C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

C68 HVAC Programmer Info & Schematic

 
Old 12-26-2011, 04:40 AM
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Cliff Harris
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Default C68 HVAC Programmer Info & Schematic

When I had the fire in my engine compartment, the plastic vacuum lines through the firewall were melted and there was nothing left but two stubs. I didn't know which was which when I got the dash apart. There is one black line and one white one. I had to take the HVAC programmer out to see what was happening. It's right above the accelerator pedal and is held in place with one 7mm screw.

I did some Internet research on this thing and found this site:

http://www.batee.com/corvette/acrepair/c68pgmrboard/

I found several problems with this site. The site owner posted this information that came from somebody else, so he couldn't check the info. He insists that the high power resistors that get hot are 340 ohms, but they are in fact 140 ohms, as his source stated. The drawing that shows the vacuum connections shows 6 connections, but there are actually 7 [ EDIT ] The number of vacuum solenoids changed from year to year. By 1996 they were down to 4 solenoids. [ /EDIT ] There are numbers molded into the plastic vacuum connector, but he shows different numbers. He also numbered the PC board edge contacts, but there are letters molded into the connector. There is a resistor network with 368 ohm pullup resistors on all the outputs of the IC, but he couldn't seem to figure this out.

I had a lot of trouble getting the vacuum connector apart and I will post pictures and the explanation later. It's not at all obvious how to do this.

The schematic. I included the "temp valve actuator" (inside the dotted box on the right side), which turns out to be a motor with a gear train and arm that opens and closes a door that determines the percentage of air that goes through the heater core. There is a feedback pot there that tells the controller the position of the temp door.

The connection between the HVAC programmer and the control head is very interesting. It uses only two control lines to control 8 outputs. The shop manual for my '86 (the first year for the C68 option) describes 5 vacuum solenoids but there are actually 6. It also describes the signals on the control lines as various DC voltage levels. The explanation is really cryptic and makes no sense. Further investigation is needed. So far I found this combination that controls the temp door motor:

T = 5 volts, U = 0 volts makes the temperature go up.
T = 5 volts, U = 5 volts makes the temperature go down.

M = 8.8 volts, N = 0 volts: temperature up.
M = 0 volts, N = 8.8 volts: temperature down.

I put in the colors of the vacuum lines that correspond to the vacuum solenoids they are connected to. The supply vacuum line is black. The heater core water bypass valve vacuum line is white. I haven't figured out the rest yet.


Last edited by Cliff Harris; 03-18-2015 at 01:19 AM. Reason: Added note on the number of vacuum solenoids.
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Old 12-26-2011, 02:19 PM
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White is eliminated by '88. Solenoids are Defrost, Front (vents), Rear (heat) and Outside Air.

GM pretty much uses the same colors for the vacuum line solenoids regardless of option (electronic or manual) or make (Chevy, Pontiac, Buick, etc): Red - Defrost solenoid; Blue - Vent; Orange - outside air; Pink - heat. Defrost has 2 lines which by default, is sucked closed and the second line sucks it open. On the Vette that 2nd line is usually Tan. Outside air can have a second orange line that doesn't hook to anything. That allows the door to move slowly and not just slam shut.
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Old 12-26-2011, 03:16 PM
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Default Now With Pictures!

This is a connector that connects the plastic vacuum lines from the engine compartment (two black lines on the right) to the HVAC programmer. The pair of black lines disappear into the wiring harness that goes through the firewall harness feedthrough. I heard that the feedthrough sealer will melt if you heat it and will then reform. I haven't tried that yet, so I can't verify that. There is no way to tell which one is coming from the vacuum source (the vacuum check valve next to the distributor) and which one is going to the heater core water bypass valve. The colored vacuum lines on the left are black for the vacuum source and white for the water bypass valve. There is a mounting barb on the bottom, but it was just floating around inside the dash. I don't know if there is a hole somewhere that this thing is supposed to mount in.



The various vacuum lines come to this vacuum hose block, which mounts on the back of the HVAC programmer box. It appears to be made of silicone. The retainer that holds this block in place is buried in the middle of the hoses and is impossible to see unless you know to look for it. More on that in the next pic.



The vacuum hose block plugs into the right side of this connector, which is mounted inside the HVAC programmer box by the screw and nut. There is a hexagonal depression molded into the left side of this connector and it is not possible to unscrew it from inside the box. The push-on retainer clip on the right is what is buried inside the vacuum hose block. This needs to be taken off to get the vacuum hose block disconnected from the box. After that is off you can unscrew the nut and take the connector out. The various vacuum hoses inside the box are connected to the left side. You normally wouldn't disconnect the hoses, but I didn't know about the clip and couldn't get this apart. I drilled out the head of the screw. Now I need to find a screw with a 7mm head to replace this one.



