The black vacuum line is the main vacuum supply to the entire climate control system. A leak  anywhere in the system, will create a leak at that point. That black line goes to a manifold under the heater system, just in front of the central console. Check page 87-104 in the shop manual for a complete schematic. Again, looks more intimidating than it really is....

From there is is routed via 5 individual solenoid valves to : 1) the flap in the passenger footwell (blue hose) 2) the infamous water valve behind the firewall (white hose) 3) the windshield air flow flap (green) 4) the flow limiter flap (comb like) behind the center vent (orange) 5) the footwell air flow flap (yellow)

If you really want to isolate individual leaky components, you need to disconnect each individual line (from the individual valve) , connect to your magic vacuum pump, then check for operation AND leaks! If it doesn't hold vacuum for at least a'll have to work on that one!

When testing from inside the car, I found that the easiest way was to plug the black hose in the engine compartment, and remove the little blue plug at the end of the manifold to connect the vacuum pump. Now you are ready to check each individual component from inside the car, without having to run the engine! Check the table on page 87-106 to verify proper position of the various components

This is how I found I had TWO leaky components: the heater shut off valve and the center vent "comb" actuator. We have already covered the heater shut off But, replacing that other vacuum actuator , would normally mean you have to open the whole heater box ....OOOUCH!

Here's what I did, as a shortcut: (I assume you have removed the glove box) - Pop the lid of that center vent actuator (before you pop yours! (-: ) - remove the coil spring - pull the neoprene diaphragm and the connecting rod towards you, and lightly twist back and forth.. it will disconnect from the flap. No need to exercise too much force, you could break the plastic tag on the vent!! - replace with the new diaphragm (taken from a new actuator...)

Now, for the REAL fun: you'll have to re-connect the new rod to the flap! Remove the passenger side's vent hose (the one bringing it to the door vents) , and looking from the passenger side windshield pillar, you'll just barely be able to see (Blink and use your right eye kidding!) the little plastic tag where the metal rod is supposed to connect. Takes some long tools to (carefully!!) push it back together (I use two 10 inch extensions, from my 3/8" socket set...worked like a charm.) You also need a really small light to see what you are doing in there (the very small Mag Lite does wonders here!) Not easy, but certainly beats taking the whole thing out of the car!!

Finally: - reinstall the spring and lid - check function I feel lucky my windshield air control flap is ok!! (That one would require to tear half the car apart to replace!!)

Here's my take on the binary temperature problem, based on a sample size of one:

There is a servomotor that controls your heat, located above the driver's right knee. You must remove the parcel shelf to get at it. It has a little arm which controls the temperature mixer flap. If the arm is all the way up, you get max heat; all the way down, no heat. Normal position with the inside temperature stable is roughly  horizontal. So the first step is to find out what that little arm is doing. With the parcel shelf out, you can feel its position while sitting in the driver's seat. If it never stops anywhere but all the way up or all the way down, you have isolated the problem somewhat.

The key input to the servomotor is pin 4 of the upper plug (there are two). It connects to the outside temp sensor (in the air duct to the alternator), which connects to the inside temp sensor (behind the slotted fitting in the dash next to the glove box), which connects to a rheostat on the temperature control slider. According to the manual, the resistance between pins 4 and 12 should be about 3.7Kohms with the slider at 18, and about 4.7 Kohms with the slider at 30. I think that assumes approximately room temperature for the sensors.

Now, if there is any break in the chain from outside temp sensor to inside temp sensor to temperature control slider, you will get the binary temperature problem. So check that reading, if you're electrically inclined. If it's wildly different, you've found the problem. If not, maybe you'll be lucky and the connector just needed reseating.
-John White-
'84 928S

Do you really want to know that your interior temperature sensor is broken?
Then get a fine cigar.  I suggest an Arteuro Fuente or Romyeo y Julietta, preferably a large ring size.  (The latest Macanudo I bought was rolled much too tightly.  Otherwise, I would suggest another of my favorites.) Smoke the cigar about a third of the way.  Start your engines.  Turn on the heater.
Hold the hot end of the cigar next to the inlet (top right side of center console).  If some smoke is not drawn right into the sensor, it is broken or stuck.

Mine was stuck.  I had loosened up the center console to install a new dash pad and instrument pod.  I touched the fan on the sensor back side, and it started to spin.  Therefore, I pulled the sensor; cleaned and lubricated it. It now passes the cigar test.

The center console needs to be loose, and 4 or 5 inches away from the dash to remove the sensor.  It is held on by 2 phillips screw.  It also has two electrical clips attached to wires that pull apart.  The clip on the sensor tube at the back side of the center console pulled straight off.  You should now have the sensor in you paw.  You can dismantle the sensor by gently springing lose clips.  Clean up the fan.  Lubricate the motor shaft ends.
Reinstall.  Try functionality before you reassemble console.  If it works now, reassemble.

Thom Ross

The Interior Temp. Sensor is on the top right side of the center console.  See the black circular vented device about 1.5 inches in diameter.  See the below procedure for removing the center console.  It may not be necessary to remove the knee shelves.  You will not need to unplug console wire clips.  You may not need to remove radio.  However, it should pull right out with the proper release tools engaged - depends on radio manufacturer.  In other words, release all bolts holding console in place to allow it to be pulled back 4-5 inches.  2 phillips screws are on back side of console that attach the sensor - remove 'em.  Pull off tube from console vent.  Unclip the 2 wire bundle connectors.  Sensor is now in your paw.  Proceed with clean and lubrication - see Interior... post below.  This will only fix a stuck fan on the sensor.
However, that may be all that is wrong.

To further check sensor, make sure that it has some resistance - with ohm meter.  I have heard that resistance will change on a good sensor when it is heated or cooled.

Thom Ross

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 so, 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. 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 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