Reliable Glow Plug Timing with the TM301 Timer
Starting a diesel engine, especially in colder climates, presents challenges that are not typically encountered in petrol-powered engines. Diesel engines rely on the heat generated by compression to ignite fuel, and when ambient temperatures are low, this process can become less effective.
That is where glow plugs come into play. Glow plugs are heating elements that help preheat the combustion chamber to improve cold starting. However, the duration of glow plug operation is important for reliable starting, battery performance and glow plug life.
The Bolec TM301 Timer Module provides a simple and adjustable way to control the preheat timing of a diesel glow plug system when used together with a suitably rated external glow plug relay or contactor.
Why Glow Plug Timing Matters
Incorrect glow plug timing can contribute to problems such as difficult starting, excessive white smoke and unnecessary battery drain.
Leaving glow plugs energised longer than required may shorten their service life, while switching them off too early may make cold starting more difficult.
For agricultural machinery, generators, industrial equipment and other diesel-powered systems, repeatable glow plug preheat timing can help improve starting consistency.
The Role of the TM301 Timer
The TM301 Timer Module is designed to control time-dependent circuits in 12V or 24V DC electrical systems.
It provides several DIP switch-selectable timing ranges, allowing the installer to configure the timer for different applications.
Available timing ranges include:
- 1–30 seconds
- 2–60 seconds
- 1–30 minutes
- 2–60 minutes
For typical glow plug preheat applications, the shorter second-based ranges are generally the most relevant.
The TM301 should be used to control the coil or control input of an appropriately rated external glow plug relay or contactor. The high current required by the glow plugs should not pass directly through the TM301 relay contacts.
This allows the TM301 to provide accurate timing while the external relay or contactor handles the high electrical load of the glow plugs.
How It Works
The TM301 offers flexible timing settings using DIP switches together with a trim pot for fine adjustment.
A typical glow plug control sequence may operate as follows:
- When the ignition or preheat control is activated, the TM301 receives 12V or 24V DC, depending on the vehicle or equipment electrical system.
- When configured for an instant-on, delayed-off timing mode, the TM301 relay output activates immediately.
- The TM301 output energises the control coil of an external heavy-duty glow plug relay or contactor.
- The external relay or contactor supplies the required high current to the glow plugs.
- The TM301 begins timing according to the selected DIP switch range and trim pot setting.
- When the selected time expires, the TM301 releases its relay output.
- The external glow plug relay or contactor then switches off, removing power from the glow plugs.
This provides a consistent and repeatable preheat period without requiring the operator to manually judge how long the glow plugs should remain energised.
Important Relay Rating Information
The TM301 contains an internal control relay, but it is not designed to carry the full current of diesel glow plugs directly.
The TM301 relay contact rating is:
- 3A at 12V DC resistive load
- 1.5A at 24V DC resistive load
Glow plugs can draw considerably more current than these ratings.
For this reason, the TM301 should normally operate an appropriately rated external automotive relay, heavy-duty relay or contactor selected for the actual glow plug load.
When switching inductive loads such as relay or solenoid coils, appropriate suppression such as a flyback diode should also be considered in accordance with the installation requirements.
Typical Applications
The TM301 can be used in a range of diesel applications where controlled preheat timing is required:
- Agricultural equipment: Tractors, irrigation pump engines and other machinery requiring reliable cold starting.
- Generators: Automatic and remote-start diesel generator systems.
- Construction equipment: Diesel-powered machinery operating in varying environmental conditions.
- Industrial diesel engines: Equipment requiring repeatable automatic preheat control.
- Special-purpose vehicles: Diesel-powered vehicles where an adjustable preheat timer is required.
Installation Considerations
The exact wiring arrangement will depend on the engine, glow plug system and external relay or contactor being used.
A simplified installation concept is:
- Connect the TM301 power supply to the appropriate switched 12V or 24V DC supply using suitable circuit protection.
- Connect the negative terminal to the appropriate system negative or chassis ground.
- Connect the TM301 relay output terminals (N/O, COM or N/C as required) to the control circuit of the external glow plug relay or contactor.
- Connect the glow plugs to the high-current side of the external relay or contactor using wiring and circuit protection suitable for the actual load.
- Select the required timing range using the TM301 DIP switches.
- Adjust the trim pot to achieve the required preheat duration.
Important: Do not connect high-current glow plugs directly through the TM301 relay contacts. The external switching device, cable size and circuit protection must be appropriately rated for the glow plug current.
Using the INH+ Input
The TM301 also includes an INH+ inhibit input.
Applying a positive voltage to the INH+ terminal resets the timer and holds it in the reset condition while the inhibit signal remains present.
This input can potentially be incorporated into a control system where another condition needs to reset or prevent the timing sequence.
The exact method of using INH+ will depend on the overall control circuit and should be designed to suit the specific engine application.
Improving System Reliability
When correctly integrated with an appropriately rated external relay or contactor, the TM301 provides a repeatable timed control signal for the glow plug system.
This can help reduce the possibility of glow plugs remaining energised unnecessarily and can improve consistency between engine starts.
It also removes the need for an operator to manually estimate the required preheat duration each time the engine is started.
Benefits of Using the TM301 for Glow Plug Timing
- Consistent preheat timing: Provides a repeatable glow plug operating period.
- Adjustable operation: DIP switch timing ranges and trim adjustment allow the system to be configured for different engines.
- Reduced unnecessary battery drain: Helps prevent excessive glow plug operating time.
- Potentially improved glow plug life: Avoids leaving the glow plugs energised longer than required.
- Suitable for 12V and 24V systems: Useful across a wide range of diesel equipment.
- Flexible control: Can be integrated with an external relay or contactor to handle higher-current loads.
External Relay Selection
The external relay or contactor must be selected according to the total current required by the glow plugs.
For example, a multi-cylinder diesel engine may have several glow plugs operating simultaneously, resulting in a significant combined current draw.
The external switching device, wiring, fuse protection and terminals must therefore be selected according to the engine manufacturer’s requirements and the actual measured or specified glow plug current.
The TM301 then handles only the low-current control side of this switching system.
Conclusion
Reliable diesel engine starting depends on having the correct preheat period for the engine and operating conditions.
The Bolec TM301 Timer Module provides an adjustable and repeatable timing solution for diesel glow plug control in 12V and 24V systems.
When used together with a suitably rated external glow plug relay or contactor, the TM301 can provide the timing control while the external switching device safely handles the high current required by the glow plugs.
This makes the TM301 suitable for applications including agricultural machinery, generators, industrial diesel engines and other equipment requiring automatic or controlled glow plug preheating.
To learn more about the TM301 Timer Module, visit Bolec Electronics or contact our team for assistance with your application.