Motec PDM15: enable outputs from ign switch

Hi all,

I'm stuck on what is probably (I think) a rudimentary problem. I am looking to turn on PDM outputs when the ignition key is on. However, the input pins appear to already have 12v on them – presumably to work with a digital switch that can route the signal to 0v.

How to do I get the PDM to wake up / enable outputs when it sees a 12v signal like the one that would come from the accessory or ignition key switch?

This is a road car, so I'm not looking to introduce race car user interactions.

Thanks in advance.

You re-wire your ignition switch (key lock) to switch to ground instead of 12V. All these inputs (Acc, Ignition, Start) would go to the PDM, and you would implement the normal key logic in the PDM. The PDM provides the "ignition 12V" wherever it needs to go in the car, not the key circuit directly.

This PDM isn't for the whole car, it's just for the engine management. Not sure I really want to rewire the whole key switch mechanism.

Although ironic, I was considering sending the ignition 12v to a relay; which then switched one of the PDM inputs to ground. What do you think?

If it's just for the engine management, why don't you power the PDM with the IGN power (or a relay/contactor controlled by the Ignition power). Now whenever the ignition is on, the PDM is on -- just like the OEM ECU.

Your idea of using a relay to "invert" the signal is a good one too.

This is an older car with questionable wiring ('83 Ferrari 512 BBi). I'm not sure the IGN key wire I have feeding into the engine bay would be reliable / sufficient to run the entire engine. I figured I'd pull from the battery for the PDM, and simply use the IGN key wire to switch a PDM input. Just seemed rather odd to have to use a relay to switch a PDM when that's what PDMs do, but maybe Motec assumes you'll be replacing the entire car's wiring as you initially suggested.

In the future, I was thinking of getting another PDM to rewire the entire car (or repurpose this one) – but I was trying to keep the scope of this project limited to just the engine. Adding EFI to a 40 yr old Ferrari is no simple project!

Thanks for the help.

Hi all,

Bringing this one back due to what I think might be a backfeeding issue. Hoping someone can point me in the right direction.

I have the PDM input 12v and PDM 0v going to a relay's contact terminals (30 and 87). I have a 12v supply active when the key is on IGN going to the relay's coil terminals (with a GND). The idea being that IGN 12v will switch the relay, which will then connect the PDM's input to the PDM's 0v; which I then use to trigger power out to the ECU, injectors, coils, and O2 sensors.

When I turn the key to ignition and back off again, I see the PDM enable and then disable power as I expect it to.

When I actually start the car, and then attempt to turn it off, the PDM is continuing to send power to the ECU, injectors, coils, O2, etc.. It's not until I physically disconnect a terminal from the relay that the PDM disables power.

When I measure the voltage on the IGN switch after attempting to stop the car, the reading is usually around 3v; which makes me think the PDM is somehow backfeeding into the main circuit; which is keeping the relay open. This doesn't happen if I simply set the key to IGN and then back off again. Only after the car has started does this happen.

Is this a setting in the PDM (or ECU) that I need to tweak? Or maybe a flaw in my circuit design?

Thanks in advance!

How is the alternator wired / controlled? Does it's field or lamp voltage come from the IGN 12V, or the PDM Engine 12V?

The PDM is wired to only power the ECU (Motec M150), injectors, coils, and O2 sensors. Everything else is the stock wiring. I have the PDM getting power direct from the battery.

Oddly, I am able to get this to work by setting the PDM to use a master shutdown when the input switch voltage is between 10v and 13v. I'm not really sure why, but it's consistently working. So basically, the outputs are always on, and the master shutdown is switched from the relay from the 12v IGN key.

Does that make sense?

The Master shutdown turns off the outputs that it is configured on, it does not turn off the PDM. The PDM is powered up whenever there is power applied to the battery stud and the two grounds are wired in.

Hey guys,

I was able to get this working with a pointer from my Motec dealer. A 1.5k ohm resistor pulls down the 12v input to near 0v; which allows me to use the 12v from the ignition wire to trigger the input as I originally wanted to. It's simpler than the relay, and doesn't have the back-feeding issue. Also doesn't require any changes to the rest of the car's wiring.

Thanks for the help.

Good tip from your dealer!