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PDM Grouping

PDM Installation & Configuration

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I've basically got all my car wired in now but I'm wondering how everyone goes about grouping outputs together to save using them all? I've attached some content including wiring diagrams of what I've done soo far the only one missing is the interior section of the car such as the heaters, fuel pump, Interior lights, heater, heater controls etc... can someone check if this all looks good? As I'm a novice to wiring!

Also the stock wiring runs the heater components on two seperate circuits, 20A fuse for the blower fan and 7.5A for the Air inlet servos... but I've wired all of these together with a 20A Fuse as I copied this idea from someone else who runs a decent racing parts shop. I don't understand how this is safe though? 7.5Amp circuit on a 20A fuse? Surely not. Or am I missing something?

Thanks guys!

Hello Reece,

Your attachments are missing.

The fuse situation is only safe if the conductors you used can support the current its fused for, for longer than the fuse being used can. If you had a 20A fuse on 24AWG conductors, you will overheat the conductors before popping the fuse... which is obviously not ideal.

Ahh right, I think I understand.

So if for example purposes only I use 6 gauge wire and fuse it at 20A on something that is usually fused at 7.5A, If the situation gets dangerous the 20A fuse will pop before the thick gauge wire melts and catches fire as it can handle more current?

So it should be safe for me to do aslong as I up the wire gauge some? Say 14 gauge?

Attached Files


I've realised that alot of my components in the car still need switched 12v power like my radio for example.

Is it safe to put multiple 15A devices onto a 15A fuse? Or am I supposed to put let's say 3x 5A devices on a 15 amp fuse max which would make 15A all together?

Hi Reece,

When you say 15A devices do you mean they were originally on a 15A fuse in the vehicle fuse panel or they draw 15A when operating?

a 15A output on a PDM is the maximum allowable constant draw on that pin. Although, you can exceed 15A in transient conditions. each PDM manufacturer have their own logic on fuse blow when exceeding the current rating of an output.

In your case if the device draws 15A then it will be the only device wired to that output.

Ideally you need to measure the actual current draw so you can allocate outputs appropriately.

No, they are originally fused to 15A... How would I measure the device output current with no wiring in the vehicle? Some stuff is also aftermarket so i may be able to look that up online.

I was hoping to have an output for constant 12v accessories and switched 12v accessories which could save quite a few outputs, items such as Gauges, radio, interior lights, switch panels you know.

I've been drawing this up all night to understand better what I'm trying to do, this is the layout of my interior and I've got a list of outputs on the left-hand side with a 'KEY' to help you navigate the diagram. Some items also have a dedicated switched 12v power to keep memory settings.

HOW WOULD I GROUP THESE ITEMS TO SAVE INPUTS? I'm guessing it's a mix of Essential, Non-Essential, Amp ratings, Live, Switched, etc....

Attached Files

Some of the current values that are in that diagram are excessive, for example, the Dome light would be a 200+ watt output based on 15A and 13.8V. If this in the standard car is on a 15A circuit, then there are other devices being powered from the same circuit.

Your Brake Lights (21W) will draw 1.6A per bulb when lit, the Tail Lights (5W) 0.4A when on, assuming a 13.8V supply. Headlights at 60W will draw 4.4A per bulb.

I get where you are coming from, I've mainly taken these from the stock fusebox values as the car is fully stripped of any wiring... Something like the headlights though I'm running in parallel as the manufacturer said they have a steady state of 11A together. What would be the best way of testing devices without them being wired in?

I'm going to compile a list this weekend of every device I have in the car, could you possibly help me set a fuse for these and group them? I'm struggling to understand the concept at the moment but everything else I'm pretty certain it's good.

Scroll to the right on the attachment and this is what I've got soo far, I'll need to find the rest of the details and then group them. It's mainly the interior I'm struggling with as the engine bay, lights and all that Jazz really needs its own Output. - https://docs.google.com/spreadsheets/d/1vbO-NkdlVcojw_Vav-0xriVJ3L-2HuRunNHcezNSLDQ/edit?usp=sharing

I may be missing something - it happens more often as I get older - but it seems you may be getting too focussed on using the PDM for everything, when it may be better used as the master control for sub/daughter/slave circuits?

For example, instead of trying to run all the "accessory" circuits directly of the PDM, use it to power a circuit that branched off to each device you want to run as 'accessories", with a fuse or, preferably, resettable circuit breaker for each that is correct for it's current. As it is most unlikely you will have several sub-circuits overload at the same time, you can run more at their 'nominal' draws while having enough reserve to keep things well within the DPM's output rating.

There are other things, like the dome (interior), engine bay, and other circuits that shouldn't need to run through the PDM as a simple direct feed, through (a) circuit breaker(s).

Other things may be better used with fused relays, coil or solid state, to manage high current loads and reduce the number of high rated out's on the PDM. For example, while I use a separate fused relay for each headlight filament (halogen where voltage drop is also very bad thing), so I don't loose al the lights with one short, you may wish to have them split to left and right side, for some redundancy if there's a problem.

Forgot, circuit breakers don't only protect the wiring, they protect the entire circuit including switches (if used), relays (if used) and the actual device(s) being powered - one example is a stalled, or partially seized, motor where the current can be much higher than normal and result in it burning out to the point of actual combustion.

This is what I'm referring to when I say 'Grouping' Although I don't want to use any relays or fuses anymore.

It's mainly the interior I want to run multiple circuits off one output as the engine bay doesn't have anything I need to power from the PDM and the Lights, fans, etc all need their own output.

These are the main things I want to group into the same output or the fewest outputs I can use. (These are all devices that will not be controlled by a switch, more always live or switched power which means they also won't need programming.)


I'm understanding the grouping idea a bit more now, essential I can have as many devices as I want on one fused circuit but if that use trips then the whole circuit/output is down unless I retry it.

1. What's the general idea of grouping devices together? Keep essential items such as injectors, fuel pumps, coils etc... on their own separate outputs? and group nonessential items like power windows, power mirrors, and interior lights on their own output?

2. Is the amp rating on each PDM output just the max fuse size I can use? Guessing the PDM can actually output more than this fuse size.

3. Would I also group items by similar Current draw? So that the fuse size stays safe for the circuit and wiring. say keep items that draw less than 10A on a 10A fuse and items less than 20A on a 20A fuse etc... (I.E AEM Gauges (6A), Radio (8A), Dashboard Clock (2A) On a 10A Fuse? - Crank Sensor (.06A), Cam Sensor (.06A), VSS Sensor (1A), Boost Solenoid (2A) All on a 4A Fuse?) Separating Non-Essential and Essential Components... Something like that anyway.

4. I also made a quick mockup of my thinking around the best way to group circuits together which I've attached, What would be the best way to test current draw? Hook ground and power up to my battery and then just use a multimeter?

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