1988 k2500 defrost problem

[kinger62]

Hi, my 1988 K2500 will not blow air through the windshield defrost vent. The actuator motor opens and classes the proper doors to get air on the floor and through the dash vents. When I push the button to get the air to defrost I can feel the air blowing on the door windows through the little side vents but I get none on the windshield itself. Does this mean the vent is blocked? Any suggestions for troubleshooting? Thanks!

[sntrym95]

The linkage is above the area between the left side of the transmission hump and the gas pedal. It's up in there and tough to get to, but you can watch it and fix it if needed. Just be patient. Sometimes the end of the linkage rod pops out of the sleeve. You'll know what I'm talking about once you see it. Make sure you have a bright skinny flashlight.

[kinger62]

Mike, do you mean at the actuator motor there? I replaced that unit and I can watch the floor vent door open and close. From the dash level (I removed the cubby next to the glove box) I can see the dash vent door open and close. Is there a third door I can't see? Thanks!

[ChevyTech]

From the dash level (I removed the cubby next to the glove box) I can see the dash vent door open and close.

That is the actuator for the temperature. Not the problem!

Is there a third door I can't see? Thanks!

Yes, the fresh air intake AKA max AC door. Not the problem!

Do as mike said.
There is more then one door being moved by that actuator next to the gas pedal. Study it as it moves.

[kinger62]

To clarify, I have found two actuator motors: one that operates the cold/hot air (this is working properly) and one that operates the bypass doors to determine where the air comes out: dash, floor, or windshield. When I replaced the actuator that operates the bypass doors I can see two doors moving...one under the dash allowing air to go to the floor, and one "mid-dash level" that lets air out the dashboard vents. When I push "defrost" both of these doors close so air cannot get through the floor vents or the dash vents. At this point air does come out of the small defrost vents that will warm the windows on the doors, but nothing comes out of the main dashboard defrost vent. Shouldn't air be diverted to the defrost vents at this point? This is why I'm wondering if the vent could be somehow blocked, unless there is a third door I'm not aware of. If so, where physically is it located?

Thanks again!

Pete

[ChevyTech]

The actuator by the gas pedal is the only actuator that you should be looking at for this problem.

The two doors moved by the actuator by the gas pedal must work in unison to direct air flow. If out of adjustment they can bind.

Do you pile stuff on the dashboard?
If something fell down the defroster vent it could be blocking movement.

One truck I helped someone with had honey on the door for the defrosters and it was stuck.

[ChevyTech]

I did some more looking and the service manual is worse then worthless, it has the actuators labeled wrong. Now I see why people get them so confused. If it is wrong in the manual, every place that uses the GM service manual as a source will be wrong.

I am still looking for more information.

[ChevyTech]

http://67-72chevytrucks.com/vboard/s...d.php?t=571882

[ChevyTech]

Here is the other actuator that you did not find.
But leave it alone.
http://www.chevytalk.org/fusionbb/sh...php?tid/170970

[ChevyTech]

[ChevyTech]

http://www.chevytalk.org/fusionbb/sh...cd9ed79a28da4/

[kinger62]

Hey, this is great! Do you have the corresponding list of part names for this diagram?

[ChevyTech]

Must be to many people on the board tonight. I keep getting this message:

“The server is too busy at the moment. Please try again later.”

I don't have the corresponding list. I have many links to check.
That is all I have found so far.
I also have some more manuals to look in.
My training manual shows the shop service manual is wrong. Typical GM screw up!!!

Move the controls from heat to defrost on your truck, while you watch part #36 move and make sure the spring on part #29 is not compressing so part #36 ends up not moving like it should.

[ChevyTech]

There is an illustration at this link where at least the actuator motors are labeled correctly.

http://www.justanswer.com/chevy/3pb6...e-control.html

Better one
http://www.justanswer.com/chevy/4ngh...ing-sound.html

[kinger62]

Thank you. I owe you!

[ChevyTech]

You’re welcome. A thank you is always appreciated.

Welcome to the message board!

[kinger62]

Thanks! I plan to make use of it. I've got a nice truck but it's got issues!

