Electronic HVAC controls

I wanted an outside air temp gauge, and had an option of installing a mirror with the gauge, an overhead console with the gauge, or an electronic HVAC head with the readout. Since I like a challenge, I decided to go with the Head Unit. I did some research on line, and found a thread concerning this swap on a Caprice. Using that info, I plunged into the project. Here's what it looks like in finished form.

BASIC INFO

The head unit controls the HVAC in a similar manner to the slide controls we have in our G-bodies. The cable in our temp slide is controlled by a stepper motor with the new head unit. The head unit decides how much to open the blend door to maintain the temperature inside the car, and the motor moves the cable in the appropriate direction to blend Hot and Cold air to maintain the temp setting. . The Vacuum motors that open and close the vents are controlled by a vacuum controller ( a series of small solenoids controlled by the head unit). The blower speed is controlled by a Pulse Width Controller instead of the blower relay and resistors in our current blower setup. The stock blower will work fine.

Installing this unit requires some minor Sheet Metal fab skills, wiring and soldering, and some dremel work on the Radio Bezel. On a 1-10 scale of difficulty, I would say this is a 6. Not a bolt in swap, but definitely doable.

PARTS

Here is a listing of the parts needed for this swap, and some direction on where to pull them

1.94-96 control head- mine is a 96 as there are some internal differences, at least this is what I was told, and it works better. Obtain from a Roadmaster or Fleetwood. Look for the ones with side mounting tabs. Some, like Park Avenues and Lesabres, had top mounting tabs which won't work that good. Get as much of the harness as you can ( this goes for all the parts!!)

2. Mixing motor- electric unit that works in place of the manual cable on the blend door. Obtain from Roadmaster if possible. However, Roadmaster does not have an Arm, so I found a '90's Eldorado typically has two of these motors behind the glove box, and they have arms. I picked them up as well for spares. The arm will transfer easily to the Roadmaster motor. When you get the Roadmaster motor, try and get the part of the HVAC housing that it mounts to. There will be two screw posts and one pin post. I "broke" that part of the housing out and used that to mount the motor in the G-body. Sure beats having to "Fab" something up.

3. Electric vacuum solenoid, in place of the manual tubes from the stock control switch. These can be found in many 90's GM vehicles, but on the Roadmaster it is located behind the glove-box, and is easy to access. Don't worry about getting all the hoses- they are the wrong size for the G-body. Try and get at least 6" of each hose so you can splice into them. Also get as much of the harness as possible. I was able to get the harness all the way back to the control head- makes for easier wiring.

4. Interior temp sensor and tube that is run behind the glove box. Tube attaches to the upper portion of the blower housing to cause a vacuum affect to pull in cabin air so the system can read interior temp for auto control. You can find this behind the glove box on the Roadmaster

5. Sun load sensor. . It is located up above the glove box on the Roadmaster in the grill as shown in the picture. May be easier to pull the grill from the top. Sensor is a twist mount into the grill, so If you can get your hand up there, you can twist it out. I drilled a 1/2" hole in the passenger Speaker grill to mount this

6. Exterior temp sensor, mounted in front of radiator.. I used the same bracket found on the Roadmaster/Caprice to mount mine, so pick this up as well.

7. Blower Control Module. Replaces the resistor module in the fan housing, and the relay. They are used on many '90's GM vehicles. Roadmaster is located up high on the HVAC from the inside of the vehicle. On Park Avenues, it is located under the hood, under the fuse/relay pack at the passenger side cowl. Get as much of the harness as you can, along with the connectors. These Controllers are very expensive as replacement parts ( $175), so pick up an extra one. I paid $5 for mine. It is to big to fit in our blower housings, so I mounted it on the same bracket that the OS temp sensor is mounted, in front of the radiator. using air flow from movement, and from the radiator fan, to provide cooling flow over the heat-sink.

8. Radio bezel from a Monte Carlo SS 86 up, or a suitable way to mount the HVAC Control. The HVAC from the Roadmaster has side mounting holes, which had to be trimmed slightly to fit.

9. If you want a wood grained Bezel, find a '90 Riviera in the pick-and-pull, and grab the wood grain panel from the dash. Cut to size to fit in the radio opening. It already is sized for the HVAC and the Radio. If you want Black, you can use King Starboard ( avail on Ebay, or from your local marine Supplier). You could even paint the Riviera Wood Grained part black. Since it is already nicely formed, I would go this route if I could find one.

