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Old 01-16-2014, 02:24 PM   #1
kinger62
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1988 k2500 defrost problem

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!
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Old 01-16-2014, 03:01 PM   #2
sntrym95
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Re: 1988 k2500 defrost problem

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.
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Old 01-16-2014, 03:49 PM   #3
kinger62
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Re: 1988 k2500 defrost problem

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!
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Old 01-16-2014, 04:49 PM   #4
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Re: 1988 k2500 defrost problem

Quote:
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!
Quote:
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.
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Old 01-16-2014, 05:34 PM   #5
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Re: 1988 k2500 defrost problem

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
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Old 01-16-2014, 08:12 PM   #6
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Re: 1988 k2500 defrost problem

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.
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Old 01-16-2014, 08:26 PM   #7
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Re: 1988 k2500 defrost problem

Thanks, ChevyTech. The actuator you refer to seems to be working properly. For the floor heat, the floor door is open and the dash door is closed. When I select the vent position, the floor door closes and the dash door opens. When I select defrost both these doors close, I'm assuming to direct the air to the top vent. Is there a third door in the system I can't see? If so, from where can I access it? I can hear the air trying to escape at the floor because it can't get out up top.
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Old 01-16-2014, 08:56 PM   #8
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Re: 1988 k2500 defrost problem

Quote:
Originally Posted by kinger62 View Post
Is there a third door in the system I can't see? If so, from where can I access it? I can hear the air trying to escape at the floor because it can't get out up top.
Yes there is a third actuator.
Leave it alone.
It is, the fresh air intake AKA max AC door. Not the problem!
If you are having trouble with the mode doors you should not even think of messing with the fresh air door.


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.

Did the linkage come out of the adjuster when you were working on it?
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Old 01-16-2014, 09:08 PM   #9
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Re: 1988 k2500 defrost problem

I thought of that but I don't think so. As I said, the floor and dash bypass doors are moving so the actuator is working.
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Old 01-16-2014, 09:22 PM   #10
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Re: 1988 k2500 defrost problem

I went and pulled a manual out and it is worthless.

I will dig and see what I have for saved links and if any of them still work and post back.
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Old 01-16-2014, 09:30 PM   #11
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Re: 1988 k2500 defrost problem

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.
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Old 01-16-2014, 09:31 PM   #12
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Re: 1988 k2500 defrost problem

http://67-72chevytrucks.com/vboard/s...d.php?t=571882
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Old 01-16-2014, 09:36 PM   #13
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Re: 1988 k2500 defrost problem

Here is the other actuator that you did not find.
But leave it alone.
http://www.chevytalk.org/fusionbb/sh...php?tid/170970
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Old 01-16-2014, 09:41 PM   #14
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Re: 1988 k2500 defrost problem

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Old 01-16-2014, 09:47 PM   #15
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Re: 1988 k2500 defrost problem

http://www.chevytalk.org/fusionbb/sh...cd9ed79a28da4/
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Old 01-16-2014, 09:50 PM   #16
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Re: 1988 k2500 defrost problem

Hey, this is great! Do you have the corresponding list of part names for this diagram?
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Old 01-16-2014, 10:04 PM   #17
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Re: 1988 k2500 defrost problem

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.
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Last edited by ChevyTech; 01-16-2014 at 10:12 PM.
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Old 01-16-2014, 10:30 PM   #18
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Re: 1988 k2500 defrost problem

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
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Old 01-16-2014, 10:32 PM   #19
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Re: 1988 k2500 defrost problem

Thank you. I owe you!
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Old 01-16-2014, 10:53 PM   #20
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Re: 1988 k2500 defrost problem

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

Welcome to the message board!
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Old 01-16-2014, 10:58 PM   #21
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Re: 1988 k2500 defrost problem

Thanks! I plan to make use of it. I've got a nice truck but it's got issues!
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Old 01-17-2014, 11:34 AM   #22
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Re: 1988 k2500 defrost problem

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
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Old 01-17-2014, 12:23 PM   #23
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Re: 1988 k2500 defrost problem

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
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Old 01-19-2014, 05:59 PM   #24
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Re: 1988 k2500 defrost problem

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.
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Old 01-19-2014, 06:32 PM   #25
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Re: 1988 k2500 defrost problem

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.
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