PDM Total Current Capacity

Zac mentions this when going through the worked example as this was the reason for adding an additional ECU Master PMU16. I have already acquired a Motec PDM30 and noticed that its total current capacity is only 100A, which is only 30% of a all channels maxed condition (see next para).

The Motec PDM30 has 8 x 20A channels and 22 x 8A channels (equates to 336A) with max current continuous of 100A, while the ECU Master PMU16 has 10 x 25A channnels and 6 x 15A channels (equates to 340A) with a rated max current capacity of 150A continuous. Therefore, the PDM32 can only use 30% of total channel capacity, where the PMU16 can use 44%

I'm using 14 of the 30 available channels and I'm at 92A of the 100A continuous but, that assumes the 14 assigned channels are all running. In reality, when the starter solenoid is used, several other functions will be deactivated (Lambda, Electric Water Pump, etc) for the duration of the start. Once the engine fires up (rpm >600), the started will drop out (as will the Alternator Field) and the deactivated electrical loads will be reactivated. So while 92A/100A is close to the limit, the reality is the current draw will be closer to a 72A max continuous.

So if, like Zac explained, you can add more than one electrical load to a single channel, the PMU16 might actually prove better suited to a certain application. My initial use of the car will be for Hillclimbs and Sprints, so I only need the wipers to work but, if I was to do the Targa series I'd need all the lights too. Two PMU16 would be a better option in the future as it would be cheaper, with more current capacity and very similar levels of control functionality.

It would pay to talk to the manufacturer about the actual limits of the PDM and your application.

That said, rather than go through the hassle and expense of changing the PDM, it may be easier to suppliment it with (solid state?) relays - this would allow you to place the higher current devices on those, minimising the loads on the primary PDM. There may even be advantages in wiring and placement, for example, one at the rear may be able to run pumps, lights, etc, and one at the front may be used for the lights, with each requiring a single high current wire and the much lighter gauge switching wiring, rather than multiple medium gauge wires.

I think you need to measure the devices to see the actual current. I used to be worried about this, but then I discovered that devices (like electric Helmet A/C units) with 40A fuses actually only draw 12-17 amps in steady state operation. Remember, your current limits are to save the wires, as they are the real current limit in the case of a short.

I think you will find the current requirements to run a car are far less than you think, but you need the capability so any one circuit can handle a load without shutting down.

Biggest current draws come from pumps and fans. Unless you have a lot of those I find 100A through a PDM is usually enough.

Gord's point about strategic external relays is a good one. I often use a 40A relay for a starter solenoid if I don't have enough outputs on a PDM, and I ran a car with electric power steering, where I used an external 60A relay / fuse. That is one I wished I had measured the actual current requirements.

Thanks for the replies. A couple of solid state relays would definitely remove a chunk of current! I also like the idea of a front and rear PDM and ‘Mini Harness’ to local loads. I’m still planning so this course has come at the right time for me!

As David said, many people really overestimate the amount of current draw they are going to experience. From simple things like the fact that in most circumstances only one coil is charging at a time, most headlights will only draw around 3A each once powered up (there is a bunch of information on this in the motec PDM user manual), wipers are fairly low current for most of the wiping phase (provided there is no stiction on the screen)

A really simple way to ballpark things is to go and look at the current output of your alternator. If you have an 80-100A alternator, the current capacity of your PDM being 150A instead of 100A won't be much use, nor will contiunous current draw of 120A for instance.

Unless you have a big fuel system with muliple pumps, or are trying to run multiple fans for heat exchangers, most people rarely meet 100A continuous output.

Thanks Nathan, you and David raise a good point. My car was originally a carby running a blow through turbo setup and many have converted to EFI over the years and I don't remember any having to upgrade the 60A alternator! I'll be adding Injectors, LS2 coils, DBW TB, a high flow fuel pump and a Davies Craig EWP. I won't be running electric windows or an anti-percolation fan (blows cold air over the fuel lines!) , so probably not much higher draw than OEM.

I could also convert some of the lighting to LED if I was getting close to limit. I'll make sure I post what the car actually draws when I get that far!