I replaced the black/blue valve at the brake booster, and the rubber 4-way connector underneath it today. Then I took the car for a drive.

When the AC was on, I could hear a constant "air sucking" noise from the center console. It changed a little when I opened or closed the center vent, and disappeared if I turned off the AC. The air was not cool at all - no difference between AC on or off.

The AC had recently started making air noises, but mainly when I stopped at a light/intersection, and the engine went into idle.

Did I pull a vacuum hose that goes from the engine compartment to the AC? Where exactly in the center console are these connections, and what needs to be removed to get to them?

 

Nicole -

When you push in the AC button can you hear the compressor come on? Is the temp sllider all the way to the left?

With the AC on can you see oil moving in the sight glass on the compressor?

The climate control system uses vacuum actuated flaps to direct hot/cold air and mix in fresh air. Over time it's common for these flaps to lose thier ability to close tightly due to dirt build-up and vacuum loss at the connectors. When you first turn on the AC it's not uncommon to hear the vacuum trying to close a flap... and then suddenly hear it "thwap" shut. The solution to that problem is called "spring cleaning": actually get in to each of the flaps and cleaning them... then checking the vacuum connections at each of the five actuators. It's all detailed in the workshop manuals.

You might also want to look through the climate control section on Greg Nichols website: <a href="http://www.nichols.nu/tips.htm" target="_blank">http://www.nichols.nu/tips.htm</a>.

 

The HVAC system can be considered as four related subsystems:
Electrical
Refigeration
Temp Control
Air Flow Control


Electrical:
Electrical operates the compressor and the blower. Most common problems are relays, fuses, bad connections and the resistor pack.

Refrigeration:
Find the line going from the firewall to the compressor, with the engine running and A/C on, feel of it. If it is cold, the refrigeration system is OK.

Temp Control:
Most common problem is a bad connection to the outside air temp sensor in the alternator cooling hose.
With the engine running and warm, A/C on, move the temp lever to full cold and feel the air temp. Move the lever to full hot and feel the air temp. Move the lever almost all the way back to cold. If there is a noticeable difference in the air temp between full hot and almost full cold, the temp system is probably OK.

Air Flow Control:
Usually bad. The most common problem is that the thin rubber diaphragms in the vacuum actuators used to control airflow split. Soon the leakage gets bad enough that the tiny vacuum line feeding the system can't overcome the leakage, and there is not enough vacuum to pull the hot water valve closed.

There are two fixes - temporary, and right.

Temporary:
Pull the left console side cover, find the vacuum manifold. One at a time, pull the colored lines off, and check each to see if it will hold vacuum. If it won't, cut a half-inch off the end, put the short end back on the manifold, and plug it. The functions controlled by the bad actuators will still not work, but they won't keep the rest of the system from working. When you replace the bad actuators, the short colored hoses show where to reconnect the lines to the actuators.

Correct:
Replace the actuators. If I were going to the trouble of pulling the console, I would replace them all.

 

Wally, great write up as always.


Nicole,

Question: was the A/C blowing cold, hot air or in between?

If I remember correctly there is only one main hard plastic vacuum line that runs from the four-way to the vacuum manifold under the center consoled. Attached to the vacuum manifold are the electric solenoids (Electric over air control valve) that controls were the vacuum is to be applied (or to which diaphragm).

You said:
“It changed a little when I opened or closed the center vent, and disappeared if I turned off the AC.”

I don’t remember off hand what part the micro switch in the center vent plays, but since the sound changes when the A/C is switched off and on tell me it is a post solenoid leak.

I’ve spent a few hours on my 89 HAVC system chasing vacuum leaks and ended up replacing the center vent control diaphragm. It’s not as had as it looks!

Here is what I would do.

1. Find the main vacuum line that goes to the dash at the four-way.
2. With your Mighty-Vac pull a vacuum on it and see if you see a leak down.
3. Pull out your “Owners Manual” and look up what controls open and close each controlling diaphragm.
4. Plug the four-way off so your not losing engine vacuum (A golf tee will work).
5. Plug your Mighty-Vac into the vacuum line under the hood (same on as in 1.) (I added a five-foot vacuum line to mine so I could do the following from inside the car).
6. Start the car and turn on the HAVC system.
7. Run the HAVC controls through their paces.
8. After you have made a switch from one control diaphragm to another you may need to reapply a little vacuum. What you will be looking for is a diaphragm or hose connection with a quick vacuum loss.


Let us know what you find


Cheers

Max

 

Thanks, guys! I guess now I have plans for the coming weekend...

The AC had been serviced by the local Porsche dealer last summer (freon leak), and was very cold when I drove the car the last time before replacing the black/blue valve. However, I had heard a different air sucking noise and a flap "slaping" when I stopped at traffic lights and the engine went into idle.

Yesterday, the AC did not blow any cool air inside, and the sucking noise was always present, no matter what I did with the AC controls. The only way to stop that noise was to turn the AC off.

I might have two problems here, an old and a new one. But at least I now have a better understanding on how to work this. Especially, after browsing through the manual (no, I did not read it in detail, yet), I was confused as to where those solenoids are actually located.

I will verify that the compressor and everything else works (I believe it does, because yesterday the idle changed slightly when I turned the AC on and off). However, it sounds like this is a pure vacuum problem.

I will keep you updated!

