Hello All,
I'm a new 89 owner (and forum member) with a problem with the blend door. I have the C68 controls and an inoperable blend door stuck in the full heat position. I've been searching for days and thought I had it but stumped again. Everything works on the controller, it's just the door doesn't move.

What I've done:

(1)I have verified that the door is not stuck.
(2)I have verified that the door motor works in both directions when voltage is applied to it.
(3)I took out the programmer and found some overheating resistors and some cold / broken solder joints. Repaired as needed.

I thought the programmer was the problem but no....stumped again. Any help would be appreciated.

 

are you sure that you repaired the solder joints in the programmer. i would bet that the programmer is your problem.

 

I thought it would be the programmer also. I'm a little confused about the R3 and R4 resistors. I have read some posts saying that they have been improperly listed as 140 ohm and should be 340 ohm. I removed both and tested them and they were both at 140 +/- a couple of ohms. I'm pretty sure I got all of the cold joints re-soldered. Even if it was a questionable joint, I hit it again. There was a bad trace on the board also. It was associated with the 1 watt resistors, I repaired that also. (used a jumper wire to bypass the bad spot). I'm not new at solder repair but I guess I could have missed something. I was just hoping that there may be another problem area associated with the blend door that I have overlooked.
Thanks for the reply.
I haven't had any luck finding a replacement programmer so I'll check mine again for a possible failed component.

I've also tried to disconnect the battery to reset the system with no luck. I've also not had any luck getting into the self diagnostics of the HVAC system.

 

The resistors are 140 ohms, 1 watt, 5%. There are two resistors, one for each direction the motor runs. Bryan Thompson incorrectly stated they were 340 ohms on his batee web site. 340 ohms isn't a standard value, so I don't know where he got that number.

I did an article on the HVAC programmer with probably way more info than you could possible use:

http://forums.corvetteforum.com/c4-t...schematic.html

Here's info on the C68 diagnostics:

http://forums.corvetteforum.com/c4-t...stic-mode.html

The blend door position is calibrated every time you turn on the ignition (no reset needed). [ EDIT ] I have found out that this is NOT true. The blend door position is only calibrated if the HVAC controller power is removed [ /EDIT ] The HVAC controller commands full heat, full cold (not necessarily in that order) and then sets the blend door to the position determined by the temperature setting. It takes about 10 to 15 seconds to do the calibration. If I turn off the radio and disconnect the blower so it doesn't run I can hear the blend door motor running. You can take out the blower motor controller and watch the process through the opening in the top of the evaporator housing.

It's possible that the connection between the blend door motor and the blend door has come apart. You can see a picture of the blend door motor and rod in the first thread. Sometimes the threaded rod comes out of the white plastic piece. I have also heard of the white plastic part breaking. I vaguely recall somebody saying that the white plastic part is available in the HELP section at auto parts stores, though I have not seen it there. It's a guide for door lock rods or something like that.

 

Temporary fix, I have a summer only 90 so I had full heat , dying inside the car, had the blend door jammed to cold only.
All good, but not fixed, just ice cold air.No need for heat for me.
MZee

 

Thanks, I'll check out the info. I did check the clip on the rod and its OK. Like I said the door moves full in each direction if wired direct, its just not receiving a power signal from the programmer.

 

I sure would like to tell you how to solve the problem but... I have an 89 with the exact same problem. I have tried three programmers, two "head" units, and two different blend door motors - all had exactly the same problem. I went through tons of trouble shooting, testing everything I could from the FSM. If my blend door was manually set to "cold" at key-on it would move to full hot and stay there. If my blend door was manually set to "hot" at key-on it would not move at all, so it is not completing the calibration run. After a month I gave up and wired a dpdt switch to the motor of the blend door so I could move it manually and get the car back on the road. You can't tell exactly where it is, so I only hold the switch for a couple seconds at a time until I get what I want. Good Luck.

 

Controls should enter Diagnostics and it it won't it's probably broken.

Check Inside Air Temp Sensor. That's the signal used by the Controls for door position. More resistance cold, virtually none hot (I don't have the exact specs at hand). I'd check it in the morning; then cook the interior for an hour in the sun with the windows up. Should be a significant change.