10. 1 ea MOSFET part number ZVN4206AV. This is used to convert the low level AC request signal from the head unit into something that will drive a normal auto relay for the AC compressor. Available from AvnetExpress.com. They cost $.35, so buy a couple of spares. Here's a link:

http://avnetexpress.avnet.com/store/em/EMController?langId=-1&storeId=500201&catalogId=500201&term=ZVN4206A&N=0&Ne=100000&action=products&x=23&y=6

I bought a few extra of these. If you want them, I will sell them for 2 ea for $4 shipped in the US via USPS. My contact info is at the bottom of the Home page.

11. 1 standard Automotive Relay. 5 amps should be enough, but they are usually 30 amps.

12. Wire, solder, shrink wrap, 1 ft 1/8" vacuum hose, in-line fuse holder ( 20 amp)

Optional- radio from a Mid-'90's LeSabre. It has an extra line out on the back that connects to the HVAC control to dim the HVAC display at night. You can put this radio in the boot or use it in place of the stock radio ( 1.5 din). Many people just live with the brighter lights at night. If you go to look for one, be sure to look for a separate connector on the back that has a PPL/Wht wire.

WIRING

I was able to re-use wires that are currently providing signal/power from the stock HVAC control to the Blower Relay and Resistor. Doing this means that no new lines need to be run through the firewall. You will have to cut the lines inside at the connector behind the glove-box, and again cut them at the Relay and Resistor under the hood. You will then splice into these wires under the dash to the new control head , and splice into the wires under the hood for the Blower Controller and Ambient Temp Sensor. I've attached a wiring diagram. It gets a bit busy, but if you follow along it will show you wire colors, etc( HVAC Wiring G-body.jpg). When you first open this diagram, you may have to Zoom in your browser to see it in more detail. I have also attached original wiring diagrams from a Roadmaster.

One of the trickier parts of getting this to work was taking the AC request signal from the new Head unit and getting the AC clutch to engage. Originally, I thought that this was a 12v signal, and I could use this to operate a relay. Couldn't get the relay to operate, and started a series of experiments to fix the problem. Found out that there is only a few MA of power available on this line. Even a small PCB relay that takes only 15 ma would not operate. Final solution was a tiny little MOSFET device ( three conductors) . This one has been designed just for this purpose, to be used in an automotive application. It has built in protection from reverse polarity and surges, as well as feedback from relay cycles. It takes the signal from the head unit ( Dk Grn/Wht) at the Center Conductor, and when it sees 12v on this line, then closes the circuit between Ground (Right Conductor) and it's other leg. This other Conductor ( Left) provides a ground to trigger a standard automotive relay. Power then flows through the relay contacts to the AC Clutch ( By way of the Low Pressure Switch on the Accumulator). The MOSFET can handle loads of 300ma. The Auto Relay draws about 150 MA, so should be no problem. The MOSFET is ZVN4206AV. Availability was detailed in the parts section.

Ignition On Power and Ground for the head Unit come from the original HVAC controller. Constant Power comes from taping into the Radio circuit, and Dimming wire from the light in the stock HVAC unit. This same Ignition-on Power source can be used for the relay for AC control. If you were able to keep the wiring harness complete for the Vacuum Controller and The Blend Door motor, then a bunch of your wiring is done.

BENCH TESTING

I found it useful to bench test all the components before installing them. I would recommend wiring up a blower motor to the controller, but be sure you secure it before power is applied. Once power is applied to the head unit, the blend door motor will cycle through full cold, and then full warm modes, before anything else starts to happen. This takes maybe 30 seconds, so be patient. Once all the cycling is done, the head unit will select where the blower output will come, and then adjust the blower speed. I also pulled the blower from a '95 Park Avenue, thinking I had scored a more modern blower. Size and fit was perfect, but it runs in the wrong direction. Don't waste your money.

Upon power-up for the first time, the electric motor will cycle full cold , and then full hot. This is to find the limits in the system, as each installation is different. Once the limits are found, then the head will determine the best location for the blower output and blower speed, and then turn on the blower. You should be able to adjust the blower speed up or down. You can adjust the temp to 60, and see the motor turn to full cold. Then adjust your temp to 85, and see the motor move to full hot. The ambient temp reading should be that of the area you are working in.