 

I would be 100% certain that the clutch engages first before you go chasing other things. Just because the AC button is IN and the idle changes doesnt really mean the clutch is engaged. If i recall, even when my AC was INOP, pressing the AC button momentarily changed the idle and the fans engaged. Get some one to sit in the car and press the AC button as you watch under the hood. start there...then you can go onto other fun detective work

My guess is the hissing is a HVAC pod &lt;evil grin&gt; comflap or the footwell, they are direclty behind the console area. I think it was mentioned before but there is a Vac manifold in the center console area. Remove the driver side, side console carpet and you may be able to see where the vac lines all run to. I basically pulled all mine off and hooked a tester to them. When i first got my car they all leaked except the DEFROST actuator!

Also, if any of those lines are off under the hood, you just introduced a leak to the entire system..MEANING, the HOTWATER valve will default OPEN, allowing hot water to the heater core.

bunch of ideas i guess.....

 

Nicole,

If all else fails, go over to:

San Carlos Radiator
<a href="http://www.sancarlosradiator.com" target="_blank">http://www.sancarlosradiator.com</a>
777-787 Industrial Road
San Carlos, CA 94070-3310

Phone: 1-888-593-3193

Talk to Mark. Mark knows 928 a/c systems very intimately. DEVEK refers their customers to him. I've been a customer of his. Twice (in different 928's). Now running an ice cold R134a system and very happy.

Cheers!

 

Could it be possible that the blue/black check valve was installed upside down? Not sure of the problems that it would cause, but it is a 1 way valve I think.

 

Thanks for all your great input!

The blue/black valve came pre-assembled with the 4-way connector and a rubber hose. I replaced the whole thing without taking that new valve out of it's assembly. It has the same direction as the old one.

 

Nicole,

Just for grins, have you tried putting the old valve and splitter back in place and see if the problem goes away?

 

Hi Nicole,

Sorry about those ac woes. I went through too much money to admit on ac service. Four sets of “qualified” mechanics including the Porsche dealership failed to correctly diagnose or solve my vacuum problems. I finally listened to Dave Roberts whom Wally P works with. Dave got me to get the vacuum pump, and start at the 4 way connector. See if vacuum holds on the hose through the firewall. Leakdown is indicated when the gauge on the pump falls off quickly toward zero vacuum after you pumped it up. That hose leads to the vacuum manifold behind the ashtray. Your leak is almost without a doubt there. Remove the left center console panel, pull the hose from the manifold, and check for vacuum in the manifold. If it leaks, pull each of the 6 hoses to find the leaking actuators. If no leak at manifold, you may just have a loose or defective hose that came through the firewall. The reason that I would start there is the air leak noise that you hear. As Wally said, brittle hose ends can be snipped off. Soft rubber should start about a half inch from the brittle end. You can also coat connection points with silicone sealant to eliminate leaks.

You are getting plenty of good advice. Mine is to start with the vacuum. As Tony said, the vacuum controlled hot water blending actuator (under the air filter) will always bleed hot water into the ac system/heater core until all vacuum leaks in that system are fixed. (If you replace that actuator, also replace the 3 inch long water hose connected to it.)

I had 2 bad actuators. The rubber diaphragms had little cracks. I pulled the center console and instrument pod to access them. If I went that far today, I would replace all of the actuators when they were accessible. See this link on Greg Nichol’s site for applicable procedures from Wally and company: <a href="http://ourworld.compuserve.com/homepages/nichols/tip509.htm" target="_blank">http://ourworld.compuserve.com/homepages/nichols/tip509.htm</a> I wrote a center console removal procedure that was put onto Greg’s site. Instrument pod removal is documented there too.

By the way, the vacuum manifold contains the solenoids. They control the actuators. The solenoids practically never fail.

You could be really lucky, and just have a bad vacuum hose connection.

Good Luck,
HTH,
<img src="graemlins/oops.gif" border="0" alt="[oops]" />

 

I'll echo others recommendation to suspect that black/blue check valve you replaced.

It is a one way valve - airflow should be allowed in the direction from the firewall to the engine - not the other way around.

 

Nicole-

Perhaps the easiset thing to do is to UNDO your recent maintenance and check that the problem is still there.

If the problem still persists, then go from there.

hth-

 

I guess I made a mistake. I threw out the old check valve after putting the new one in. You guys are right - I should have done a direct before and after test...

However, you will be delighted to hear that I invested in a MightyVac today. With that and your instructions I am making progress towards my 928 mechanic degree. I guess I'm going to flunk on the engine and transmission rebuild class...

I'll keep you updated!

 

Nicole,

You inspired me to check out my vacuum leak issues on my HVAC tonight.

I pulled the line from the spliiter (fed by the infamous black/blue valve) and pulled the other end at the console. Easy to get to by removing driver's side console carpet. I plugged the console side and ran a vacuum check with the mityvac. That line was leaking. Yes, the line itself, not the rubber ends that the line connects to. I could wiggle the line where it goes into the firewall and the vacuum would bleed out or drop faster. I put vacuum on the console connector at the manifold that the bad line feeds and it held steady. No leaks on any colored lines. My problem was the vacuum feed line from the check valve to the console.

I ran a new rigid line from the check valve (through the firewall, taking same route as original line). Now the selector works much better, switchig from vent to floor to defrost.

There are two other lines which run off of the splitter at the check valve. I also have a leak in those lines. They go into the wheelwell area. I need to research the archives and the manual on what those lines feed. I mention this because this leak is causing the vacuum at the console to be lower than I think it should be. If I plug those lines to the wheelwell, vacuum as metered on a vacuum guage reads higher at the console.
My point is that you may find a vacuum leak feeding th HVAC as I did, or a leak at the manifold behind the console, but be sure you have sufficient vacuum feeding the entire system.

Greg