After spending quite a bit of time troubleshooting the infamous A/C blowing from defrost I thought I would post some troubleshooting tricks I have learned to isolate the problem.
The problem is caused by vacuum not operating the blend door motor, ie vacuum leaks etc.
My first step was to locate and test each vacuum connection under the hood. Most common areas are behind the battery box at the vacuum canisters following that forward to the passenger side firewall where the check valves are located and then to the source at the engine.
Once all that is checked and found to have vacuum (vacuum gauge is very handy) it's time to move inside to the EATC unit (the climate control head)
I believe that there is a good How To already on the 4 o rings that are on the solenoid plungers inside the unit so I won't duplicate that here. FYI - the o rings are 5/16th OD and 3/16 ID and are available at Lowes for about $2.00.
I had already replaced mine and it worked properly for several months, but for $2 I did it again, just in case.
On the back of the head unit are the vacuum connections and I started by plugging the guage into the black vacuum line and it measured engine vacuum there which proved no leaks to the head unit. The red hose is the one that opens the blend door via the blend door motor straight over the gas pedal.
I should point out here that you can also check the head unit itself for trouble codes. Press the Off & Floor buttons simultaneously & then quickly press the Automatic button. You will see a little circle indicating that it is performing the diagnostic - no codes at the end are what you are looking for and all lights on the panel will be lit.
In my case there were no trouble codes, but A/C still blew at the defroster.
My final step after verifying that the blend door was not stuck and the motor worked properly (be careful, you can stick your hand in there but it's SHARP on the edges.
I got 2 straight vacuum line connectors and a length of vacuum hose and inserted them in the black & red sockets on the connector, basically connecting the vacuum source (black) straight to the blend motor (red)
Leave the electrical connections in tact on the head unit.
Started the truck, turned on the A/C and holy Shat, blend door opens and air is blowing from the panel.
I have now confirmed that the head unit is defective. Since the o rings have been replaced twice, I feel that it must be an electrical issue inside the control head.
For now, while I look for a new unit at a price that doies not involve a reach around, I will leave the vacuum line "jumper" in place and run it that way.
If that had not opened the blend door, it would have indicated the head was OK and the blend door motor was defective.
Hope this helps someone else as summer is coming!
Last edited by HiTechRecneck; 04-27-2012 at 12:31 PM.
I'm bumping this thread because it contains great information and saved me from pulling my dash apart.
Long story short... I replaced my heater core this past weekend. While the dash was torn apart I decided to replace the heater blend door (Dorman part# 902-207). Everything was fine when I got the dash reassembled but I did notice that the blend door in control of the transition between the main vents and defrost vents seemed sluggish. Two days later the air would only blow out of the defrost vents.
The EATC o-rings are only two weeks old so I was highly skeptical that they were the cause. I was afraid I was going to have to pull the dash apart to check for binding of the new heater blend door. I found this thread in a search and was able to bypass ("jumper") the EATC head unit as shown in the pic above. Doing this ruled out the EATC o-rings but reinforced my fear that the blend door was binding.
Luckily the OP mentioned the location of the blend door diaphragm. I climbed under the dash and found the red vaccum line that runs from the EATC to the diaphragm was pinched by a dash support bracket when I reassembled the dash. I repositioned the vacuum line and everything is back to normal operation.
I have the same issue with my truck it only blows from the defrost, I've had it like this for a while now and just dealt with it. But now after reading all this great information over about 16 times I will go experiment with it this weekend and see what I can do myself before taking it somewhere.