For the vacuum controller, resistance between Terminal 6 ( Brn Wire) and each of the five other terminals should be between 70-120 ohms each. Resistance of Outside Temp sensor should be between 12,000 ohms and 1400 ohms- Higher temps lower resistance . Resistance of Inside Temp sensor should be between 100K ohms and 282 ohms. Higher temps =lower resistance. 75F= 3000 ohms approx.

I had always wanted a 12V power supply for the bench, and found instructions for taking an old AT Computer Power Supply and converting it for bench use. I'm including it here because it is a useful tool. http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

ON BOARD DIAGNOSTICS

You can also access on-board diagnostics. If you hold the TEMP ( up Arrow) switch and OFF at the same time and hold, the head will enter diagnostic mode. It will perform a self test of the display segments, and then will display "-00". Pressing blower speed ( Up Arrow) to select "-02" will display any AC system diagnostic codes. Press MIDDLE switch, and the codes, if any, will be displayed. The codes are

On the one I got, there was a hole in the arm approx. 1" from the center of the motor pivot point. In this hole I inserted a 1" #6 machine screw, with a self locking nut. A second self locking nut is used on the end of this screw to hold the end of the actuating cable captured on the screw. If you want to use the Roadmaster motor, you can "tap" this arm out of the Caddy motor using a 1/4" bolt and hammer. Just be sure to support the motor appropriately. The shaft that runs through the motor is splined, and there are two small notches on the front side. These notches align with the "Stops" that are on the back side of the motor. The arm should be aligned in line with these notches. The motor will be positioned in the car upside down from what you see above. Full cold turns the motor clockwise to the stop, while full hot turns it counter-clockwise to the stop. In the G-body, full cold requires the cable to be pushed towards the passenger side of the car, hence the upside down position.

The first picture show the motor with my original arm. The second picture show the motor with the new Caddy arm. If you can't tell, the motor mounts to the HVAC box behind the Radio, and a little to the left and below. It is nearly behind the ALDL connector , if that helps. I was able to get behind the panel and use nuts and bolts. You can also use self tapping screws. As you can see, having the cable attached allows you to see the exact angle and position the motor needs to be mounted. Not much leeway here without the possibility of cable binding. Also, lube the cable prior to installation. You can unhook from the heater box end through the glove box opening, if that helps.

10) Install the Inside Air Temp Probe. To do this, you have to drill a hole in the side of the air box. I think this can be done through the Glove-box opening. I drilled a 1" hole, but I think you can get by with a smaller one- maybe 5/8" or 3/4". The Venturi mechanism had 4 small tabs on the bottom to mount in the original car. I cut these off, and used a couple of Self Tapping screws to mount it.

The hose attaches to the bottom of the venturi port, and extends the sensor to a remote location. I used the gap between the Glove-box and the Lower Sound Dampener for my placement.

11) Install the Vacuum Controller. I used a piece of flat sheet-metal to make a simple bracket. You want to attach it to the screw post that can be seen in the picture above, just to the left of the venturi Mechanism. You want it to sit as far back as you can achieve, almost touching the Venturi.

Now you will cut and splice all the rubber hoses. The ends of the Roadmaster hoses are too large to work on the g-body. The color codes are basically the same. Purple is the vacuum source from the engine. Orange is recirc valve. Tee into this orange line the hose that goes out to the Water Control valve. When the system calls for RECIRC, it will shut off the flow of hot water through the heater Core. The Grey line gets Plugged Off and is not used. Red goes to Red, Blue on the controller goes to Tan, and Tan on the controller goes to Blue. This is the only difference, as it will open the middle , right and left vents when AC is on. If you hook them up with direct color match, the vents don't work as they should. Use a short piece of vacuum hose ( 1-2") for the splices. A little silicon spray makes them easier to insert.

12) Mount the Sun Sensor. I drilled a 1/2" hole in the passenger Speaker Grill. A little dab of RTV or hot glue on the back side will keep it in position.

