Technical Description of OBD-II Trouble Code
Circuit High in Glow Plug Control Module
What does that signify?
The OBD (on board diagnostics) code P0384 is a generic code that applies to all brands of modern diesel engines, including Ford, Dodge, Chevrolet, GMC, and VW Volkswagen vehicles. To fully comprehend the meaning, implications, and symptoms of this code, it is crucial to understand the underlying dynamics.
Unlike a traditional gasoline engine, a diesel engine does not rely on a compressed fuel mixture and an electrical ignition source. Diesel engines have significantly higher compression than gasoline engines.
This high compression causes the air in the cylinder to reach temperatures over 600 degrees, which is sufficient to ignite the diesel fuel. When the piston reaches the top dead center of the cylinder, high-pressure fuel is sprayed into the cylinder. It immediately ignites upon encountering the super-heated air, and the expanding gases drive the piston downward.
Glow Plug
Since a diesel engine requires super-heated air to ignite the fuel, a problem arises when the engine is cold. When starting a cold engine, it is challenging to super-heat the air because its heat is quickly lost to the cold cylinder head.
The glow plug serves as the solution. Installed in the cylinder head, the pencil-shaped plug heats up until it glows for up to ten seconds. This causes the surrounding cylinder wall temperature to rise, allowing the heat of compression to reach a sufficient level for ignition.
A typical diesel engine glow plug:
Glow Plug Circuit
The circuit is common to all diesel engines, except for the component used to time the glow plug operation. The vehicle will either have a glow plug control module or the PCM will handle this function. If a service manual is not available, you can inquire at an auto parts store if they sell a control module. If not, then the computer controls the timing.
Operating Principles
When the ignition is turned on, it supplies power to the glow plug relay. The computer or control module will provide a ground to activate the relay. The oil temperature sensor plays a crucial role in this process. When the computer detects a cold engine, it will either activate the control module or relay by providing the ground.
Once activated, the relay supplies power to the glow plugs for a duration determined by the computer or control module.
If the vehicle has a control module, its sole function is to provide a timed ground to the relay. It will have a fused power source, and the computer supplies the ground to turn it on.
A P0384 code is triggered when the PCM (powertrain control module) detects a high circuit condition in the glow plug control module electrical circuit.
Symptoms
The glow plug warning light will illuminate, and the engine will be slow to start in warm weather or fail to start in cold weather.
If the engine does start, there will be a noticeable knocking noise until the engine warms up to operating temperature. White smoke will be visible from the exhaust as the excess fuel from the hard start burns off. The engine will have a noticeable misfire until the cylinder head temperatures rise sufficiently to support complete combustion.
Glow plug warning lamp illuminated:
Potential Causes
The glow plugs have a life expectancy of 30,000 – 40,000 miles and may have reached the end of their useful life, requiring replacement. Poor injection timing can cause excessive wear on the glow plug. Additionally, a stuck glow plug relay or timer module can cause them to burn out faster.
Diagnostic Steps & Possible Solutions
Other Glow Plug related DTCs: P0380, P0381, P0382, P0383, P0384, P0671, P0672, P0673, P0674, P0675, P0676, P0677, P0678, P0679, P0680, P0681, P0682, P0683, P0684.