 

One of the 140 ohm resistors controls the motor when it moves from cold to hot. The other controls the motor when it moves from hot to cold. One of those resistors is open (or there is a bad solder connection <-- more likely).

Take out the HVAC programmer and check it out. The previously referenced thread will help you get it out.

 

I've already had the resisters out and they both checked out. Correct me if I'm wrong.....doesn't both resistors supply the circuit post and neg and then switch polarity as needed? So if there is a problem on one side or the other of that circuit, the door will not move in either direction? I suspect the problem is in one or more of the other components of that circuit.......possibly the transistors.

 

Have you tried a "start-up" with the blend door moved to the cold position? You have to disconnect the battery, move the door using jumper wires and then reconnect the battery and key-on. While you are doing this check that the blend door motor assembly is sending the proper feed-back resistance to the control unit, this is outlined in the FSM.

Have you checked the inside temp sensor as SunCr suggested? I found these values on line, 4400 ohms at 60 degree and 2200 Ohms at 85 degree.

Otherwise back to the Programmer, and they are getting hard to find and expensive. Mine works for everything EXCEPT the blend door.

 

I don't have a FSM but I have already tried that exact procedure with negative results. I have not checked the inside temp sensor. When I looked in the programmer the first time, it was pretty cooked in the area where the 140 ohm resisters are located. There were even some traces on the PCB that where cooked. I had to repair them with some jumpers. I'm going to remove all of the components associated with the blend door, test and probably just replace them. I will report back with any results.

Thanks again for all of the replies.

Any chance anybody has the schematic for the programmer PCB?

 

It's in the link I posted earlier. I put it here for convenience:

http://forums.corvetteforum.com/c4-t...schematic.html

That one is for my '86. They have changed over the years. A year or two later they eliminated the heater water control.

I have one from an '88 Cadillac and it has transistors instead of the 140 ohm resistors.

 

I did read your link from your earlier posts (awesome information), I just didn't realize there was supposed to be a schematic in it. There is a broken image link in the first post that I assume is the schematic. The only other schematic I get from that post is the link to batee.com and his are all hand drawn and "reverse engineered"......

 

Here's a direct link to the schematic:

http://www.misterpeachy.com/VettePic..._Schematic.pdf

I clicked on the link to make sure it worked and while I was there I looked at the circuit. It appears I was wrong about one resistor being used for one direction and the other resistor being used for the other direction. The resistors are actually in series, so they're both in the circuit at all times. It's the transistors that determine which direction the motor turns.

[EDIT] I screwed that up. It actually IS true that the motor will only move in one direction if one resistor is open (or the circuitry associated with one resistor is open). [/EDIT]

 

Thanks, I will wonder about unjamming the blend door when the nights get cold before the snow falls, some say I should just buy a new a/c control digital unit ,then it should work again.
MZee

 

Do the diagnostics on the control head and see what you get:

http://forums.corvetteforum.com/c4-t...stic-mode.html

 

I feel foolish coming up with a non-electronic fix, but here goes.
My 93 had the same bled door problem, but the electronics are different. After many attempts to repair I took it to my favorite Corvette shop (Tony's in Gaithersburg MD). Turns out the PO had a heater core replaced and the place that did the work forgot to plug the blend door motor back in! Obviously that's not your problem!!

Until I resigned myself to spending money on the fix, I had seen posts for earlier model Corvettes that suggested using a manual choke cable -attached to the blend door flap- to bypass failing electronics and get your car cool again. The post I saw came with pictures and detailed instructions. I was going to do it, but it wouldnt work on my model year.

You're doing the right thing by trying to fix it the right way, but there may be an alternative if all else fails. Just a thought! '

 

Cliff, your information has been wonderful. Here is what I've come up with so far. After removing the components and testing them individually, I found that one of the switching transistors marked "8399" has failed. New problem......I can't find any information on these things at all. I know they are NPN, I just can't identify them for replacements. All other parts of the circuit function as designed, I'm just not getting the negative signal I need to move the door. Any thoughts or further information?
They did use that same transistor for all of the vacuum circuits as well. I'm going to borrow one of those to confirm for now.

 

EUREKA!!!! One transistor and two diodes............problem solved. Now all I need is a suitable replacement transistor cuz I stole one from one of the vacuum circuits. Anybody know the specs?