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MoTec PDM30 Fuel Pump & Fan Wiring

Practical Motorsport Wiring - Club Level

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Hey guys,

I'm going to be using a MoTec PDM30 to wire up my S13.

I've got a couple of questions in relation to sending ground inputs through the PDM.

Both the fuel pump trigger from my ecu and the fan trigger from the Davies Craig EWP/Fan controller are ground inputs to the PDM.

How do I go about wiring these to PDM so they work correctly?

I've been told I'll need a 1k pull up resistor for each circuit but not sure how to build these into the loom?

I've also read that the PDM30 has 10k pull up resistors built into each channel so not sure if this negates the need for the 1k resistors mentioned above?

Finally, I'm wondering if I can run the alternator charge light wire through the PDM so I can send it to the light on the dash as well as my Astra power steering pump?

Thanks in advance.



Just connect your grounding-type relay triggers to the PDM. It will work. Where did you see the requirement for a 1K pullup resistor?

Regarding the alternator charge light -- why do you want to make this a separate circuit from "ignition power", normally you would just connect the charge light between ignition power and alternator field input. So when the ignition was on and engine stopped, the light would be on. Once the alternator is turning, the light should go out.

How much current does your Astra power steering pump use? Seems like that would need separate circuit(s) or a relay, since some of those electric power steering setups need up to 100 amps!

Thanks heaps for the reply David. The 1k pull resistor advice was from someone apparently in the know. But makes it a lot more simple just connecting these triggers directly to the PDM.

Firstly, I probably should have explained the power steering pump situation better. It has a main power feed running through a 60 amp fuse from memory.

I'll only be running the two triggers through the PDM, one to a switch on the dash and the other to the alternator charge light circuit.

My thinking was that I could run alt charge signal wire from my engine loom (I'm using a wiring specialities engine loom which has an interface connector with flying leads that I'll use to connect the engine loom to the PDM) to an input on the PDM, then out of the PDM to the light on the dash from one output and another output to the power steering pump. The light on the dash will have igntion power on the other side so it turns off when the alt is charging.



I don't think your alternator charge scheme will work. Because the current flows to the alternator to get it started and it's essentially a low-resistance coil. It will draw too much current if you just directly connect that to a PDM output (thus the lamp, which limits the current, and automatically turns off once the alternator is running). If you connect that to a PDM input it's won't supply sufficient current (provided by the internal pull-up resistor) to get the alternator charging.

You can always add the 1k resistors if you are having trouble detecting the "off case", that could be ECU dependent, but I would expect the internal pull-up on the PDM inputs to work fine.

Thanks David, that makes sense.

The power steering pump essentially needs a signal to know the engine is running/alternator is charging, so that it doesn't drain the battery.

Have you got any suggestions for how best to achieve this?

If I take what you said and run the charge wire direct to the light and have an output from the PDM supplying ignition power to the other side of the light, could I splice the charge wire between the alternator and light to send voltage to the pump?

Hope that makes sense.

There may be variations on this, but as a rule.

With ignition on, and alternator not operating (engine not running), 12V from the ignition goes to the warning bulb and is earthed through the field windings of the alternator. the current through the windings is required for the initial charging to occur.

When the alternator starts to charge, there's a 'back EMF', or voltage produced and the bulb is extinguished because there is 12V on both sides - sometimes there's a small imbalance, which can be seen as a faint glow.

As such, I expect you can take the voltage signal from the wire between the bulb and the alternator to 'turn on' the power steering, as there should only be a 12V signal from there when it's actually charging.

I would probably just control the output with a condition that used the Engine Speed from the CAN bus. But the wiring you and Gord both suggest should work as well.

Thanks again guys.

My ECU sadly doesn't have CAN bus David.

Here's the pinout for the interface connector from my engine harness.

Would I be able to run the tach signal (I'm not running a tacho on the dash so this wire is redundant) to an input on the PDM30 and use that to trigger the run wire for the power steering pump?

So, I'd have my button on the dash to turn the pump on and off however it would only actually power up if the engine was running and the tach wire was providing voltage to the PDM.

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