[ChevyTech]

I am going to see if I can hot link an illustration in, so we can us this thread for future reference.
Source of illustration if linking does not work:
http://www.justanswer.com/chevy/4ngh...ing-sound.html

[kinger62]

Thanks again, ChevyTech. I'm going to head out now (its warm enough to work outside!) and see if I can track this down. Looks like, but one of the diagrams, the door (valve) actuator motor is controlling three doors, not two. I'd only looked at the bottom two. I'm going to check the third now, as that must be the culprit.

Pete

[kinger62]

Well, thanks to your diagrams I discovered the problem. By laying on the floor and looking up under the dash with a flashlight while pushing the defrost/heat button I could see the defrost door wasn't opening. I sprayed some lithium grease up there while moving the door worth a long screwdriver. It worked. Now it opens and closes like it should. Good thing I don't need to replace the door as it seems the only way is to remove the dash first! I really appreciate your help.

[ChevyTech]

Excellent!

Thanks for letting us know what fixed it.

Like I said, a previous problem like yours was caused by honey on the door.

Yes, the defroster door it is not the easiest item to get to. Not the kind of a repair that someone wants to do, in the north, in the winter, unless they have a heated shop.

[tanman722001]

I know this is a old post. But one link they was said a thumping noise. As i have. They say cuy to two wire why cut the wires. What to do after you cut them. Or is there a different fix.

[ChevyTech]

Originally Posted by tanman722001

I know this is a old post. But one link they was said a thumping noise. As i have. They say cuy to two wire why cut the wires. What to do after you cut them. Or is there a different fix.

If you use the cut the wire suggestion, the actuator (motor) will no longer work. That will stop the noise because it no longer works.

That actuator is there to make the AC colder, when the recirculation door is closed. The recirculation door opens so the vehicle doors close easier, and it does not pop your ears when you shut the door. It is open more then it is closed. In the defrost mode it is needs to be open to bring dry air into the vehicle to keep the windows more clear.

If you have a truck and the AC does not work, you could unplug the motor so it does not make noise.
It is hard to get to so that is most likely why they said cut the wire.

[88lowrider]

I see this is a Old thread but not finding any updated information or more information about my problem. So, going to try here first. ChevyTech, you have always helped me in the past so, here goes.
1988 GMC C1500 Sierra, Gas V8 with EFI, auto trans, AC, had the fancy stereo in it that got replaced.
It has the fancy heater/ac controller in it so its I guess digital they might call it?
I will get a picture of the truck and the controls so I am not wasting anyone's time.

I had let the truck set for almost a year, outside. Started to bring the truck back as the price of the new ones were not possible, so, fixing this on my own the best I can. Just turned 70 but been a mechanic in my early year for two dealerships, both GM. Grew up in a wrecking yard taking things apart and building hot rods. I do understand Innovative fabrication and have had to be creative as most of my rigs are older. 91 Geo Tracker, 76 Dodge Van, 79 chev 1500 4x4, and my 88 GMC lowrider which the heater quit working.

I have been though this thread looking and all the links, printing out pictures of the heater design, location of the actuators and well, blah blah blah.

I think its best to show you what I have and the big question would be, How do you Test the Temperature Door Actuator motor ?

I have had the instrument cluster out and able to access to the heater/ac controller. The book just says how to Replace it, not test things.

I would like to test the heater controller as well as the actuator motor before I just start replacing expensive items.
Hope someone can help me please.

[ChevyTech]

Hi Lowrider.

This is from my notes and should help.

HVAC Temp door actuator testing

With everything connected, and the key on, the dark blue wire (which shows the position) with door at full cold should read about 4 volts. At full hot the Dark blue wire should have about .5 volt on it.

The light green and light blue wires are the power and ground to move the motor. It should show a voltage between 6 and 7.5 volts on these wires when the motor is trying to move the door position. The polarity depends on which direction the controller is trying to move the door.

Here is a link to a thread with a schematic in it for a 1994.
the 1988 system is different then an 1994. A 1988 thas a position sensor circuit for the recirc door and a 1994 does not. I would think there are other differences too.


http://www.truckforum.org/threads/ac...-silver.20021/