HVAC issue, AC does not work

[sntrym95]

So this has been an interesting problem. My 95 Chevy's A/C will sometimes work, sometimes not. The compressor and all under hood components are fine, they kick on properly. The issue seemed to be with the recirculation door having moved too far, and the control panel could not move it back to the proper position using cabin air.

Got a new recirc door, installed, and voila! it worked. Once. Then the new door moved to a point where it could not be moved back unless I put a screwdriver in the opening and manually put it back. Then it will work. Once.

The control panel is newer-ish, maybe 4-5 years old. I don't remember if it's a Delco or not, but it seems to work fine.

In looking at schematics, there is a wire running from the recirc motor to the blend door motor. I'm trying to figure out the communication path for the system, and if a signal from the control panel or the blend door motor is causing the recirc door to operate incorrectly. I did some searching here but didn't really find anything specific to this issue.

Does anyone have a flowchart or similar to show how the signals from the motors work together?

[sntrym95]

Found this paper that describes the different motors and how they function. http://www.askatech.com/AskATechLive...er.ashx?a=7532

So based on what I've seen, the blend door sends a signal to the recirc motor when the temp is set all the way to COLD so that the recirc motor closes the door to allow max cooling.

So it seems the climate control unit in the dash could be bad, not the motors.

[ChevyTech]

I don't have a 1995 schematic and that is the year the system changed.

I thought I would take a look at the link you posted but it does not work for me. I get a Chrome message.

Earlier years would run the recirc door motor for a specific amount of time to open the recirc door, and a specific amount of time to close the door. It was common for the gears to strip and then they would click instead of moving.

1995 style control heads were known for the transistors failing on the board.

[ChevyTech]

Ok I went to a different computer and got the PDF download.

There was no year specific information.

I will see what I can find for a schematic.

[ChevyTech]

I did not find a schematic but when I was looking through my files I found information on repairing the head by replacing the transistors.

a/c problem fixed
* * * #348460 - 02/26/03 07:38 AM

Hi there!
I hope this info is valuable to someone perplexed by defect a/c.

If you have following symptoms in your truck
1) A/C button won't work
2) Recirc won't work
3) Mode of operation knob has no effect (air comes only to floor for example)
4) Rear defog button works normally
5) Temp and fan speed knobs works normally
then you might have a similar problem I had. The fix is both easy and inexpensive. Culprit is bad transistor Q1 on your control head. By changing it (cost: 6 cents at Radioshack) everything works as usual. Here in Finland GM dealer asks 275 Eur (about same in US dollars) for the head, so there is your motivation to fix it by yourself.

To take the head off, remove dash and gently slide the head out. There are clips on both sides of the head, you need a screwdriver to tweak them slightly inwards. Disconnect all cables.

The transistor is MPSA56 (bipolar pnp) and you can locate the faulty one easily. It is the one under the bracket just next to mode of operation knob axis (NOT the easiest one at the edge of the board). Install the new one to the other side of the board, so you don't have to fit it into that extremely confined space. Make sure to connect the legs to the right holes. Emitter, base and collector are marked on the faulty transistor.


Another reply to the thread:

Thanks gani for the info. My ac is now operational and the cost was only a little time and $1.75. Most of the time was spent looking for the transistor. Radio shack did not have MPSA56 so I used a compatible substitution NTE 159.

Source:
http://www.chevytalk.org/threads/sho...e/0#Post348460

The photos do not show in the thread that is very old but I saved the most important photo to my hard drive years ago. I will try to upload it here.

[sntrym95]

Thanks ChevyTech! I hope you back up that hard drive, that's some valuable information.

I'm going to see about finding a junkyard unit for now instead of spending $130 on a new one. I'll try to repair my existing one with the info above. Thanks!

[sntrym95]

Finally got the transistor in the mail a few weeks ago, and finally got around to pulling the A/C panel. Soldered the new one in, and voila! it works! I also found a semi-loose connection to the A/C panel, so I fixed that as well.

Thanks ChevyTech!

[ChevyTech]

You’re welcome, and thank you for the update.

You saved yourself a lot on money and hassle.

[Jeepwm69]

Originally Posted by ChevyTech

I did not find a schematic but when I was looking through my files I found information on repairing the head by replacing the transistors.

a/c problem fixed
* * * #348460 - 02/26/03 07:38 AM

Hi there!
I hope this info is valuable to someone perplexed by defect a/c.

If you have following symptoms in your truck
1) A/C button won't work
2) Recirc won't work
3) Mode of operation knob has no effect (air comes only to floor for example)
4) Rear defog button works normally
5) Temp and fan speed knobs works normally
then you might have a similar problem I had. The fix is both easy and inexpensive. Culprit is bad transistor Q1 on your control head. By changing it (cost: 6 cents at Radioshack) everything works as usual. Here in Finland GM dealer asks 275 Eur (about same in US dollars) for the head, so there is your motivation to fix it by yourself.

To take the head off, remove dash and gently slide the head out. There are clips on both sides of the head, you need a screwdriver to tweak them slightly inwards. Disconnect all cables.

The transistor is MPSA56 (bipolar pnp) and you can locate the faulty one easily. It is the one under the bracket just next to mode of operation knob axis (NOT the easiest one at the edge of the board). Install the new one to the other side of the board, so you don't have to fit it into that extremely confined space. Make sure to connect the legs to the right holes. Emitter, base and collector are marked on the faulty transistor.


Another reply to the thread:

Thanks gani for the info. My ac is now operational and the cost was only a little time and $1.75. Most of the time was spent looking for the transistor. Radio shack did not have MPSA56 so I used a compatible substitution NTE 159.

Source:
http://www.chevytalk.org/threads/sho...e/0#Post348460

The photos do not show in the thread that is very old but I saved the most important photo to my hard drive years ago. I will try to upload it here.

So just to clarify, it's the one marked 2N4403 PNP in the middle of the board pictured?

[sntrym95]

Yes, there are two. One on the end (not the one in question), and one in the middle that is just above the bracket. I could not remove the entire board, but I could easily get to the solder joints. Using a bulb, I was able to suck the solder out enough to remove the transistor.

I then soldered the new transistor on the opposite side of the board (where I desoldered the old one), so it's easier to replace if need be.

I'd grab a pic but I put everything back together already. If I need to remove the dash again (which will probably be soon to replace the stereo), then I'll take a pic and post it.

[ChevyTech]

A transistor can be mounted on the back side but make sure you mount it so you don't have a mirror image on where the connections should be.

Base emitter and collector connections need to stay the same.

You can loop the leads so the transistor is in the same position as if it were on the other side of the board so the lead side it up with the cap side toward the board.

[sntrym95]

Yeah, I made sure to flip mine around when I mounted it on the other side.

[ChevyTech]

Originally Posted by sntrym95

Yeah, I made sure to flip mine around when I mounted it on the other side.

Excellent!