Updated: 11-Jul-2023
Quick link: HVAC vacuum diagram
Black Balls on 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.
The position and orientation of the reservoir as installed at the factory is seen in the photo above/at right.
Vents
HVAC Guide: Components, Control Functions, & Troubleshooting (A/C-equipped cars): 
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 in the menu.
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.
HVAC Controls
Cars with Air Conditioning:
Please download the HVAC Guide for complete explanations.
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).
Retrofitting A/C into 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.