13) Wiring. Time to Git-er-done! Use the Wiring diagram below (HVAC Wiring Gbody.jpg). Take your time, solder all the connections, and use shrink wrap. Run your harnesses neatly under the dash, and also under the hood. A piece of 1/2" 10' split loom will run from the front of the car to the blower area, where the connections are made to the inside of the vehicle. Best to disconnect the negative Battery Cable while all this is going on- better safe than sorry, and it will prevent a drain on the battery from having the doors open. The diagram uses existing wires from the stock HVAC system, so no wires have to be run through the firewall. Cut the wires at the Relay and Resistor connectors, and Cut them inside the dash at the connectors that attach to the stock HVAC Control head. Leave enough slack inside for wiring, and enough on the connectors so they could be re-spliced into the system if you ever wanted to go back to stock.

The only tricky part is wiring in the MOSFET. They are easily damaged by static electricity, so be careful. The Device has a flat side and a rounded side. When the flat side is down and the leads are pointed at you, Ground goes to the right connector, AC request from the HVAC head unit goes to the middle connector, and the left connector provides ground for the Automotive relay that will drive the AC clutch. This is a low voltage device, but be sure the Ground and Relay wires are large enough to carry the 150MA load- 18 or 20 guage should be OK. A convenient place to mount the relay and MOSFET is on the HVAC unit below the new Vacuum controller. There is another screw stud there for the relay mount, and the MOSFET can be shrink-wrapped in the wiring harness. You might even be able to hot glue the MOSFET directly to the side of the relay- just don't know about heat dissipation. I have mine in a small plastic box mounted to the HVAC unit with a push in "Christmas Tree" fastener.

Double check all your wiring, re-connect your battery, and test. Be sure the Blend Door is fully closed when the temp request is at 60. You can find it behind the Glove Box up high on the HVAC housing. For reference, this is where the Adjustment Cable terminates. You may have to adjust the door using the threaded rod on the blend door bracket. Push on the end of the rod while holding the round plastic part it slides through ( I used a pair of channel lock pilers) to move the door to it's fully closed position. This is the most important position, as the car can make plenty of hot air, but cold air on a hot day is at a premium.

Radio Bezel Modifications

My vehicle has a later model Monte Carlo SS (86-88) bezel for the radio. I cut the HVAC opening to match up with the side of the Radio Opening. The way the original opening was made for the HVAC controls, the upper edge is slanted. When you cut across where the Cig lighter now resides, you can't get the same slant- just no material there to work with. I decided that the best solution would be a cover plate that fit on top of the bezel. The rest of my dash is wood grained, and I just happened to find that a '90 Riviera has the same wood grain in it's dash. I popped the wood grained portion off, and used that to build my cover. If your dash is black, I might recommend King Starboard as a substitute. You can find it on Ebay or at a local Marine Supplier. or just paint the Riviera piece black. I cut the Riviera dash to fit the opening. I have a pattern that is attached that you can use for the cutting process regardless of which product you choose. Because I had also installed a dash cap, the upper two screws of the Bezel were no longer functional. I drilled a third hole in the bezel and cover in the upper left hand side to hold the Bezel firmly in place. I could have done the same on the right side, but there was nothing to screw into without fabing something, and I guess I was getting lazy. I attached it to the stock Bezel with some Plastic Weld epoxy, but you could use hot glue, double sided tape, or maybe even Velcro.

If you are using the Buick Radio, so you can get Dimming on the display, then the Ppl/wht wire that is on a separate connector on the back will connect to the same color wire on the head unit. One additional wire will be needed to the radio- Lights On. This is different from the Dimmer Wire (grey). It is a yellow wire, that gets 12V when either the parking or headlights are on. I connected this wire to the Brown Wire coming from the Headlight switch.

You can see the PPL/Wht wire on the upper left connector. The green wire on that connector is for steering wheel controls- maybe another project???

WRAP-UP

Hope this works as well for you as it has for me. A couple of points on operation. Setting 60 degrees on the temp request activates RECIRC. Any higher, and the RECIRC is off. When RECIRC is on, no hot water will flow through the heater core. When off, the controller will "Blend" hot and cold air using the blend motor to achieve requested temperature.

The OS air temp works fine when the car is moving, but at a stop light, it will creep up. I discovered this is a problem that was addressed in the PCM of the Roadmaster, holding the temp if the car was not moving. I may try making a shield to surround the sensor, but I can live with it for now. In retrospect, if you can mount this away from the heat of the engine you will likely get a less "responsive" reading.

For more reading, and as a credit for much of the knowledge here and with helping me through this process, you can view this thread:

http://www.impalassforum.com/vBulletin/showthread.php?t=230505&highlight=roady