size, no significant additional performance.

Sidenote- There is still plenty of R12 out there. I sold my last 30lb can for $125 last summer-demand is dropping since there are fewer R12 cars in service and those that are are getting rustier. May be tough to ind, since most shops are droppingtheir servide equipment.They don'tsee it often enough to maintain the equipment. End of sidenote

Three big areas of consideration-

1) Unit Fit
2) Controls
3) Plumbing

Starting at the top....

Yous got to fit it in the IP. Best thing to do is CAD- Cardboard Aided Design Find out where and what size voids you have available to you, because there's one piece you pretty much need to stuff in there unaltered, the distribution housing. We can play games with most the other stuff, but if you start fooling around with the distribution housing you increase the time and effort by.... alot

Now to your resources. There are in general two system configuration types- lateral flow and center stack. I'd guess you want to look at lateral flow units. Even though center stacks take less volume, they need a deeper IP to fit. A lateral flow unit is identified by a blower motor above the passengers outboard foot, with a duct feeding air into an Evap housing area (expansion volume) or directly into the distribution housing.

So you've found yourself a big chunk of real estate you plan to park the distribution housing in- so what is a distribution housing? It usually contains the heater core, mode (outlet) and blend (temp) control doors and actuators (cable, vacuum, electric- a couple types of electric.) Sometimes it will come right off the face of the evap, sometimes the evap will be located remotely. If it comes off the face of the evap, you'll most likely have a freeze control device there also. If it comes off the face of the evap, you probably should consider keeping it that way.

Let's assume for the sake of argument the evap is located in the expansion volume (as area of the duct coming off the blower cutoff expands, it causes the air to slow and it's pressure to rise.) So take the HVAC unit and hack it into about 3 pieces- you have a blower section, an evap housing/expansion volume, and a distribution housing. It's OK to get in touch with your inner Dexter here. Just make sure the blower is sliced past the cutoff (the small portion near the outlet.) You would do well to also investigate before you do this- WHERE IS THE BLOWER COOLING CHANNEL? A blower motor has a set of brushes, these are carbon/copper and a bunch of other stuff. When they pass current, they heat up, and need a stream of cooling air over them. When that doesn't happen.... I've been told it looks like sparklers.... Anyway, include the section of housing that has a rubber tube or a part of the blower motor housing molding containing an air channel (can I word this any clumsier?) in the blower section of the corpse.

Decision point-planning on running reirc, fresh, or both? The blower housing will have a fresh/recirc door. If you want to run either exclusively, the blower air inlet (usually a separable part of the unit) can be modified. If you want both, you need to get creative tying the fresh section to the implante's fresh air section. then screw it into position. I know I'm treating this a little cavalierly, but it's significantly a function of trial and error, sawing off bracket sections that don't work, making new ones, and making what you have fit. Downside is the housing will most probably
20% talc filled polypropylene. It's almost universally used by OEM's and it's almost perfect for this use- but there's almost no structural adhesive that will bond to it, meaning you need to plan on making mounting brackets and mechanically fastening it to the housings- while insuring you don't interfere with the motion of doors or kinematics.

My fingers are tired, I'll type more later

[MIKE67]

Subscribed. I have been thinking about doing this for a while. Greg, I appreciate your input .

[rb70383]

I was just thinking of this the other day. Question si what about the controls? Some are servo and others are still cable driven. My 01 dodge ram is servo driven, at least the blend air door is. Doesn't seem to be integrated into the body at all. Wonder if something like this would work for retros?

[lsx80monte]

Some of these newer units are pretty big.I tried to use a rear unit from a 2000 tahoe but just couldnt get it to go in my monte.

[Twentyover]

Spoke briefly about cables and vacuum door control, think these are pretty obvious.

Electric actuators come in two basic types- feedback and non-feedback, and variations in control strategies result.

