Glow plugs


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Discover the critical role glow plugs play in diesel engine performance and learn how to maintain them for optimal efficiency on our website.

What are glow plugs?

Glow plugs play an important role in diesel engines, especially during cold starts. Diesel engines are compression ignition engines, which means that the injected fuel ignites without the need for an ignition spark. In order for the injected fuel to ignite during a cold start, the cylinder combustion chamber in the engine must be heated to 700-900 °C (ca. 350-450 °F). However, for the conventional operation of an engine, the glow plugs are not needed, once the engine is started. The high compression in a diesel engine (compression ignition) heats the air in the combustion chamber itself until it burns.

glow plugs

However, this does not mean that the glow plugs in a modern direct-injection diesel engine are only needed for starting, as the glow plugs also glow during engine operation as soon as the engine control unit detects that the combustion chambers are too cold for optimum combustion during part-load operation, which is why it should not be ignored if a glow plug is listed as defective in the fault memory. Even if the vehicle starts normally when cold, a defective glow plug can cause further damage, e.g. no regeneration of the diesel particulate filter, which can result in damage to the turbocharger due to the high exhaust gas pressure in the diesel particulate filter.

How does a glow plug work?

As soon as the ignition of the vehicle is switched on, the preheating of the combustion chamber by the glow plug starts.

Now, a high current flows via the glow control unit to the glow plug connection. As a result, the glow plug heats up very quickly and begins to glow at the lower end. If the glow system is intact, this process normally takes only a few seconds. On a car’s dashboard, you can see after a few seconds the pre-heating indicator light turns off.

If the temperature in the glow plug increases, the electrical resistance also increases. As soon as a certain resistance value is reached, the current to the glow plug is also reduced. This prevents the glow plug from becoming too hot and being damaged.

However, if the engine is not started, the glow plug is switched off by the glow control unit after a certain time.

Components of the glow plug system

Wiring harness:

To connect the glow plugs to the glow control unit, a cable harness is needed. This wiring harness goes from the glow control unit to the glow plugs. Usually, this wiring harness is located under the intake manifold or above it. On some vehicles, this is very difficult to reach.

Glow control unit:

In order to control the glow plugs, the glow control unit is used (also known as the glow time control unit).
On the one hand, the glow control unit has a connection wire to the glow plugs and on the other hand, it’s connected to the engine control unit. In most vehicles, the glow control unit is simply a controlled timing relay (in very modern vehicles, the glow control unit is more complex).

Glow plug control unit

The glow control unit does not have its own fault memory as a rule. If there is an error in the glow system, the error is stored in the engine control unit.

Cost of repair on the glow plug system

As a rule, the costs for a defective glow system are very manageable. For most vehicles, there are installation sets with everything needed to replace the glow plugs. There are also sets that include the glow control unit. In average, the prices should be located in between US$ 50 and US$ 250.

Pay attention that the cost to fix your glow system can be significantly higher in case you are not able to lose the glow plugs. Glow plugs can easily break!

In the worst case, the cylinder head has to be dismantled in order to drill out a stuck glow plug. This can cost you quickly US$ 2000-US$ 3000.

Keep in mind, that every car has different software and modules built-in. Therefore specific features will vary with every model.

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Discover the critical role glow plugs play in diesel engine performance and learn how to maintain them for optimal efficiency on our

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