byrdman454

Decided to check the vacuum lines on the '88 S4 to find out why there was hot air blowing under acceleration.

Black line-holds vacuum
White line-holds vacuum
Yellow line-holds vacuum
Blue line-leaks
Orange line-leaks
Green line-leaks

From one of Wally P's previous posts it seems like I can plug the orange line without any problems. The blue line actuator is located under the blower fan and must be replaced. He does not mention the green. Can I plug it? If not where is this actuator located and how do I change it.

Thanks

okbarnett

I forget which colors go where. But you can plug them and see if it fixes the hot air coming in. The overview is; they all come from the silenoid strip in the console, you need to diagnosis further why and which ones may be leaking to solve the problem. I would suspect one of the vacuum bottles at the end of one of those lines has a cracked open rubber diaphram, thus the vac leaks out. You can also check the outside air 'in' door which is upside down over the fuse block. If if is staying open with the ac on , then hot air is coming in and overpowering the ac. Maybe block it closed. Push it closed toward the firewall.

byrdman454

The blue line is the recirculation actuator which controls the flap above the fuse block. Wally's writeup indicated this is not one line that can be blocked off.

The orange line is the center vent line. Wally indicated this one can be blocked off.

The green line is the the defrost. I do not know if it can be blocked off or not. I do not even know where this actuator is to replace. Jon Pirtle's website simply states "best of luck" when discussing replacing this one.

I need some insight into the green line actuator (defrost)

Thanks

okbarnett

White I believe goes to the hot water line cut off. This is a troublesome unit and cooks and cracks open its diaphram, and causes leaking vac. And this one leaks water too. Replace it frequently. The Blue (if you say so, goes to the outside air in above the fuses) can be blocked if you manually close that flap. It like most of the other ones is normally in the heat / or open position. I guess because in Germany they never had ac and all systems are go for the heating. So block that flap closed and block off the blue line. the others are in the air handler box. You can get to them all when you rebuild the box. Maybe a couple if you take the console off. they operate the flaps in the box. which ones I wouldnt remember w/out looking at a box. But you should be able to further diagnosis the problem after bolcking some lines and maybe getting to the vacuum silenoid strip.

SharkSkin

On my 78 at least, the orange goes under the hood to the heater valve. Try blocking this line at the engine bay and try different temp settings. The diaphraghm on the hot water valve should be checked to make sure it doesn't leak. If the vacuum holding the heater valve closed is marginal during idle and sedate driving(due to leaks in the diaphraghm and/or hose) then it might well open the valve when you open the throttle,which causes a loss of vacuum.

Come to think of it, if your check valve & vac reservoir are doing their job, the vacuum in the HVAC system shouldn't change based on manifold vacuum so check those too.

byrdman454

The white line does go to the heater valve. It holds vacuum and I have already replaced the valve. No problems here.

I need info on the green defrost line. Can I plug it without problems and where is the actuator located?

shaaark89

you can plug the green line, but it will leave you without the ability to use the defrost vents since the actuator will stay in the closed position.
that actuator is very tought to get to to replace. as i recall its way up under the dash on the drivers side of the center console.
the blue recirc line can also be plugged, which will leave you without the recirculation function. this means that fresh air will be used s the incoming air to the climate control. a bit harder to cool than recirculated air, but still should work fine.
orange linie can be plugged as well with no major consequence.
black line is the supply line to the climate control from the 4 way splitter under the hood so it should hold vacuum. if you block that one you will have no climate control at all.

okbarnett

the remaining lines go to vac units in the air handler box. The box consist of flaps and diverters to direct the air in different directions. There is one flap on the bottom for the footwell . It opens and air goes out to the L&R footwells. One flap goes to the top windshield vent. It opens and the air goes to the top dash windshield vents. The other (if I remember) goes to one that opens the main L&R side/door vent. So the climate control slide bar with the symbols for bar/up/down/up&down etc.. control each different vacuum silenoid electric accutiator (on the strip), which opens vacuum to that particular line and vac bottle. Each one should function as you slide from symbol to symbol. The hot/cold slide bar operates the electric motor that opens the main metal flap in the air handler box. It opens like a clam shell... When full cold = full open flap, max air flow thru ac evaporator strait thru the dash center vent opening.

touque

speaking of vacuum lines, as a temporary work-around, are there any problems 'manually' wedging open both the comb flap (center vent) and re-circ flap for the summer?

okbarnett

temporary means the whole summer? OK, you can block the outside air in/re-circ flap closed and not even block the vac line if that vac bottle doesnt have a cracked open diaphram. If its diaphram is open, then the vac will still be leaking there. You would have to block that line. That would keep the out side air out of the car. The center vent / main metal flap is a different story. It wouldnt have any reason to be malfunctioning unless the motor or some part of the controls where malfunctioning. It is moved by the electric motor. If you block it, you would burn up that motor. Unless you unplug the wire to it. That would screw up the whole climate control system. You couldnt even get to that flap w/out opening up the air handler box.

touque

I'm not sure we're talking about the same flap. Here's a picture, 'borrowed'
from Tony's rennlist web page here (hope that's OK Tony?)



