Body Electrical
Preglow Time Relay
A preglow time relay controls how long the glow plugs heat before and after a diesel engine starts. We carry 19 relays from $7 to $253 by Huco, Febi, AC Delco, Vemo and Hella, mostly for Mercedes diesels such as the 300D, 300CD, 300TD and E300, plus Chevrolet Colorado and GMC Canyon.
Shop preglow time relay by vehicle
Why diesels need glow plugs
Diesel engines ignite fuel with heat from compression rather than a spark. When the engine is cold, that heat is not enough on its own. Glow plugs heat the combustion chambers so the engine fires on the first try. The preglow relay switches the heavy current the glow plugs draw and times how long they stay on, often based on engine temperature.
Signs of relay trouble
Hard starting when cold is the main symptom, along with long cranking times, white smoke and rough running until the engine warms. The glow light on the dash may not come on, may go out too quickly or may stay on.
Bad glow plugs cause the same symptoms. Test each plug's resistance or current draw, and check the relay's fuse and connections, before deciding which part has failed.
Matching the relay
Preglow relays are specific to the engine and the number of glow plugs. Older Mercedes diesels use different relays than later engines, and some relays have built-in fuses for each plug circuit. Compare the part number on the old relay and the pin layout with the listing.
Disconnect the battery before removing the relay, because glow plug circuits carry high current. Clean any corrosion from the connector during installation.
Preglow Time Relay questions
How do I test a glow plug relay?
With a helper turning the key to the preheat position, check for voltage at the glow plug feed using a test light or meter. If the relay gets its trigger signal but sends no power, the relay is likely bad.
Should I replace glow plugs and the relay together?
Only if both test bad. A failed relay can leave good plugs cold, and a shorted plug can damage a relay. Testing each part first avoids buying parts you do not need.
Can a diesel start without glow plugs working?
In warm weather a diesel may still start, though it can crank longer and smoke. In cold weather it may not start at all. Repair the system before winter.