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MoTeC's Power Distribution Modules

Forget using fuses and relays

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Replaces fuses and relays
  • Smart, programmable switching
  • Lighter and uses less wiring
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The distribution of electrical power in a car through the use of switches, fuses and relays is about as old as the car itself. However, Australian aftermarket electronics manufacturer MoTeC has now released a range of Power Distribution Modules (PDM) that replace much of this traditional wiring. The advantages include lighter weight, much better electronic protection of circuits, and the ability to implement smart switching.

The cheapest PDM costs AUD$1870.


The PDM comprise software-configurable systems that use internally mounted, high-power output transistors.

The base unit PDM15 has 15 outputs (eight 20 amp outputs and seven 8 amp outputs) and 16 switch inputs. Also available are the PDM30 (30 outputs, 16 inputs), the PDM16 (16 outputs, 12 inputs) and the PDM 32 (32 outputs, 23 inputs).

Note that ‘like’ outputs can be paralleled for extra current capability. For example, three 20 amp outputs can be wired in parallel for a 60 amp load.

The PDM15 and PDM16 use magnesium alloy cases and weigh just 260 and 270 grams, respectively. The larger PDM16 and PDM32 use alloy cases and weigh in at 330 and 405 grams.

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As this diagram shows, the PDM completely replaces the fuse and relay boxes.


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The PDMs are designed to be interfaced with other MoTeC products like the company’s Electronic Control Units (ECUs) and electronic dashboard displays.

In fact, to achieve full functionality, either an ECU or dash needs to be fitted. This is the case because the PDM can accept a CAN (controller area network) bus connection, allowing it to receive (and then act upon) sophisticated data streams originating from these other devices.

However, the PDMs can also accept simple on/off inputs from switches, allowing them to be used as standalone devices – they just can’t do as much.


The easiest way of seeing how a PDM can be used is to compare traditional and PDM wiring.

Click for larger image

This diagram shows the conventional wiring for a fuel pump and electric radiator fan (thermofan). Each circuit requires a fuse and a relay, with the heavy current cables running right back to the ignition switch (or main fuse box). The low current side of the radiator fan relay is controlled in this diagram by the ECU – but this could also have been done by a temperature switch.

Click for larger image

This diagram shows the same system but using a PDM. As can be seen, the PDM removes the need for relays and fuses, in addition to simplifying the heavy current wiring.

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Darren Reynolds, a MoTeC development engineer, runs a PDM in his SR20 turbo-engined Nisan Bluebird. In that car the PDM controls items like the lights, wipers and horn. This is the factory wiring loom that was surplus after the PDM was installed.


Click for larger image

So why use a PDM?

As the high currents are switched by transistors, there is no need for relays. Relays can be unreliable in harsh environments and weigh more than using the PDM.

The PDM replaces the need for individual circuit fuses. Instead, current monitoring is carried out by the PDM.

Each circuit can have a software-specified maximum current (programmable in 1 amp steps). The PDM uses internal software to simulate the temperature rise of a wire subjected to current overload. This allows the system to cope with short-term current gulps (like those that occur when switching on electric motors) but still be sensitive enough to protect wiring in the event of a true overload.

In addition, the PDM can be programmed to try reconnecting the circuit after a fault condition has caused a shut-down.

The PSD can be programmed to perform special switching functions. These include:

  • Turbo timer (the engine continues to run after ignition switch-off)

  • Alarm system / immobiliser (requires the input of a second switch before the car will start – that could be the horn switch)

  • Operate the indicators and hazard flashers without requiring a flasher unit

  • Courtesy light delay

  • Battery saving (disables certain functions if battery voltage falls below a configurable value)

  • Auto-off headlights

For example, the immobiliser / alarm system can be configured like this:

To start engine

  • Turn ignition key switch to ignition position

  • Press and hold the horn button (the horn doesn’t actually sound!)

  • Turn key to start

Alarm triggers

  • When key turned to start position without pressing horn button

Alarm Output

  • All lights and indicators flash and horn beeps for 3 minutes

Clearing Alarm Condition

  • Turn on hazard lights for 2 seconds

Horn Operation

  • Horn works normally except when key is switched to ignition and engine not running

Because all of these outcomes are programmable, any of these aspects can be changed. For example, an existing switch other than the horn button can be used.

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In addition to the outputs being programmable, the inputs can also be software-configured. For example, anti-bounce and variable hysteresis functions can be implemented to provide better reliability in accepting switch inputs. Switch inputs are required to connect to ground (that is, when the switch is operated, the input to the PDM is pulled to 0 volts) and so switches need to be configured in this way. (Note: many standard switches in cars don’t do this, instead connecting to battery voltage when closed.)

While the inputs to the PDM can be configured to switch when a certain voltage is reached, the inputs are not currently ratiometric, meaning that as battery voltage fluctuates, so can the switching point. Thus, at this stage, sensors (eg coolant temp sensors) cannot be connected to the PDM to allow direct operation of the radiator fan. (Instead, in this example, a temp switch would need to be used. We understand that ratiometric inputs are under development.)

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With the connection of the PDM to a MoTeC ‘hundred series’ ECU or dash, much more sophisticated logic can be implemented. For example, as shown here, the radiator fan can be controlled on the basis of engine temperature, ground speed, engine rpm and a cool-down switch input.

In addition, with CAN connection, single pushbutton engine ‘auto start’ can be implemented (as occurs in some current cars that use smart pushbutton start), and current logging and dashboard fault indication can occur.

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Another function of the PDM is monitoring. A laptop PC can be used to instantly see the status of:

  • Input switches (on/off)

  • Outputs (on/off, current draw and calculated load – the latter taking into account modelled wire heating)

  • PDM total current

  • PDM temperature

...and some other information.

The PDM is rated to run at a continuous 100 degrees C, a temperature that will be attained with all loads running at maximum current. The mounting location should take this maximum temp into account.


The PDM has potentially great value in a variety of cars.

We can see it being very useful in racing machines (its primary design purpose), but also in ultra-lightweight vehicles and home-builts. We’d like to see better, flowchart-based software for configuring the internal logic functions, and also PWM control of outputs (allowing for example light dimming, fuel pump speed control and radiator fan speed control) – and we understand that such improvements are being actively considered.

But even as it is now, the PDM has the potential to revolutionise power wiring in cars.

The user manual and software for the PDM15 is available from

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