At this point, a precisely measured amount of diesel fuel is injected, causing combustion. However, during a cold start, the self-ignition temperature is not reached due to the low starter speed and high heat loss to the cylinder head, which makes preheating via glow plugs necessary.
With the introduction of automatic pre-glowsystems and pencil-type glow plugs, cold have been significantly improved. A temperature sensoron the cylinder head sends a signal to a relay, whichsupplies the glow plugs with the necessary heatingcurrent.
Since the electrical current is considerably higher due tothe cold heating coils, but the relay still needs to switch offsafely, glow time relays are often designed as dual relaysto distribute the load across multiple circuits.
Once the operating temperature is reached, the relayswitches off. A time delay ensures that the glow plugsremain active for a few seconds after the pre-glowindicator light turns off.
If the engine is not started immediately after the indicatorlight goes off, the relay switches off to preserve thebattery.
Modern systems have an electronic glow time control unit,which adjusts the heating power to the engine’srequirements. The control unit regulates the pencil-typeglow plugs using pulse-width modulation, based on themapping parameters in the engine control unit, in severalphases.
In many cases, pre-glowing to operating temperaturealready begins when the vehicle is unlocked, using a shortpush voltage. Afterward, a lower nominal voltage ismaintained. When starting the engine, the voltage isincreased again to compensate for cooling caused by coldintake air, which is detected by the mass air flow sensor.
Even though modern direct injection engines improveignition capability, extended post-glow while the engine isrunning remains essential at lower temperatures. Anotherimportant aspect is intermediate glowing, which supportsdiesel particulate filter regeneration.
The same rule applies to glow time control units: the cause of failure must be identified! Leaks in intake manifolds and defective oil separators allow aggressive oil vapors to enter the control unit, leading to itsdestruction.
Very often, the vehicle-side connector for the main power supply to the control unit is brittle and deformed.If the glow time control unit is replaced due to a failure, the connector pin can be pushed inward during installation.Thesmall contact surface creates a transition resistance, leading to increased current draw and high temperatures. The controlunit may overheat and burn out – posing a fire hazard! Therefore, the cable we supply should always be replaced duringeach installation to avoid follow-up complaints from the customer.
