If you drive a diesel-powered Mercedes, your car has a glow plug relay. It’s quite common to have glow plug timer relay issues to develop in a Mercedes-Benz. If you notice your Mercedes refuses to start, especially in cold weather, you should bring your car in for an inspection because there might be a problem with the glow plug relay. In this article, we’ll discuss at length what symptoms to look for, and why you might be having glow plug relay problems.

Understanding the Glow Plug Circuit and Relay

In order to understand the glow plug relay, we need to explore what the entire glow plug module is all about and its purpose.

  • Glow Plug: These are spark-plug-like devices found in diesel engines. Every cylinder of a diesel engine has a glow plug. When the glow plug is sprayed with an air-fuel (air-diesel) mixture, it heats up to extremely high temperatures. When hot, the elements at the tip of the plug glow. These (usually) ceramic plugs help the engine start. This is especially necessary in cold weather.
  • Glow Plug Control Unit or Module: The glow plug control unit or module monitors and regulates the glow plugs. The module scans and analyzes data from the ECU (engine control unit) to decide how much electrical current is required for the heating process and turns the current on and off accordingly. The module uses a glow plug relay to achieve this.
  • Glow Plug Relay: This is part of the ignition system of your diesel car that is responsible for intermittently turning the current on and off to heat the glow plugs. In order to decrease the amount of current used, the glow plugs are distributed between two circuits. Each has its own glow plug relay. After the engine successfully starts, the glow plug relay switches off the glow plugs, so that they don’t overheat.
  • Purpose: The purpose of this system is to ensure the engine achieves effective combustion, fuel burns efficiently, and with low emissions.

Why Does the Glow Plug Relay Go Bad?

Considering there is no manufacturing defect, or your Mercedes doesn’t have poor-quality parts, like any other component in a car, the glow plug relay is subject to normal wear and tear. The electrical parts may deteriorate over time.  Due to usage, connections and terminals are bound to become unstable. Eventually, the system will reach a point when it will begin to fail.

Indications That the Glow Plug Relay is Malfunctioning

Your Mercedes will give you ample signs that the glow plug relay timer may not be working as intended. Here are some examples:

  • Difficulty Starting: The most common indication that the glow plug relay is having problems is when your Mercedes is having trouble starting, especially if this happens in cold weather.
  • White Smoke: If you notice white smoke wafting out of your tailpipe when you are starting your car, particularly in the cold, it’s a sign that the glow plugs are deteriorating.
  • Engine Misfires: If your engine misfires, you can experience a multitude of problems, the most critical being your car may refuse to start.
  • Rough Idling: If your Mercedes jerks and vibrates during the warm-up phase, this is strong evidence that the glow plug relay may be compromised.
  • Poor Fuel Efficiency: A faulty glow plug relay will cause you to get low mileage because of all the engine issues it creates.
  • “Wait to Start” Light: When this light on your dashboard remains on longer than intended, that is pointing to a problem in the glow plug unit.
  • Check Engine Light (CIL): When this light is illuminated on your dashboard, don’t ignore it. It can imply any number of problems, glow plug relay issues being one of them. It is imperative that you take your Mercedes to a professional mechanic who can diagnose the reason.

Mercedes Glow Plugs

Diagnosis and Maintenance at MotorHaus

There is really only one surefire way to keep your car running at its peak. You need to have regular maintenance done on it. Problems such as the glow plug relay issue can be caught early, during routine maintenance, and need not become a huge problem. A professional mechanic can diagnose it and make the necessary repairs or replacements.

At MotorHaus, we are highly trained mechanics with years of specialized experience with European cars. Call us or make an appointment to visit us. We are located in West Palm Beach, Florida.

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