This is what the back of the box looks like. You can see the vacuum connector with the screw and nut that hold it in place. The black and silver part is the rear window defroster relay. Just underneath it is a place molded into the box for it to mount. I mention this because this relay was taped to the harness. There is no interference when it's mounted to the box so I don't know why they didn't mount it where it was obviously designed to go.



The is the vacuum connector mounted inside the box. The head of the screw fits into a depression in the connector and you can't unscrew it from this side. I tried to unscrew it, and finally ended up drilling it out, which is why there is a circular mark in the center of the screw head. The vacuum hoses from the vacuum solenoids go on this connector. As I said earlier, you wouldn't normally disconnect these.



The vacuum solenoid board mounted in the box. This is the position it normally sits in, with the electrical connector on the bottom. The shop manual says the board has to be in this position for testing, so apparently the solenoids depend on gravity to work correctly. The horizontal black object is a manifold that connects the vacuum source to all the solenoids. The output vacuum lines are all loose here. Normally they would be plugged into the vacuum connector. The two tabs on the top of the box slide into slots and the box is held in place by one 7mm screw in the lower left corner. The screw that holds the PC board into the box has a 1/4" head, another little weirdness in a mostly metric car. The infamous overheating resistors are located in the lower left corner of the PC board. You can see that the PC board is discolored from the heat. I bent the leads to lift them off the board and then put 5 minute epoxy under them to hold them in place. That was after I heard the "snap!" of the circuit traces popping off the PC board. The copper traces are glued to the PC board and overheating them will weaken the glue bond.

These resistors are 140 ohms, 5% tolerance, 1 watt. They are under-rated for the application, as the actual power they dissipate is V * V / R = 13 * 13 / 140 = 1.2 watts. They should be replaced with 140 ohms, 5% tolerance, 2 watt resistors.



This is a picture of the top of the temperature door motor housing. More non-metric weirdness, as all the screws that hold the housing to the bracket and the motor in the housing have 1/4" heads. The arm on the left with the threaded rod attached is what opens and closes the temp door. The threaded rod snaps into the white button. It's supposedly adjustable but I couldn't find any instructions anywhere on how to do the adjustment.



This is what the temp door motor looks like. The shadow is on the top because I flipped the picture upside down because this is the position it normally sits in. The part sticking up on the top left is a knurled bushing that the arm is pressed onto. Above the motor is a gear train that lowers the speed WAY down. The arm moves very slowly. At the bottom left is the potentiometer that tells the controller the position of the temp door. The only other parts on the PC board are a couple of wire jumpers. There are traces and holes for a 14 pin IC and several discrete components but no markings, so I have no idea what would go there. I have heard that these motors are also used in Cadillacs with the automatic climate control, so that is a possible source of parts if your unit fails.


Last edited by Cliff Harris; 08-31-2013 at 06:31 PM. Reason: Added info on 140 ohm resistor power dissipation.
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Old 12-26-2011, 03:43 PM
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I did find the following page in the shop manual. There is another picture on page 8A-201-12 that is more pictorial but it doesn't show any vacuum line colors. I don't understand what they mean by "front mode" and "rear mode" door motors. They are located right above each other according to the illustration in the C68 section. I haven't located these guys. They appear to be behind the radio (which I haven't removed yet). It's not at all clear what they do from this picture.


Last edited by Cliff Harris; 12-28-2011 at 01:54 AM. Reason: Added reference to page 8A-201-12
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Old 12-27-2011, 11:57 AM
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Front is Vent; Rear is Heat. That's how the Designers see it coming out of the Box.
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Old 01-13-2012, 01:26 AM
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I tested the HVAC programmer to see which solenoids are activated in the various modes. Here's the result:

Code:
        L1     L2    L3     L4     L5     L6
        ORG   PINK   TAN   WHITE   RED   BLUE
OFF     --     --    --     ON     ON     ON 
AUTO    ON     ON    ON     ON     --     --
BI-LEV  --     --    --     --     --     --
ECON    --     ON    ON     --     --     --
HTR     --     --    --     --     ON     ON
DEF     --     ON    --     --     ON     --
The shop manual's description of how the AC control head communicates with the HVAC programmer is totally wrong. The names of the signals and solenoids keep changing and it turns out that the voltages are all wrong also. Apparently they changed the design between the time the shop manual was written and the system was manufactured. From the 1986 FSM, page 1B1-4:



I couldn't see any change in the voltages on the inputs (pins T & U) with my DVM, so I used my oscilloscope to see what was really going on. Very surprising. No DC voltages here. It's all a series of bursts of pulses. The yellow trace is the voltage on pin T. These pulses are continuously changing. The blue trace is the voltage on pin U. There are various combinations of number of pulses and pulse widths for the various modes. Here's the DEFROST mode:

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Old 01-13-2012, 10:05 PM
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Default Pwm

Pulse Width Modulation (PWM) is used to control blower motor speed. Could this be the waveform you're seeing?

