Mercedes OM602 Glow Plug Problems and Diagnosis
Introduction
Glow plug faults are one of the most common starting problems encountered on older Mercedes-based Hymers fitted with the OM602 five-cylinder diesel engine.
Many owners confuse glow plugs with spark plugs. Diesel engines do not use spark plugs. Instead, glow plugs act as electric heaters, warming the combustion chambers during cold starting. Once the engine is running, the glow plugs have little or no further function until the next cold start.
A healthy glow plug system is essential for reliable starting, particularly in winter.
How the System Works
When the ignition key is turned to the pre-heat position, the glow plug relay supplies power to each glow plug for a few seconds.
The dashboard glow plug warning lamp illuminates while the plugs are heating. Once sufficient temperature is reached, the lamp goes out and the engine can be started.
Mercedes used a relatively sophisticated system on the OM602 engine. Each glow plug has its own connection from the relay box rather than all plugs being connected to a common bus bar. This allows the control system to detect certain failures and warn the driver.
Typical Symptoms
Common symptoms of glow plug system problems include:
Glow plug warning lamp behaving abnormally.
Warning lamp staying on after starting.
Warning lamp not illuminating at all.
Difficult starting in cold weather.
Excessive white or grey smoke immediately after starting.
Rough running for the first few seconds after start-up.
Gradually worsening starting performance over months or years.
A single failed glow plug may not prevent the engine from starting, particularly in warm weather. However, once one plug fails, the remaining plugs are often not far behind.
Glow Plug Life Expectancy
In normal use, glow plugs typically last many years. Experience suggests that around ten years is a reasonable lifespan.
When one plug fails, it is generally sensible to replace the complete set rather than changing individual plugs one at a time.
Cheap pattern parts often prove to be a false economy. Bosch and Beru plugs are generally regarded as the safest choice for long-term reliability.
Testing the Glow Plugs
A simple resistance check can often identify failed plugs.
Disconnect the output connector from the relay box and measure resistance between each glow plug terminal and earth.
Typical readings are approximately:
0.7 to 1.2 ohms
A reading significantly higher than this, or an open circuit, usually indicates a failed plug.
The Glow Plug Relay Box
The relay box is mounted in the engine bay and contains the heavy-current switching required to power the glow plugs.
Although generally reliable, relay failures do occur.
Common faults include:
Burnt relay contacts.
Failed internal electronics.
Blown strip fuse.
Corroded power connections.
Damaged wiring.
A large strip fuse is mounted on top of the relay box. This is easily inspected and should be checked before condemning the relay itself.
With the ignition switched on, battery voltage should be present at the glow plug outputs during the pre-heat cycle.
An Interesting Real-World Fault
One owner experienced progressively worsening starting problems over the course of a year.
Testing showed:
All glow plugs measured correctly.
Temperature sensor tested correctly.
Relay appeared to operate.
No glow plug warning lamp fault was displayed.
The relay pulled in but immediately dropped out whenever load was applied.
After extensive investigation, the cause was found to be a nearly broken power connection at the starter motor terminal. The cable had fractured inside the crimp terminal and could pass enough current to operate the relay, but not enough to power the glow plugs.
The fault was eventually cured by repairing the terminal connection.
This serves as a useful reminder that wiring and power-supply faults can mimic relay failure.
Replacing Glow Plugs
Replacement is straightforward for an experienced mechanic but can be awkward on older vehicles.
Points to note:
Access to the front cylinders can be difficult.
Correct sockets and extensions are required.
Old plugs may be tight.
Excessive force can occasionally break a glow plug during removal.
Although broken plugs are uncommon, extraction can become a major repair if it happens.
For this reason, replacement is often best carried out by a garage familiar with older Mercedes diesel engines.
Emergency Starting
If several glow plugs have failed and the engine will not start in cold weather:
Avoid flattening the starter battery through repeated cranking.
Allow the remaining plugs as much heating time as possible.
As a last resort, starting fluid may get the engine running.
However, starting fluid should be regarded as an emergency measure only and not as a repair.
Useful Part Numbers
OM602 five-cylinder engine:
Glow Plug:
A001 159 17 01
Glow Plug Relay:
A007 545 99 32
Always verify part numbers against the vehicle VIN before ordering.
Practical Advice
When glow plug problems first appear:
Check the strip fuse.
Check power supply connections.
Test the glow plugs individually.
Inspect relay operation.
Consider replacing the full set of plugs if one has failed.
In many cases the fault will be a failed glow plug. However, relay box faults, poor connections and wiring problems should not be overlooked.
A methodical diagnosis will usually identify the problem without replacing unnecessary parts.
References
Mercedes Glow Plug Info and Issues
The Mercedes Glow Plug Relay Box
Part Numbers Mercedes Glow Plug
More Mercedes Glow Plug
Deep Dive Into the Relay Box Mercedes Glow Plug
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