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PDM Installation & Configuration: What Is a PDM?

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What Is a PDM?


00:00 - In this section of the course, we're going to cover some of the general theory related to using a PMU and the design of the power management section of the electrical systems in your automotive project.
00:10 To start with, let's answer the question, what is a PMU and why should we use one? To understand what a PMU is and why you might want to use one, we need to have a quick talk about the conventional devices that they replace.
00:23 When we're designing and building the electrical systems in our vehicles, we need to get power to the various parts of the vehicle at different times, distributing it to different components in response to particular conditions.
00:36 To do this safely, we also need a way of protecting your wiring in the vehicle in the event of a crash or component failure.
00:43 Traditionally this has been done with banks of separate relays and a fuse box or circuit breakers.
00:49 The relays take a main power supply from our battery and when they're triggered to do so, send that power out to the connected fuses or circuit breakers and then onto the connected devices.
00:59 Throughout this course, we'll refer to a setup like this with discrete relays and fuses as a conventional automotive electrical setup.
01:07 A PMU replaces these relays, fuses and circuit breakers with solid state electronic switching, current monitoring and feedback.
01:15 While the conventional approach absolutely still has a place, particularly in budget constrained projects, using a PMU has many benefits.
01:23 The first of these is a reduction in bulk.
01:26 Building and wiring a bank of relays and fuses quickly becomes very bulky and is a surprisingly difficult thing to do tidily.
01:34 Conventional relays have individual pins designed to accept either individual female spade terminals or for that relay to be plugged into a specific housing.
01:43 The problem here is that either way, every relay in the system needs a power supply and trigger wire run to it.
01:50 This can put us in the tricky situation of needing to splice out a large gauge central power supply wire to the many smaller gauge wires for each relay.
01:59 The relay output then needs to be run to a fuse and then finally from that fuse, out to the connector devices.
02:06 This increases the number of splices and connectors in the wiring harness dramatically.
02:11 All these individual connectors and splices can and often do add up to a lack of reliability.
02:17 You can probably ask anyone that's worked in the performance automotive world, myself included and they'll have more than one story of electrical gremlins being traced back to an intermittent connection to a relay.
02:28 Now this isn't to say that individual relays and fuses can't be reliable.
02:32 They absolutely can and using specific integrated relay and fuse boxes goes a long way towards solving this problem.
02:39 With that said, using a PMU tends to eliminate these issues entirely.
02:45 PMUs do this by giving us one main power supply input connection to the device and then specific connectors for the power supply outputs and triggering wires.
02:54 These connectors are specified for automotive use and are generally fare more reliable than spade terminals or relay housings.
03:02 Most PMUs also let us turn outputs on and off via CAN bus messages, often eliminating the need for trigger wires altogether.
03:10 This makes the wiring harness design and build much more straightforward and tends to result in a cleaner, more reliable outcome.
03:17 The reliability benefits are not restricted to just the connectors though.
03:21 Traditional relays are mechanical switching devices with moving parts and are prone to failing in higher vibration environments.
03:28 Race cars are the definition of a high vibration environment so switching to a PMU can give us another reliability increase here.
03:36 In a conventional setup with relays and fuses, if there's an intermittent problem with a powered component in a vehicle somewhere and it blows that connected fuse, that component is dead until the fuse is physically replaced.
03:48 This can leave you stranded on the side of the road or track.
03:51 PMUs can be configured to disable an output when it senses too much current draw but then retry that connection after a time delay.
03:59 If the connected component is in the process of failing but still working intermittently, this can be enough to limp the vehicle home back to the pits or even finish the race.
04:09 Because PMUs are programmable, this means they give us a lot of flexibility also.
04:13 Conventional relays have individual trigger wires, meaning when they switch on or off is hardwired into the harness.
04:21 So changing things around later can be very tricky.
04:24 With a PMU, we program the outputs to turn on or off in response to input conditions.
04:30 This means that we can easily change those conditions in the future if the vehicle is modified.
04:36 Related to this is the ability for a PMU to give us some body control functions.
04:40 This does vary a little bit from brand to brand but the majority of modules on the market will be able to control things like indicator lights and wiper motors.
04:48 Further simplifying the system by not needing to have separate flasher modules or changeover relays for different wiper speeds.
04:55 You can see from all this, that PMUs have some pretty big upsides but we'd be remiss if we didn't mention the downside.
05:01 And that, predictably is the cost.
05:04 A PMU setup will almost always be more expensive than a conventional relay and a fuse setup.
05:10 However the added flexibility, ease of the wiring harness design and construction and that proven reliability more often than not outweigh this extra cost.
05:20 Personally I've completely switched to using PMUs these days as just a single missed track day due to a power supply related issue is a much higher cost than that initial extra financial outlay.

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