W126 climate control - centre vent not blowing cold - blend door servo diagnosis

Rohan - the W126 climate control centre vent on the 1986-1991 (Push-Button Climate Control) is fed from the centre blend door servo, which is a seperate vacuum-actuated unit from the side vents. Common failure modes:

• Centre blend door vacuum line cracked - most likely. The line runs from the climate control head behind the dash, down the centre console, to the blend door servo behind the centre air duct. After 35+ years the rubber junction at the servo cracks.
• Centre blend door servo diaphragm failed - second most likely. Test by applying vacuum directly with a Mityvac. If it holds 10 inHg for 60 seconds, the diaphragm is fine. If it leaks, replace.
• Climate control head failure - least likely at this age but possible. The internal vacuum solenoid stack can fail.

Genuine MB centre blend door servo: 126 800 12 75, $380. Aftermarket equivalent from Pelican: $145. The Pelican unit is acceptable and lasts approximately 8 years.

Diagnosis tools: a Mityvac vacuum pump (Repco $65) and 60 minutes of patience.