Originally Posted by Cliff Harris View Post
I tested the HVAC programmer to see which solenoids are activated in the various modes. Here's the result:

Code:
        L1     L2    L3     L4     L5     L6
        ORG   PINK   TAN   WHITE   RED   BLUE
OFF     --     --    --     ON     ON     ON 
AUTO    ON     ON    ON     ON     --     --
BI-LEV  --     --    --     --     --     --
ECON    --     ON    ON     --     --     --
HTR     --     --    --     --     ON     ON
DEF     --     ON    --     --     ON     --
The shop manual's description of how the AC control head communicates with the HVAC programmer is totally wrong. The names of the signals and solenoids keep changing and it turns out that the voltages are all wrong also. Apparently they changed the design between the time the shop manual was written and the system was manufactured. From the 1986 FSM, page 1B1-4:



I couldn't see any change in the voltages on the inputs (pins T & U) with my DVM, so I used my oscilloscope to see what was really going on. Very surprising. No DC voltages here. It's all a series of bursts of pulses. The yellow trace is the voltage on pin T. These pulses are continuously changing. The blue trace is the voltage on pin U. There are various combinations of number of pulses and pulse widths for the various modes. Here's the DEFROST mode:

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Old 01-14-2012, 12:52 AM
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This is the HVAC programmer. No connection to the blower motor. See the schematic in post #1.

From what I could tell, the signal on pin U (blue trace) controls the vacuum solenoids and the signal on pin T (yellow trace) controls the temperature door motor.

Last edited by Cliff Harris; 01-14-2012 at 12:53 AM. Reason: Clarified location of schematic.
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Old 02-04-2012, 02:16 AM
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You might want to take a look at a later shop manual. It sounds like the documentation in your year's manual is sketchy.

The '94 manual describes the operation pretty well including much of what you've found experimentally.

If there are any specific questions you have about the system that I might be able to find answers for in my manual, I'm happy to check for you. I don't know what changed over the years, but at least the mechanical bits seem to be the same.


I'm curious about the signals between the HVAC head unit and the programmer. Given the way most other things in this car talk together I expected a serial data stream, but then when I saw two wires I wondered - differential serial?

But your o-scope traces negate that thought.

If you have traces in different modes that you could post on your thread, I'm very interested to see them.

You theorized in your last post that the signals from the head unit are controlling the solenoids and motor. In the 94 system there is no control from the head unit - all the smarts are in the programmer - the HU just provides parameters.
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Old 02-05-2012, 01:27 AM
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Default Lots of Waveforms

As requested, here are the various waveforms I found. I only put one in the original post because I didn't think very many people would be interested in this geeky stuff. Left to right as they appear on the controller faceplate.

OFF:



AUTO:



BI-LEV:



ECON:



HTR:



DEF:

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Old 12-04-2014, 03:54 AM
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Note that the HVAC system changed completely in 1990. The smarts were moved from the HVAC control head to the programmer. The control head is mostly a bunch of switches that tell the programmer what to do. The digital display can do much more than the earlier units.
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Old 09-26-2018, 03:16 AM
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I took mine apart also saw the issue w/rubber tips. Then I remembered my hobby fuel cars use brass fuel line tubes, so I went to the hobby store and bought 3" tubes and inserted tubes
right thru the rubber connectors by warming bras tubes w/heat gun , and inserting them with needle nose pliers right thru the middle of existing rubber tips in each connector, the reconnected and bolted back and that is the final time this ugly monster reared it's head.!!!
HERE IS A LINK ON SITE FOR 1/8 BRASS TUBING_https://www.etsy.com/listing/494568975/brass-himmeli-25mm-tubes-24-raw-brass
HOPE IT HELPS.
JACK

Last edited by raid007; 09-26-2018 at 04:43 PM.
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Old 12-23-2018, 05:52 PM
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I had about given up, thanks for reference to factory shop manual to install Programmer in its proper position for it to work.
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