A feedback actuator gets an input from a control head, moves the actuator into demand position, and tells the control head 'OK, I'm here" usually this is through a potentiometer (rheostat) built into the actuator.There is an input voltage sent to the pot, and it (the pot) replies with a voltage between 0 and full input voltage to the control head. the control head may use this feedback to trim the actuator into a better correlation with the desired position programed into the control

A non-feedback actuator gets an input from the control head, moves the actuator into demand position, and tells the control head....nothing.
So how does the control head know actuator position? It doesn't KNOW, it ASSUMES. I'm most familiar with pulse count actuators. These non-feedback actuators actually do provide indirect 'close enough' feedback. Here's how they work- The small DC motor in the actuator, just like every other DC motor,has brushes and commutators to generate electromagnetic fields in the armature. .As the motor is turning, the current flow to the armature is made and broken as the commutator bars move past the brushes.This causes a change in current - call it a spike or pulse. In a 12 commutator motor,there will be 6 pulses each motor rotation (12 comm bars divided by 2- 1 positive and one negative.) So each motor rotation yields 6 pulses,say you have a 50:1 gear reduction in the actuator, 300 pulses will be 1 rotation of the actuator output shaft. Control head counts the pulses, and knows that for a given door position, it needs say for instance 4.2 revolutions of the output shaft- so it counts 6 x 50 x 4.2= 1260 pulses- and assumes the door is in position. It also knows how many pulses it needs to go somewhere else when the control is adjusted to a different position.

Based on the differences in style of control, i think you can see the risk in mixing and matching control heads. Also note that a control head from one supplier (TRW, KDAC, there's a bunch out there) can service several different platforms, and internal programming and pcb will be different between the platforms and blower speed control strategy. Just cause it looks the same, doesn't mean it is.

Another point on pulse count (and I assume other non-feedback actuators)- pulse count is 'accurate enough' for most applications, but if many changes in setting are made, it will start to loose control of position, as it will miss counts, actuator location will not be exactly as described in the programming, clearance and hysteresis in the kinematics, and normal variation between parts. To prevent system chaos, most manufacturers will park the doors. At my prior employer, 4 minutes after key off, doors would go to park position, recirc, floor, hot, regardless of the control location at key off. This indexed the actuator position so the control head knew where to start counting pulses at the next key on.

Since this kind of morphed into control heads, i'll address interface with body control modules. In the main, if the platform has body module, you need to assume it's probably interfacing with the control head. commands go from the control head to a CAN bus prior to execution.CAN bus feeds back information for executing stuff like warmup strategies on ATC- the control head uses existing input from CTS, IAT, etc on the bus to perform these 'special teams' strategies. On test trips we would read door position, outide air, and coolant from the bus- just like the control head does- from a display tied into the CAN bus (at least where the control head thought the door was- it was accurate enough).

While working at an OEM,you will end up going on test trips.These climatic trips are used to determine a couple things- Does the control head programming do what we've instructed it to do? Did we write the correct instructions- is it doing what we instructed it to do, but we make a mistake somewhere in what we were asking it to do? And finally,are our outlets up to the task? Are we directing air where we think it should be, are we encountering any obstructions preventing air from reaching it's target location?

Lest anyone think this is in the least bit glamorous- You roll out at 4:30 am in January in the Keweenaw peninsula- car has set outside all night, it's 20 below. As the car master, you go down a half hour before the trip start, open the doors on the car, go into the trunk to light up the data acq, plug in and fire the laptops used for recording and secure the BCM interface/display. 15 minutes before start the team starts to show up. everyone stands outside the car- a single body in the car will raise interior temperature about 1 degree per minute. Load 4 bodies in too soon and you're moving the baseline.You're not wearing snowsuits here, usually a light boot, jeans and T-shirt or golf shirt with sweatshirt and a winter coat. Approximating what one would wear going to work. 5-15 minutes standing or walking around to emulate walking through a parking lot.

Or summer-August afternoon in Phoenix or Tuscon. Car was driven, parked in the sun. windows up for an hour. You're wearing business casual.4 guys pile in and drive, windows up the entire time.




Looks like i drifted off-topic a little sorry. Next chapter will be kinematics, then maybe back to plumbing and loop back on unit fit


More later

[MIKE67]

Thanks Greg. This helps a lot (and also when trying to troubleshoot the daily driver).

[Twentyover]

I'll go back and maybe emphasize something i touched on earlier- size doesn't really matter. Just keep telling your girlfriend/wife/companion this is the case- it's worked for me for 25 years. Stay away from pickup truck units or full size units. There is little significant performance improvement, they usually just bigger and harder to package. Look at fwd intermediates or compacts, maybe a fox mustang or smaller Japanese SUV's. Any performance reduction (and there won't be much) will be mitigated by the ease of packaging

OK, I'll venture into controls.

