Diesel Glow Plug Relay Failure (OM617)
The OM617 and OM616 diesel engines in classic Mercedes-Benz models rely on electrically heated glow plugs to pre-heat the combustion chambers for cold starting. The glow plug relay controls the high-current circuit that powers all the glow plugs simultaneously. This relay handles significant current (up to 60+ amps) and its contacts burn out over time, leaving the glow plugs unpowered. Without functioning glow plugs, diesel engines can be extremely difficult or impossible to start in cold weather.
Which models are at risk.
Warning signs to watch for.
Hard starting or failure to start in cold weather, while hot starting remains normal
Glow plug indicator light on the dashboard does not illuminate during the pre-heat cycle
Engine cranks normally but will not fire in cold ambient temperatures
Excessive white smoke on cold start if the engine eventually fires with partially working glow plugs
Glow plug light illuminates but glow plugs are not actually receiving current (relay contacts welded or burned)
Why this happens.
Relay contact burnout from the high current draw of four or five glow plugs cycling repeatedly
Age-related failure of the relay coil or internal components after decades of thermal cycling
Corrosion at the relay base socket or wiring connections, increasing resistance and heat
One or more shorted glow plugs drawing excessive current through the relay, accelerating contact wear
How we diagnose it.
- 01
Check whether the glow plug dash light illuminates when the key is turned to position two — no light suggests a relay or circuit issue
- 02
Measure voltage at the glow plug bus bar with a multimeter while the relay is activated — should read battery voltage (approximately 12V)
- 03
Test individual glow plugs by measuring their resistance — each should read 0.5 to 1.5 ohms; an open circuit or very low reading indicates a failed plug
- 04
Remove the relay and test it on the bench by applying 12V to the coil terminals and verifying the contacts close with an ohmmeter
- 05
Inspect the relay socket and wiring for corrosion, melting, or heat damage from high-current arcing
How we fix it.
Replace the glow plug relay with a new unit
Simple repair that can be done in minutes; new relays are readily available; restores reliable cold starting immediately
Does not address potentially worn glow plugs that may have contributed to relay failure; does not address socket corrosion if present
Replace the glow plug relay and all glow plugs as a complete service
Addresses the full starting system; new glow plugs reduce current draw on the new relay; best long-term reliability; prevents repeat failure
Glow plug removal can be difficult if they are seized in the head from corrosion; higher total cost; one or more plugs may break during removal
Keep it from happening.
Replace the glow plug relay proactively every 10-15 years as a maintenance item
Test glow plug resistance annually and replace any plugs that are out of specification
Keep the relay socket and wiring connections clean and free of corrosion
Apply dielectric grease to the relay socket contacts to prevent corrosion buildup
Address any glow plug failures promptly to prevent the remaining plugs from overloading the relay
Common questions.
Can I start my diesel Mercedes without working glow plugs?
In warm weather (above approximately 60°F / 15°C), a healthy diesel engine may start without glow plugs, though it will crank longer and produce more white smoke initially. In cold weather, starting without glow plugs is extremely difficult or impossible, as the combustion chambers do not reach the temperature needed to ignite diesel fuel through compression alone.
How do I know if it is the relay or the glow plugs themselves?
The quickest test is to measure voltage at the glow plug bus bar while turning the key to position two. If you see battery voltage at the bus bar, the relay is working and the problem is likely individual glow plugs. If there is no voltage at the bus bar, the relay is not passing current and should be tested or replaced. You can also test the relay independently on the bench.
What is the correct pre-heat time for an OM617?
The pre-heat time varies with ambient temperature. At around 50°F (10°C), the glow plug light will stay on for approximately 10-15 seconds. In freezing conditions, it may stay on for 20-30 seconds. In very cold weather (below 0°F / -18°C), two glow cycles (turning the key off and back to position two for a second pre-heat) can help ensure reliable starting.
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