PDM output grouping

How do people tend to group outputs together, in order to make the most out of the available ones?

Regarding EFI: I was thinking of grouping all "mission critical" together, thinking that regardless of where the problem is, you cannot go any further, without some of them. For example ECU and injectors, but not necessarily coils and definitely not wideband, since you can limp it back without constant output reset.

Body: looking in grouping stuff that have a dedicated switch, like horn, brake light and park lights, or maybe indicators together. If any of them fail and the system resets the output, if you don't reuse it you should be good to go, as long as the issue is not between out put and switch

Are there any best practices known or applied for this?

It seems a bit tricky to fit them all in a "common" 16~20 output PDM, if you need to factor in some additional stuff like demisters, rally intercoms, cameras ect. With that in mind, the 32 output AIM unit seems interesting.

Yeah, I've worked out many possible permutations on paper, and it's really a personal preference how you want to group these.

My conclusion is that a typical 16 output PDM is just not enough to run a car that needs to be road legal. 16 is enough for most track-only cars though.

A ~20 output can manage everything for a road legal, but there will be compromises. 30+ is what's needed if you want a PDM to run everything.

My perfect ideal system would have 48 outputs (or 3x 16outputs), then I'd have enough to individually fuse EVERYTHING. No splices in the harness, and epic limp-home capability!

Yeah, that's broadly my conclusion too. Assuming a reasonable budget, this brings up the next question.

Grouping outputs "kind of" not individually fused?

Add fuse boxes for the above, ending up again relying on conventional components, but with a bit more logic?

Or just leave the OEM setup, or a conventional one for that matter, for anything that you don't mess with during the car development, like lights, horns etc and only use the PDM for the modified or the new stuff?

Another aspect, is that in the unlikely event that a PDM fails, you are pretty much done, whereas a fuse is always changeable. This is hard to swallow for a rally car. I know the pros have PDMs for a good reason, but they also have a couple of spares with them too.

I know there is no right or wrong, just thinking publicly here.

I was hoping for some more clarification on this in the course too. There is a couple of minutes in the worked example that talks about grouping, but im still not really clear on it. On paper I have planned, to group for example the ECU (1A), Gauge (5A) and a 12v socket (10A). It seems reasonable to group these under a 25A output, as they all come on with power to the car (ACC), however, it wasn't clear to me how to wire and configure for this? The other group would be the 'ignition on' as with a normal car, to have on the Injectors, coils and lambda heater. My thinking is similar to yours, some clarification and worked example around this subject would be good.

I would be a bit reluctant to group the lambda heater together with injectors and coils.

1) There seems to be a benefit in delaying the wideband warm up a bit, so I would switch it on after engine start, even with some additional delay.

2) in case it fails for whatever reason, it would cause the engine to stall, for a reason that is not 100% necessary. in the worst case, if there is a "consistent" failure, like a short or something, you would not be able to start an otherwise running engine.

Totally agree on the number of outputs. 16 is usually barely enough to run a dedicated track car much less a street car.

Also agree that the decision making process is subjective and application dependant. I prioritize the engine critical stuff and would run fuses for things that aren't if I run out of channels. I often look to what OEM's do when it comes to grouping, stuff like the ECU and injectors and engine sensors (12V ones) are pretty much always grouped and this is done so that the ECU is getting the same voltage as the injectors for voltage drop. Ignition coils are always a separate fuse and a separate relay, possibly for noise or just amp draw. So thats just 2 examples.

Things I don't like doing would be grouping an ECU and a non essential device like a 12V socket, socket has a bad device plugged into it or even just improperly and now your ecu is offline. While PDM's make resetting the circuit easy, still not a grouping I'd do.

I also isolate any channels that I intend to use PWM or soft start etc.

If you're sticking to a 16 channel on a street legal car I'd prepare to run fuses for some of the less critical stuff

One more thing I'll note, the lambda heater is ecu (or wideband controller) controlled and it does so via the ground path not the 12V side. It just needs a switched 12V supply and the controller does the rest.

But like Armaki said, open discussion no bad ideas.

I'm hesitant to group dissimilar things together - even if they're mission critical - as I use current consumption as a diagnostic tool very often. As has been said it's a weird balancing act and most solutions have their pros and cons.

I also thought "channel grouping" was a disappointing omission from the course, although the elephant-in-the-room seems to be the number of discrete channels required to run mundane components like lighting assemblies, wiper motors and thermo fans. Then if you need to double-up outputs for unexpected items like electric water pumps and compressors, then a 16 channel PDM will almost certainly need to run parallel with some traditional fuses and relays to remain cost-effective for club racers. The more affordable 32 channel PDMs (such as AIM and Black Box) seem to be limited to 120A, which seems barely enough for 16 channels.

120A is still quite a lot of current.

If you are running a PDM at a constant 120A, you would need to upgrade the alternator as most common alternators put out less than 100A at full load as well as upgrading the battery to support that load being placed on it.

The off road races cars that I have dealt with have all run multiple alternators (one had more than 500A of alternator on it) to provide the power required for the multiple fans, pumps and lighting systems that they run, as well as careful programming of the PDM's to reduced load at lower engine speeds.

Good point Stephen. I was sizing for worst-case scenario, which was getting unrealistic.

I am considering the Syvecs PDU as they make one with 34 outputs which is enough to run reasonable road legal setup- I couldnt make a 16 or 20 be that feasible/helpful vs doing a new fuse panel

Just throwing the HP Electronik 8441 out there as a 34x output option as well. It has low current limits on the 16x low current outputs (2.9A) and has limits on the pin groupings (8A per group). That's not likely to be a deal breaker for the low demand stuff, but a consideration.

Ah, must have commented on another, similar, thread.

Is there any particular reason you HAVE to have everything switched by a single PDM?

You may find it's simpler to have additional relays, whether mechanical or solid state, switched by the PDM - this will also allow the lower amperage circuits to be used for high current control.

Alternatively, use a secondary PDM for the additional circuits you wish to control?

Armaki, most ECU already have a delaying strategy to heat up the lambda sensor, so putting its 12v together with ign and inj wouldn't be a problem.