Climate Control

Vacuum Testing

Vacuum Testing

>I've seen various and sundry posts on the topic of vacuum and the many
>functions it controls in our cars. I'm fairly certain, after having changed
>out my vacuum-controlled heater valve and that since warm air is still
>entering through the center vent, my 88 S4 has some sort of vacuum leak.
>Where to begin?

Let me refer you to the excellent posts from Scott Shambaugh [below] regarding the recirculation flap and vacuum testing the climate control system.

Also, keep in mind that when you shut your engine off the heater valve returns to the "open" position and allows hot coolant/H2O to flow to the heater core - this helps the engine cool down - but the side effect is that the heater core will now be warm, so if you restart your engine within a few of hours you are likely going to get warm air coming through the center vent... You can cool the air down by simply turning on the a/c for a minute. After that, you should have fresh (cool) air coming from the center vent.

Hope this helps. (Thanks Scott!!)
- michael

Scott's original post:

I verified this problem with the Mytivac by attaching it directly to the line that feeds the HVAC system (the line that attaches to the X connector near the break booster and feeds into the fire wall). With the ignition on, engine off, the test port vacuum held fine (a test I had performed earlier thinking the HVAC vacuum system tested good) as soon as I pressed the A/C button the system lost all vacuum.

So unless I am missing something I appear to have a vacuum leak in the HVAC control system managed by the vacuum manifold, but only when the A/C compressor is turned on. This would also explain why my cruise control wouldn't hold speed while the A/C was on (same vacuum system).

Scott's follow-up post:

For those of you who remember and have interset in this subject I now have extreamley cold air from the center A/C vents and my cruise control works perfectly. The vacuum problem was in the climate control area, specifically the fresh air recirculation flap bulb. After removing the bulb and taking it apart there was a hair line fracture of the rubber embrane, thus when vacuum was applied the flap did not function properly. This also allowed a vacuum leak for the entire system managed by the vacuum manifold to malfunction. I replaced the bulb (928 Specialists approx $16.00) and the A/C works to well, I have to adjust the temp setting away from cold in 90+ temps here in Nashville with the rear A/C off!

For those of you who are having A/C woes and suspect a vacuum problem, I would suggest this simple test that will isolate whether or not you have a problem in the climate control area of the system (all vacuum components in the dash to include the vacuum line that feeds the infamous hot water valve) this will require a Mitivac or similar vacuum test devise, if you don't have one you can get one from any auto supply store ($25.00) it is very easy to use, I never used one before and figured it out in 5 min. of messing with it.

1) From the brake booster extends a vacuum line that has a check valve (small 1 inch in dia. valve) the other side of this valve is a line that extends to a X connector.

2) The other points of the X have a
a) test point plug
b) line into the wheel well that goes to the cruise control
c) line that goes into the firewall that is the feed for the entire climate control,
including the hot water valve.

3) Remove the line that goes into the fire wall from the X connector, attach the vacuum tester directly to this line, with the egnition off apply vacuum to the line, it should hold vacuum if it doesn't you have a vacuum problem in the climate control, now turn the ignition (you don't have to start the engine) and turn on the A/C (this actuates flaps) now apply vacuum to the line, it should hold vacuum, if it doesn't you have a problem in the vacuum manifold area.

This is how I isolated my vacuum leak, if you find a leak here you will have to get into the dash behind the area of the radio and isolate your leak from the vacuum manifold by simply pulling off vacuum lines one at a time to check that each will hold vacuum.

Hope this will help anyone willing to attempt their own fix!

Scott Shambaugh
86 928 Auto
Nashville, TN

>I'm getting a constant flow of warm air out of my side vents at all temp.
>settings. I've replaced the heater valve,but the problem persists. I
>assume it's a vacuum problem...what should I look at first?
>Rich '81 5-speed

This is a generalized vacuum test. Your system may vary from this. BTW, we sell an inexpensive kit that includes the proper 4-way connector, check valve and lines.

1) Set the temp lever to the minimum temp. Pull the vacuum line off of the heater valve, and crank the engine. Vacuum closes the valve, so you should have strong vacuum at the disconnected hose. If not, reconnect the hose and go to step 2).

2) Pull the hose off of the inlet side of the vacuum check valve located near the brake booster. There should be a strong vacuum. If not, trace the line back and check for a disconnected or collapsed line. If there is a strong vacuum, reconnect the check valve and pull the line off of the other end. There should be the same level of vacuum at the valve connection. If not, either clean the check valve with spray carb cleaner and low-pressure compressed air, or replace it. If there is a strong vacuum, go to step 3).

3) Carefully check the rubber 4-way connector for leaks. If any are found, repair or replace the connector. If no leaks are found, go to step 4).

4). Find the vacuum line that disappears into the fender, and the line that goes the other way and disappears into the firewall. Pull the line off that runs to the firewall. There should be a strong vacuum at the 4-way connector. If not, pull the line off that disappears into the fender and plug that opening on the connector. Recheck the vacuum level on the connector that goes to the firewall. If the vacuum increased, remove the fender liner and check the vacuum reservoir and its lines. Repair or replace as required. If there is good vacuum, reconnect everything and go to the next section.

Remove the carpeted panel from both sides of the center console. Locate the vacuum manifold. It has six colored vacuum lines attached. If possible, get a vacuum tester, such as a MityVac hand pump. (Given the number of vacuum problems you will face over the years, I would suggest buying a MityVac.) If you can't get a tester or pump of any kind, you can use the self-contained oral vacuum source (put it in your mouth and suck).

Remove and test one line at a time, replacing each line before removing the next.
The lines are:
A - Yellow - Footwell Flap
B - Green - Defroster Flap
C - Orange - Center Nozzle Stage One
D - Brown - Center Nozzle Stage Two
E - Red - Mixing Flap and Heating Valve
F - Blue -Fresh Air Bypass Flap

The Red line is the most likely suspect if it is a vacuum problem.

If the problem is not a simple vacuum problem, check pages 87-104 and up in Volume IV of the shop manuals for the A/C troubleshooting. The setting motor (temp control motor) and the heater valve are two likely suspects.

Wally Plumley
928 Specialists

928 Tips Home     Greg's Home