I thought this flap was controlled by the orange vacuum line and not an electric motor? If not, then I definitely don't want to burn anything out...

okbarnett

Well I thought you refered to the inner flap that is metal and is controlled by the motor. Thats the front flap, which is vacuum controlled. It could be blocked open. I am questioning what sort of problem you might have that would require those 2 flap problems. If you have poor enough vacuum to cause those problems. It would be a tracable problem. I would suggest to start at the master cylinder. Check thru the blue check valve, black vac line, to the selinoid strip. If no leaking there, check each colored vac line by plugging off till the vac is retained.

bd0nalds0n

The orange line runs to the pod that controls the comb flap. If this pod leaks, the comb flap will stay down all the time. I actually prefer it this way; with the comb flap up, a disproportionate amount of air comes from the center vent, and almost none from the door vents. The comb flap down seems to create some pressure, forcing more air from the door vents.

The blue line runs out of the drivers firewall, across the cowl, down past the blower motor, and into the recirc box. Removing the recirc box is a royal PITA, since there's all the tarry undercoating to contend with, not to mention unfastening the fuse panel. Also, one of the blower motor mounting screws mates to the recirc box, and has to be removed from under the cowl. I found this out by breaking it.

The green line runs to the defroster pod, and is located behind the instrument pod. Since none of the writeups mention this, I was intimidated by it. However, it's easier to switch out that the blue pod or the orange pod.

The orange pod was difficult because it's hard to get the actuator arm connected to the flap, and IIRC, the arm is sized or bent such that it doesn't lend itself well to the swap-out-the-guts-but-leave-the-original-housing-intact approach. I ended up using a hole saw to cut a small hole next to the orange pod, so I could get into where the actuator arm mates with the flap arm. I have seen pix of a similar approach, 'cept the guy used a rubber stopper to plug the hole. I was more ghetto: used a nice piece of sticky duct tape.

The green pod fails "open." If this line doesn't hold vacuum, your defroster flap is always open, sapping flow from other more desirable areas.

To change the green pod, you must remove the instrument pod. While it's out, take scotch-brite to the three ribbon connectors. Your voltmeter and fuel gauge may work better. When reinstalling, be sure you re-connect the small wires leading to the rheostats for the instrument dimmers and wiper delay. Oh, also note that the connectors for the odometer reset and driving lights are identical fits. The other switches/connectors seem to have unique shapes to prevent against this. The driving light harness has more wires. Be careful not to switch these upon reinstall. BTDT.

There have been writeups about the design philosophy of failure modes: fresh air, max heat, defroster open. It seems Hans and Fritz didn't want your windshield to fog up on the drive home through the Black Forest. You may not fully appreciate their sensibilities as you experience this during summer in a Midwest river town having 90 degree days with higher humidity.

My advice would be to buy all four vacuum pods (yellow - feet, orange - comb, blue - recirc, and green - defrost) and then:

Pull the parcel trays,
Pull the steering wheel,
Pull the glove box,
Pull the console,
Pull the instrument pod,

and bang out the job all at the same time. This may take more than a full day, given all the stuff that has to come out--then go back in--after you figure out how to actually replace the parts that need replacing.

But if you don't you'll always be chasing something, and it's no fun to keep taking all this stuff out and putting it back in.

You may also wish to contemplate adding a separate switched power lead to the blue recirc flap solenoid. Later model 928s have a "smog" switch, I believe, which accomplishes exactly the same feat--recirculated air on demand.

Having recently done exactly what I've outlined above, I still have one more gremlin to work out: All my pods hold vacuum, all my solenoids work, and the motorized blending gizmo moves from top to bottom depending on the temperature slider. My hot water valve closes when the climate control is "off," however, when it's turned on it opens (solenoid loses power), even with the a/c switched on and the temp slider on max cold. If I can't get it sorted out soon, the white solenoid is another candidate for a switched power lead.

Good luck.