Black Balls on the Hood

The black thing is a vacuum reservoir for the A/C system since the system works via vacuum pressure rather than via cables.  In conjunction with the check valve, the reservoir retains vacuum in the HVAC system so that the vent flaps remain open under periods of low engine vacuum.  A/C cars have 3 vent flaps to operate (separately or in combination with each other depending on the setting), compared to non-A/C cars' 1 vent flap.  

Vents

How the Vent System Works

Refer to diagram at right:

1 = Windshield defrost vents: fresh, cooled or heated air

2 = Side window defrost vents: fresh, cooled or heated air

3 = Side dash vents: fresh or cooled air

4 = Footwell outlets: fresh, cooled or heated air

5 = Center dash vents: fresh, cooled or heated air

All vents are temperature controlled, except for the side dash vents (#3).  

Only Cold/Fresh Air is Coming out of Side Vents

That's how the system was designed.  See above.  To modify the side vents to output heated air, see the DIY Guide link at left.

No Air out of Center Vent

• Vacuum leak somewhere in the system.
• Faulty vacuum servo (inside the dash, just left of the glove box).
• Faulty air distributor (inside the dash under the radio, right side).

No Heated Air out of Center Vent

• The control valve is not opening: The control valve is faulty, or is incorrectly adjusted.  The control valve is located in the engine bay, near the brake fluid reservoir.
• The control valve cable is faulty.  The cable is attached to the temperature selector on the dash and to the above-mentioned control valve.  Check to see if the cable has become detached at one or both ends, or is binding somewhere.
• There is a blockage in the heater core and/or related hoses, or heater core itself is faulty.

Only Heated Air out of Center Vent

• The A/C system needs a recharge.
• The control valve is not closing: The control valve is faulty, or is incorrectly adjusted.  The control valve is located in the engine bay, near the brake fluid reservoir.
• The control valve cable is faulty.  The cable is attached to the temperature selector on the dash and to the above-mentioned control valve.  Check to see if the cable has become detached at one or both ends, or is binding somewhere.

Troubleshooting Steps for the Above Issues

The HVAC system is composed of these main components: vacuum hoses/tubes, vacuum reservoir, vacuum distributor, vacuum servos which operate vent flaps and a vacuum check valve. If any of these components fail, air flow will be affected.  It is highly recommended that you purchase a vacuum pump, such as a MityVac, to test the system and its components as outlined below.  Please refer to the HVAC vacuum diagram for the following steps.

Step 1

Remove the check valve in the engine bay and apply vacuum to the port on the "motor" side of the check valve while a finger plugs the opposite port.

Does it hold vacuum?  Yes: The valve is good; reinstall it and move to step 2.  No:  Valve is faulty; replace it.

Step 2

Remove the radio (and instrument panel bezel, if desired).  Using the diagram, carefully remove the center vent/defrost servo hose from the vacuum distributor (hose "A" in the diagram).  Place the hose onto your vacuum pump and apply vacuum.

Does it hold vacuum and open the center vent flap?  Yes: Reinstall the hose and move to step 3.  No: The center vent/defrost servo is faulty; replace it (this requires removing the dash).

Step 3

Carefully remove the yellow vacuum tube from the vacuum distributor, connect it to your vacuum pump and apply vacuum.  Move the top selector lever in any direction.

Does the distributor hold vacuum?  Yes: Reinstall the hose and move to step 4.  No: Vacuum distributor is faulty; replace it.

Step 4

Make sure all vacuum lines and connections are properly connected and are not leaking; replace as needed.

HVAC Controls

Cars with Air Conditiong:

Cars without Air Conditioning:

Blower Fan

The Fan Works on Only 1 or 2 Settings, or Does Not Work at All

The fan motor is worn out, the fan switch is faulty, wires are faulty, or the resistor is faulty.  Consult the Bentley Service Manual for repair information.  Also see this thread and/or this thread on VWvortex for further information.

Blower Fan Does not Work on Speed 4

• If the blower fan works on speeds 1, 2 and 3, and the A/C works on all three settings, the culprit is most likely a faulty A/C relay.
• If the blower fan works on speeds 1, 2 and 3 and the A/C compressor is not engaging, the culprit is most likely a blown A/C thermofuse (located on the relay panel).

Installing A/C into a non-A/C Car

Even though A/C was an option, in the United States non-A/C Cabriolets are more rare than A/C-equipped Cabriolets.  On the flip side, air conditioning was a very expensive option outside the States and, in most instances, not really necessary.

All of that being said, if anyone in the USA requires A/C but has found a fabulous Cabriolet to buy without A/C, keep looking.  Swapping A/C into a non-A/C car just simply isn't worth the expense or the trouble.  If you already own a Cabriolet and want to install A/C, look into all of your options.  Anything is possible, as the saying goes, but installing A/C is a somewhat complicated endeavor.