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1999 Dodge 1500 Laramie, 318. The air will only blow out of the a/c vents, no matter where I set the control switch. The fan changes speeds according to where I set that switch, and it will blow hot when I turn the thermostat far enough to the right. A few months ago, after a mechanic fixed another problem, I asked him about the HVAC problem. When he tried all of the settings, he concluded that the blend door actuator was bad. A few Youtube videos indicated that the actual problem might instead be the plastic piece that attaches the actuator to the blend door (alternately called connector and coupler), which commonly cracks. Here's a series:

View: https://www.youtube.com/watch?v=S3U2X04aXdA


View: https://www.youtube.com/watch?v=D5m4O8S9uLw&t=481s


There's a third video, but these two are the major ones. He's right about it being time-consuming, and a design that wasn't completely thought out. Rather than an adjustable crescent wrench, I used a needle-nosed vice grip. I bought the plastic part from a dealer for about $19 after tax. The one in the truck was cracked:

Blend door connector 2.jpg


But air still only comes out of the a/c vents regardless of the setting.

I ran the test on the actuator with the battery, and it engaged when I touched the wires to the motor, like it did in the video. I'm guessing this means that the actuator is working.

Months ago, I replaced the vacuum lines from the firewall to the intake manifold with a set that I found in a junkyard that seems in good condition. I don't see any cracks in those lines. Is there a way I can test the check valves?

What could be causing the problem? The control switch? A vacuum leak? A bad wire or connection? Something internal? Something else?
 

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A lack of vacuum will default the doors to blow out the defrost to keep the windshield clear (a safety mode).
Having them blow out the dash vents sounds like a HVAC control, vacuum actuator or door problem.
The electric feedback motor actuators began in 2001 on the DR trucks. The older BR/BE were still vacuum controlled doors.
 

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You'll need a vacuum tester such as one of these.
I recall using someone else's some time ago, but I forget who or where. I'm heading up north tomorrow, so I'll ask around there.

A lack of vacuum will default the doors to blow out the defrost to keep the windshield clear (a safety mode).
Having them blow out the dash vents sounds like a HVAC control, vacuum actuator or door problem.
Thanks, this narrows it down. I can find an HVAC control in a junkyard, though there's no guarantee it will work. Maybe I'll also buy a back-up or two, if they're cheap enough. The only doors I checked were the one above the fan, which sprung back to shut after I pulled it open, and the blend door, which seemed a bit stuck when I first tried to pull the old plastic connector off, but it did move side to side soon enough. When I tried to put the new part on, it moved up a bit before I was able to push it completely on. but after it came back down, it moved side to side like it did earlier, so I'm guessing that it stayed on its hinge. I've yet to see any other doors. Where is the vacuum actuator, and how would I test it?

I'm also wondering if a piece of the dash panel could have fallen into the HVAC system and wedged itself where it's causing the problem.
 

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. . . The air will only blow out of the a/c vents, no matter where I set the control switch. The fan changes speeds according to where I set that switch, and it will blow hot when I turn the thermostat far enough to the right. A few months ago, after a mechanic fixed another problem, I asked him about the HVAC problem. When he tried all of the settings, he concluded that the blend door actuator was bad. . . . .
This was an incorrect diagnosis. The blend door controls the amount of air that goes through the heater or bypasses the heater so therefore regulates temperature of the air leaving the HVAC plenum at any port. It uses an electric stepper motor to control the blend door position. Since you can regulate air temperature the blend door actuator is not the issue. You can control the fan motor speed so that is not the source of the problem.

There are 2 vacuum actuators associated with the mode feature of the HVAC control. There is a panel - defrost door actuator and a floor - defrost actuator. Either one or both of these vacuum actuators are stuck / not moving as intended. Also the vacuum line to each of these actuators could have a problem and not supplying intended vacuum to move the internal diaphragm. See attached images. I would suggest you check these actuators first before obtaining another HVAC control assembly.

You can attach a hand held vacuum pump to the ports on the actuators. Apply some vacuum and note if the actuators move. That will indicate if the problem is with the actuator or binding in the door that the actuator controls. If the actuator will move smoothly then the issue has to be with the vacuum line (broken, disconnected) or a problem at the vacuum control part of the HVAC control assembly in the dash or a problem with the source vacuum line that originates in the engine compartment.

HVAC Mode Floor Defrost Actuator.gif HVAC Mode Door Actuator.gif
 

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This was an incorrect diagnosis.
Which I guessed after running the test with the battery. But I did replace a part that needed replacing.

Thanks for the other info. I think I saw one of the actuators when I looked at the fan and the door above it, which might be something like a fresh air/recirculation option. It was closed, which I'm guessing means it's on recirculate. I don't see a selection for that option on my dash panel. Does opening it depend on the heater/defroster/air selection?

I'm in Michigan, so I'll ask around up here for a vacuum tester.
 

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. . . . I think I saw one of the actuators when I looked at the fan and the door above it, which might be something like a fresh air/recirculation option. It was closed, which I'm guessing means it's on recirculate. I don't see a selection for that option on my dash panel. Does opening it depend on the heater/defroster/air selection? . . . .
In default status situation the recirculation door will close the opening on the HVAC plenum to block cabin air from being recirculated. When the door is open and the vacuum actuator enabled, the system is drawing air from the cabin along with some outside air. Recirculation on these systems is never 100% cabin air only.

Your HVAC control panel does not have a separate setting to enable cabin air recirculation. The only way to get recirculation is to set the control to MAX AC. In MAX AC setting the air conditioning compressor is engaged and vacuum is applied to the pneumatic actuator to open the HVAC plenum to interior cabin air.

Do an online search for hand operated vacuum pump with gauge. You should be able to find something in the $20 - $30 price range.
 

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On later vehicles, the recirc door closes after shutting off the engine. This helps keeps the varmints out, so they can't build nests in your HVAC.
With the vehicle running, vacuum available and a normal HVAC setting, the recirc door should be open.
 

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Thanks for the recirculation info. Varmint control makes sense. I've seen some South Main Auto Repair videos where the electrical problems were caused by mice chewing on the wires.

I ran into another problem or two that will require a junkyard run. Will try to return to this one during the weekend.
 

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It took a trip to the junkyard to clarify the problem. I saw a Dodge truck of similar vintage with most of its dash out and the HVAC system more exposed. The other vacuum actuator is on the driver's side of the transmission tunnel. When I pulled on it, the door it attached to was stuck, but opened with a bit of effort, and without breaking anything.

So I looked at that one in my truck at the house this morning, and though the door moved freely enough, the actuator was off the plastic connector. There was also a wire dangling, which I removed -- apparently, a previous owner tried to jerry-rig something, and it didn't last. Anyway, I put the circle in the actuator bar over the circle extension on the plastic connector, and now air blows through the heater and defroster vents when those selections are made with the heater control dial. One question remains: is there some sort of clip or washer that holds the actuator bar in place? If so, what? (Ok, that's 2 questions.)

None of the answers were exactly right, but IC and Allan both pointed in the right direction, and provided info that might be helpful to another Dodge truck owner with a similar problem. While IC's answer came first, Allan's was more detailed, and without it, I still might be looking for the solution, so he gets Best Answer. Unfortunately, I don't see a Best Answer selection in this new software. And the suggestion to acquire a vacuum tester is a good one, so everyone gets a like.
 
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