Hi Folks,
I'm trying to work through some electrical and nonfamilarity issues with my 89 S4 automatic. I have replaced all the bulbs in the climate control unit and replaced the diahragms in the two upper vacuum switches inside the car while I was in there. One was bad.

Why Porsche has to be so obtuse/opaque? with their ventilation system is beyond me.
Every other German car I've had from the 80's has the ability to let in direct outside air while either the heat or A/C being engaged or not. Most owner manuals list it as an anti drowsy feature.
The 928 owners manual does not mention anywhere the ability to draw in untreated outside air. Mine only states that with the temp slider all the way cool and the outlet slider in the next to furthest left position, the air is not temperature treated. I guess that means from the outside?

Using that as a starting point, the air then coming from the center column and doors is a fair bit warmer than the outside air. After a bit of solid confusion I noticed that the upper and lower center mounted vents were passing different temperature air?! The bottom was warmer than outside and the top seemed to be direct from outside. This is not mentioned in the owners manual. More experimentation and I find with the outlet slider set to the combined up and down positions, outside temp air seems to come from the upper center vent while nothing comes out of the bottom center vent. Is this proper function?

While still in this setting warmer than outside air continues to come from the door vents. As the top center vent alone is really insufficient to "vent" the whole car, Is it also possible to get outside temp air through the doors and center vent if so what is the setting?

Perhaps most surprisingly when I turned on the A/C it actually provided cooled air to 60 degrees or so with cooled air coming out from the upper center vent and the doors. Definitely need at least a recharge. In trying to find some answers I came across a site showing another vacuum solenoid, similar to this inside the car under the rain cover in the under hood scuttle. What is the function of that one and should I also look to replace that diaphragm?

thx all.

Mike

 

Hi Mike --

Guessing my 1989 car is somewhat similar to yours... Put the vent slider under the square symbol next to the off position, so one click from left. Make sure the vent lever in the center is up, so the switch is not true. Make sure that the comb flap is operating correctly. Set the temp slider somewhere less than ambient temp. Fresh air will dispense from the top half of the center vent, less flow but same air from the bottom half of that same vent. Note that "fresh air" isn't the same as air that's passing from outside directly to the center vent. In fact, it passes through the blower and air box including the AC evaporator if everything is working correctly. There's a seal on the blend door flap in the box that ratios flows between the evap plus heater core side and the evap-only side. With heat through the cowl and maybe through the firewall, heat soak in the cabin and the HVAC air box and such, the "fresh air" tends to be warmer than ambient air.

There is no HVAC vacuum solenoid or diaphragm under the cowl cover in the engine bay. Closest is the heater control valve with a vacuum actuator, mounted/connected at the rear of the right side (passenger side on US cars) cylinder head under the lower air cleaner box. These valves tend to leak coolant through as they age. I had one that had a failed vacuum diaphragm. Know that any leak in the HVAC vacuum system risks not closing that heater control valve completely, so you have heat from the engine helping you warm the cabin.

Search will get you a lot of info on testing and servicing the vacuum system and components. You've dipped into that already it seems. The system MUST hold vacuum before you go into refrigeration issues, heat flow and balance. One tiny and otherwise innocuous HVAC vacuum leak will give you the temp performance you report.

Mine blows ice cubes. The cold is in there, but it sometimes takes a lot of careful work to get to that level of system performance.

 

Bob,
thanks a lot for the detailed response. I had forgotten (funny how your mind can blank out traumatic experiences) when I had the center stack apart that the vents are simply a pressure fit.
I pulled them out and tested as you mentioned and could see the comb door closing but not being able to hold vacuum well enough to stay closed even though I replaced both major solenoid diaphragms. It just opened again slowly,
With the vents removed you really get nice air flow through there which is the way the car should have been if you ask me.

I have ordered a Mity Vac and will begin looking for leaks etc.

During my searches I had run across an extensive article on a very early car which had one of the solenoids under the rain cover in the scuttle. But it led me to think that I should
preventatively replaced the coolant flow valve and the often dodgy one way vacuum valve of which I have many spares from my Audi days.then I should have a better baseline.

You mention "Make sure the vent lever in the center is up, so the switch is not true". While all apart I checked the switch to make sure it was good. When the vent is closed and the switch triggered, can you tell me what signal it sends or disrupts to the rest of the system?

thx
Mike

 

That center vent switch seems to affect the way flow is shared among the other vents but particularly the "blend door". There's a servo-operated mixing valve that ratios flow between the heated and un-heated pathways through the air box. Operating the little vent lever down diverts more air through the heated side. Effectively, you are restricting flow through the top half of the vent similar to what the comb flap does for you automatically. As you restrict the cold-side-only flow at the airbox outlet, the blend door operated by the servomotor adjusts the airflow at the inlet to the two sections. It's a double duplication of redundant function (that's 8x!...) dreamed up by a Behr engineer who had never actually worked on an older automobile. The design (and designer) ASSumed that every servo and actuator will always and forever work perfectly. And of course, they always do in our 928's.

The heater control valve is a wear/service item. Our better suppliers here have identified a specific replacement that's a lot more durable than the generic knock-off pieces you can find at low-cost outlets. Replace it, along with at least the short connecting hose from the cylinder head hose nipple. If the system is drained and you have the coolant out, replace the rest of the heater hoses and the stub that connects the bottom of the reservoir to the steel return Y pipe that runs along the right side of the engine bay on the inner fender wall. I think there are four rubber sections between the heater valve and the front water manifold, in addition to the head-to-HCV connector hose. In my limited experience, it's ever so slightly more convenient to change all of those in the relative comfort and convenience of the workbay, vs. the side of the road in east nowhere, 110º plus in the blazing sun, with the toolkit screwdriver and those last couple bottles of drinking water to refill the system. Those are thirty five year old hoses that fortunately have never been hot or stretched, only stored safely away in the cocooned car in the climate-controlled storage museum.

 

I drove the car yesterday and made some temp adjustments and moved the vent lever around. I could hear the little switch click between closed and open, but I didn't hear the setting motor move at all, and couldn't feel any changes in airflow or noises regardless of the switch position. I need to go back through the system descriptions in the workshop manual to see what it says about the switch.


Scratches head...

 

You mentioned having replaced all the diaphragms but are still seeing leaks.* One thing I found while going through mine is that the diaphragms are not the only leak point...the electrically-operated vacuum solenoids (all in a row under the center console) can fail as well. Somewhere there is a description of the test procedure for them, but basically, you need to think about their behavior in both on and off states, and make sure they are not introducing a leak which would allow the little pods (with the diaphragms) to lose vacuum. I ended up replacing two of those with used ones from 928 International (new are either NLA or very expensive I think).

*I just re-read your post and you said "both" of the major...diaphragms. I think there are 4 or 5? So if you just did two, you may still have a leak in one of the others.

 

Success on 2 fronts. The heater control valve was an original all plastic type that had failed, so in went the new metal one. I also replaced the one way vacuum check valve by the brake booster. I had never seen one so deteriorated that the housing was actually suspect. I had several spares so in it went. Bonus that it stopped the annoying buzzing/vibration noise I had that seemed to emanate from the dash pod. I could not place it, less of course someone had said how about the buzz from a bad one way check valve! I hope to drive it soon to see the difference, though visually the comb filter moves and stays both engaged and or disengaged. It does seem to take a longish time, 10-15 seconds for the heater valve to open or release after the HVAC change is made but at least it works. Full systems check will happen when I take the car offline for the season. Thanks for the help!