When your 2019 Ram 2500 diesel (equipped with the Cummins 6.7L) won't start and the glow plug light is showing abnormal behavior, the cold start assist system may not be adequately preheating the combustion chambers. Diesels rely on compression heat to ignite fuel, and glow plugs provide supplemental heat when cold.
Understanding Diesel Cold Starting
Unlike gasoline engines that use spark plugs, diesels ignite fuel through compression heat alone. When the engine is cold, the cylinder walls and incoming air are too cold for compression to reach ignition temperature. Glow plugs heat the combustion chamber to enable cold starting.
Wait-to-Start Light Operation
The glow plug indicator (sometimes called "wait to start") illuminates when you turn the key to ON, then goes out when the glow plugs have heated sufficiently. In cold weather, this may take several seconds. Attempting to start before the light goes out can result in hard starting or no-start conditions.
Glow Plug Failure Symptoms
The 6.7L Cummins has one glow plug per cylinder. A single failed glow plug may cause rough starting but usually won't prevent starting entirely. Multiple glow plug failures result in very hard starting or no-start, especially in cold weather. In warm weather, even multiple failed plugs may allow starting.
Glow Plug Relay and Controller
The glow plug relay supplies power to all glow plugs simultaneously. A failed relay prevents any glow plugs from heating regardless of their individual condition. The glow plug controller determines how long to heat based on temperature - it may also fail, providing incorrect heating cycles.
Diagnosing the System
Check if the glow plug light illuminates at all - no light may indicate a blown fuse or controller failure. Individual glow plug resistance can be measured - typically around 0.5-2 ohms when cold. Current draw can be measured at the relay to verify plugs are heating.
Other Cold Start Factors
Beyond glow plugs, diesel cold starting depends on good compression, proper fuel delivery, adequate battery power for slow cold cranking, and correct fuel grade for the temperature. Winter diesel fuel is formulated for cold weather starting.