First, this IS NOT my area of expertise- I work downstream on the HVAC module, I simply obey the orders the control head sends. That said, there are generally three types of HVAC controls- cable, vacuum, and electrical actuator. Pick a unit that has a control head you can live with, because you're going to do just that. I'll try to detail an option that I've seen to let you keep your existing control later in the post.

Cable controls are used forever until maybe the 60's. At that time, there was no fresh/recirc option there was only mode (outlet) and blend (temp) controls. The two cables, along with a fan switch, moved the blend and mode doors to control airflow direction and temp. Cables were generally abandoned as systems became more sophisticated, although they are still currently used on lower end systems for blend and less frequently mode control. This is because a single zone system (no dual zone option) will have a single blend door, but may (meaning will probably) have several mode doors. When controlling a single function like temp, a cable is still viable. If the system uses cam type kinematics, a cable can control mode also

In the 60's and 70, vacuum motors were introduced. These are binary, a door is either open or closed. requires the use of multiple mode doors to control airflow.Nothing wrong with them,but they dictated multiple mode doors and are no good for controlling transitions.You get hot or cold, defrost or floor,unless you cleverly (and complexly) generate intermediate stops. Vacuum works well for fresh/recirc and other remote controls.

This kind of takes us to state of the current art. Most current systems use electric actuators to position fresh/recirc,blend, and temp doors. Many also use blower control modules,rather than resistors, to control fan speed.

I'll take a swing at blower control first. Typically, a resistor or a blower control module is used. Resistors have several resistances, that control voltage delivered to the blower motor. They can be wire wound, pcb,or ? I won't claim to know all types. They an be identified by having a fixed resistance between terminals. Fan switch controls circuiting hence resistance,and thus blower speed. The resistor must live in the airstream for cooling- it can convert up to maybe 150 watts into heat, will toast easily if provoked.

More recently, blower control has been addressed with a blower control module. Known by several names, I'll refer to it as an LPM (liner power module) since that's the acronym I'm most familiar with. LPM's may look like a resistor on the outside, but will have aluminum fins to dissipate heat on the airstream side. The LPM converts a low current pulse at a fixed frequency and varying width to steady voltage. As pulse width increases, so does voltage and current delivered to the blower motor. Same as your fuel injectors- a longer pulse delivers more wattage (fuel) to the blower motor, and it spools faster. An LPM also needs to live in the airstream- at about half throttle it's maybe kicking 125-175 watts heat into the system, Because I'm writing the next paragraph, I'll refer to this as a pwm/LPM control.

Coming soon to a car near you will be a true PWM/PWM system.Unlike an LPM system, a PWM/PWM will use the low current pulse from a control head to switch full voltage and current off and on, rather than dumping some as waste heat. You end up with an effective voltage, the arithmatic average of full voltage times % time on divided by full voltage. Since this has a big potential for RFI, location of electronics (engine management, body control module, infotainment) becomes a question. Some of these PWM/PWM are separate modules like the LPM, some are piggybacked into the motor to reduce RFI antennae length. Don't need to worry about seeing these in a boneyard for a few years, but the plan is to implement some platforms in 2014.

OK,covered blower control. With the information above, you may want to think about whether to use a resistor or an LPM blower speed control when adapting a prod car system.

I'm closing this missive here and will open another on actuators, because assure as I'm sitting on my ouch, if I don't, my computer will crash and I'll loose what I've typed... err, keyboarded

[MonzaRacer]

Honestly if your underdash system is functional all it takes is swapping out the hoses. As for me I have been scrounging parts from salvage cars, collecting the aluminum lines, fittings and such. I have used an inline orrifice tube repair adapter commercially available, slide in a AAOT in place of the CCOT(automatic adjusting orifice tube in place of the cycling clutch orifice tube or fixed orifice) Honestly updating the currently installed parts isnt hard, you just have to understand AC and fundamentals. after that knowing parts and operation you can swap in just about any part.
On older GM cars getting away from POA/STV helps a lot. NOW if your system uses a sensing bulb to control the H valve, then you only need to flush, swap hoses, add ester oil or ROC oil and recharge till pressure numbers come to normal standards.
Also dont forget some of newer cars have such tiny systems that they use very little refrigerant, and similar to vintage air, this actually over works the system a lot. Been there, worked on them.
On my 71 Monte I replaced high side line with compression fitting type fittings crimped to galaxy barrier hose, even with NO fan my 71 would throw 38 degrees out vents .