Genuine OEM 1238000078 - HVAC Control Vacuum Control Valve

Product Overview

This Genuine OEM 1238000078 HVAC control vacuum control valve is a factory-original Mercedes-Benz component engineered to regulate vacuum-operated HVAC functions across a broad range of classic and modern Mercedes models. The valve manages vacuum distribution to heating and air conditioning blend doors and mode selectors, ensuring precise climate control operation. Fits Mercedes-Benz W116 (450SEL, 450SEL 6.9), W123 (230, 240D, 300CD, 300D, 300SD, 300TD), W126 (380SE, 380SEL, 380SL, 500SEC, 500SEL), and W463 (G55 AMG, G63 AMG) models from 1973 to 2018. As a Genuine OEM part, this valve meets factory specifications and is backed by Mercedes-Benz quality assurance.

Symptoms of a Failing HVAC Control Vacuum Control Valve

• HVAC blend door stuck in one position, preventing temperature adjustment
• Mode selector unresponsive; unable to switch between vent, heat, defrost, or floor modes
• Hissing or whistling sounds from the dashboard when the HVAC system operates
• Inconsistent heating or cooling despite thermostat adjustments
• Vacuum leak audible near the HVAC unit under the dashboard

Cross Reference Numbers

This Genuine OEM 1238000078 is equivalent to or replaces the following part numbers: 123-800-00-78, 1238000078.

Vehicle Compatibility

This part fits the popular models below. For Fitment Confidence, please use the vehicle selector tool above to verify exact compatibility for your vehicle.

• Mercedes-Benz 230 (W123): 1977–1978
• Mercedes-Benz 240D (W123): 1981–1983
• Mercedes-Benz 300CD (W123): 1978–1985
• Mercedes-Benz 300D (W123): 1981–1985
• Mercedes-Benz 300SD Turbo (W126): 1980–1985
• Mercedes-Benz 300TD (W123): 1981–1985
• Mercedes-Benz 280CE (W123): 1981
• Mercedes-Benz 280E (W123): 1981
• Mercedes-Benz 380SE (W126): 1984–1985
• Mercedes-Benz 380SEL (W126): 1981–1983
• Mercedes-Benz 380SL (R107): 1981–1985
• Mercedes-Benz 380SEC (C126): 1982–1983
• Mercedes-Benz 450SEL (W116): 1973–1980
• Mercedes-Benz 450SEL 6.9 (W116): 1977–1979
• Mercedes-Benz 500SEC (C126): 1984–1985
• Mercedes-Benz 500SEL (W126): 1984–1985
• Mercedes-Benz G55 AMG (W463): 2005–2011
• Mercedes-Benz G63 AMG (W463): 2013–2018

This may also fit other applications. Please use the vehicle selector tool to verify fitment.

Often Replaced Together

For a complete HVAC system service, many owners replace the following at the same time:

• HVAC blend door actuator or vacuum actuator
• Vacuum hoses and connectors throughout the HVAC circuit
• HVAC control module or dash-mounted climate control unit
• Heater core or evaporator core if refrigerant or coolant leaks are present

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Frequently Asked Questions

How do I know if my Mercedes-Benz W116 or W123 needs a new HVAC control vacuum valve?

The most common sign is loss of HVAC mode control. If you cannot switch between vent, heat, defrost, and floor modes, or if the blend door remains stuck in one temperature setting, the vacuum control valve is likely at fault. You may also hear hissing from the dashboard or notice a vacuum leak smell. A professional diagnostic scan or manual vacuum line inspection will confirm whether the valve or another component is responsible.

Can I replace this valve myself, or do I need a technician?

Replacement difficulty depends on your vehicle's year and model. On W116 and W123 models, the valve is typically mounted under the dashboard near the HVAC unit and may be accessible to a skilled DIYer with basic tools. However, vacuum line routing can be complex, and improper reconnection will prevent HVAC operation. Professional installation is recommended if you are unfamiliar with vacuum system work. Consult your service manual for exact location and routing diagrams.

Is this a Genuine Mercedes-Benz part, and does it come with a warranty?

Yes, this is a Genuine OEM part manufactured to original Mercedes-Benz specifications. When purchased through eEuroparts.com, it is backed by our fitment guarantee. Warranty coverage depends on your vehicle's age and condition; most Genuine OEM parts carry a 12-month limited warranty. Always verify warranty terms at the time of purchase.

What is the difference between this valve and aftermarket vacuum control components?

Genuine OEM parts are engineered and tested by Mercedes-Benz to exact factory tolerances and material specifications. Aftermarket valves may have looser tolerances or different internal designs, which can result in incomplete HVAC mode switching or premature failure. For climate control reliability on older classics like the W116 and W123, Genuine OEM components are the safest choice.

Will this valve work on both gasoline and diesel Mercedes models?

Yes. This valve is used across all powertrain variants within the compatible model years, including gasoline V8, I6, I4, and diesel I5 engines. The HVAC vacuum control system is independent of engine type, so fitment is based solely on the vehicle chassis and model year.

How long does a vacuum control valve typically last?

Vacuum-operated valves can last 20 to 30 years or more if the vacuum lines remain intact and free of leaks. However, rubber seals inside the valve can harden or crack over time, especially on vehicles from the 1970s and 1980s. If your Mercedes is experiencing HVAC control loss, the valve is a prime suspect. Preventive replacement during a major HVAC service is often worthwhile on older models.

Order Genuine OEM 1238000078 today. In-stock orders ship same or next business day from our U.S. warehouse, and our fitment team is one call away if you need help confirming the right part for your Mercedes-Benz.

Note: All information on this page and in the FAQ is for general reference only. Verify fitment and consult a professional as needed. eEuroparts.com is not responsible for incorrect part selection or installation.