AC switches from vent to defrost

[kwiksilver]

on my 89 lebaron convertible
while the car is idling the air blows out the dash vents. the instant you accelerate air switches to the defroster.
what are some of the common causes for this? I assume there is some sort of vaccum valve or possiblely a vaccum holding resivoir.
where do I start?

[contraption22]

The vacuum line for the heater controls might be hooked up incorrectly. It should only be able to see vacuum. Make sure it is hooked up to the power brake booster, or put a check valve in line.

[kwiksilver]

thats an idea... I did have to re-do those lines.
I will check it out

[Turbo3Iroc]

The check valve is connected to the brake booster. That could be bad also.

[kwiksilver]

check valve is installed correctly and is holding 25inches of vaccum when I use my hand held vaccum pump.

when I hook up the check valve and apply vaccum to the line going into the car it wont hold vaccum... I assume this indicates I have a leak somewhere?

[tommyturbo]

not sure if this helps, but on my 88 shelby z, there is what appears to be a check valve under the dash on the drivers side where the vacuum source for the a/c controls comes through the firewall. I know on my car that often if I go into heavy boost for too long the check valve blows off the vacuum line and it just dangles under there until I reconnect it, and when that happens the car goes to defrost, since that must be the default for the system.

I know on all my turbo mopars the system is slow to shift modes unless you have alot of vacuum, trying to switch modes with any boost doesn't happen.

[Turbodave]

check valve is installed correctly and is holding 25inches of vaccum when I use my hand held vaccum pump.

when I hook up the check valve and apply vaccum to the line going into the car it wont hold vaccum... I assume this indicates I have a leak somewhere?

sounds like there is a leak

[chilort]

sounds like there is a leak

+1

That's generally a sure fire way to know you've got a vacuum leak is when the climate control goes from any position other than defrost to full defrost.

[kwiksilver]

yeah... I get the fact it has a leak.
Not having much experience with these cars, I was hoping someone might point me the right direction as to some of the more common problem areas and/or parts.
Instead of ripping the entire dash out of the car and testing everything.

[kwiksilver]

not sure if this helps, but on my 88 shelby z, there is what appears to be a check valve under the dash on the drivers side where the vacuum source for the a/c controls comes through the firewall. I know on my car that often if I go into heavy boost for too long the check valve blows off the vacuum line and it just dangles under there until I reconnect it, and when that happens the car goes to defrost, since that must be the default for the system.

I know on all my turbo mopars the system is slow to shift modes unless you have alot of vacuum, trying to switch modes with any boost doesn't happen.

I stuck my head under dash and don't see any sort of check valve on the line coming into the firewall.
I can see several vaccum operated servos, so will test those first.

[raccoon]

I know at one point Chrysler moved the check valve to under the hood from under the dash.
from my memory you should have 2 lines that come out under the heater core lines, one gray one black. one goes to a vaccum source and has a check valve, the other goes to a coolant control valve. on a 89 I think the coolant valve thingy is under or around the battery.

think you could just add another check valve?
is this the digital climate control system?

[kwiksilver]

thats a good point... my heater control valve is leaking coolant, it needs to be replaced anyway.

[chilort]

Yes, two lines, at least up to 88 (that was the newest one of these I owned) and one is gray and the other black (as already stated). The same two lines are there regardless of digital or manual climate control. And they come out where the heater core lines go.

If the control valve is leaking, replace with haste. They are plastic and once they get old they break if you look at them funny. Many chain repair stores will replace that valve whenever they do any repair work to the heater system (that's how brittle they are!).

The lines that route under the dash are almost (almost) impossible to mess up. The plug (for the red, green, yellow, etc) under the dash only goes one way. I've never seen a servo go bad (but that doesn't mean it doesn't happen). Even if the lines are plugged into the wrong servo or the wrong way, it still won't do what you are experiencing. Instead, you get odd combos of heat and vents when you shouldn't based upon the position setting at the dash. If you punch the throttle and the thing goes to defrost, it isn't messed up lines in the dash.

Generally, if you have the system switching to defrost, the vacuum leak is somewhat large and under the hood.

Good luck, and where are you located in GA?

[kwiksilver]

I worked on this some today... I can find no vaccum leaks in the engine compartment. heater control valve is holding vaccum as well as the cruise control servo.
I tested the 2 actuators I could access from under the dash and they both held vaccum.
everything else requires pulling the heater box out of the car, which wasnt in the mood to do.

anybody have any ideas or suggestions of what to check next?

[kwiksilver]

I havent confirmed it yet.... but I suspect the brake booster is not holding vaccum.

either the check valve is faulty or the booster is leaking internally.

[chilort]

Oh, that's right. That stupid thing has a check valve in it as well. Been trying to hunt down the source of a very very very very small vacuum leak. Thanks!!!!