What causes the AC to only blow cold out the defrost vents but sometimes switch to the regular vents and blow out the floor vents if the heater button is pushed on a 1994 Buick LeSabre?

Answer 1

AC controller suspectI had a '98, basically same engine system, so my shot...The default position is "defrost" because if the system fails you want to keep the windshield clear.

Does the unit go to defrost whenever the enginee is started and ran for a few minutes??

The AC controller is a enclosed box.. should be on firewall someplace..it has multiple vaccum lines hooked to it from various valves/swicthes that actuate doors that direct the air flow. Will also have a fairsize electrical plug. These puppies are expensize... around $600 at a dealer. I'd go looking for a junk yard. Check physical cdition of both yours and the junker because...

My problem was that the nipples for the vac connections were crumbling and had blocked the system. No one could tell me why plastic should do that, including a GM zone guy...

So if I could get a "cheapie" junk unit, I'd replace it and see what happens..

Good luck

Mystery Black Box the culprit - vacuum distribution manifold?

My wife's 1994 Park Avenue Ultra had both the defrost-only and the warm-on-one side problems. The former turned out not to be the actuator, nor the control unit. There are two vacuum operated actuators, the one you can see that's attached to the vent-box where the heater air comes out, which has two vacuum lines, an orange and a white, attached to it, and the one you can' see that's up above all that tucked out of sight behind the dash directly behind the control unit. That actuator has a blue vacuum line on it. I took the orange line off and hit the heat button. I could then feel vacuum. I put it back, then took the blue line off the upper actuator and hit the Auto (AC) button - nothing. I put the orange line on the upper actuator, hit heat, and voila! the vents came on.

So, I put everything back where it belonged and followed the blue line all the way over to a black box to the right of the glove compartment. I don't know what it's called, but it has a sticker marked "MODEL 16197174" and below that "DELCO ELECTRONICS KOKOMO, IN" (if anyone can name it, I'd love to know what it is) This box is about 7" tall, 3" wide and about 5" deep, has a big orange gang plug on the front, but most importantly has about five vacuum lines coming out the bottom front that control all those actuators. There is one black line coming into the box from the engine compartment and there is a kind of manifold where all the vacuum lines from the dash join up to it, and each individual line is controlled via its own little relay. I pulled the gang plug and another electrical plug at the top, removed the two screws that hold it in place (one right in front and another underneath and eased that sucker loose. There is a cover held in place by several tabs and it just has to be winkled loose, but removing that allows the vacuum lines to be removed. Those are on kind of a harness to keep them in the proper order and it just slides loose. After I took the cover off and discovered the relays, I stuck the orange gang plug in, started the car, activated the "blue" relay by poking it with a screwdriver and out came cold air from the dash vents. Fancy that. So, a quick trip to the junk yard and $7.46 later I'm putting in the new unit. Fired it up and tested good for air at the dash as well as the other climate control settings.

Oh, and did I mention that I had to remove the lower dash to get to the mysterious box? You'll need a 7mm socket and also I think a 10mm to remove the screws, but the lower dash pieces come out pretty easily. Instructions for that are available elsewhere online so I won't go into that process.

But the trip to the junk yard led me to fix the passenger side air problem, too. I found a Park Avenue that already had the glove box and dash removed, and as I was disconnecting the wires at the top of the mystery box, I looked up and guess where it leads? Right to the actuator for the PA actuator motor! So, I pulled that while I was at it. Two little screws on top, one at the front that you can see, and one in the back that you can't, but you can get it by feel with a 7mm socket (1/4" drive). Put that in while I was swapping out the mystery box. Buttoned everything up, and as Emeril would say, BAM!!! Works like a charm. Up and down, side to side, hot and cold.

So, if you're having this problem I recommend checking for good vacuum at the actuators, because if that's your problem swapping the box beats the hell out of trying to take all that Plumbing apart to swap the actuators, check the doors, etc. and then try to get it all back together.

The amazing thing is that I've seen all kinds of answers to this problem on this board and many other places, and everybody suggests bad vacuum lines, bat actuators and so on, and NOT ONCE have I even heard mention of this black box with the vacuum distribution and the relays. But it definitely was the source of the problem. I fixed it myself and we figure I probably saved a hundred bucks on the parts and no telling how much $$$ in labor (8 hours, but I didn't know what I was doing ahead of time the way a mechanic presumably would) by figuring this out and doing it myself. So, now you know!