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PDM installation and Starpoint earthing

PDM Installation & Configuration

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Hey guys,

So I figured this would be the best section to put this question in as it applies to a PDM installation. I am working through my NEXUS install and im doing a full concentric twist harness throughout the car. I am trying to figure out the best way to do my starpoint grounding method while still keeping it tidy for the concentric twisting. The rear harness I have already done and ran grounds up to the battery (under dash), for the rear lights and fuel pumps. From this point im not really sure where to go. I plan to use radlocks to pass negative terminal through the bulkhead to the block (13b-rew). I would like to do the front harness the same, IE run my ground for lights, fans and EWP through my harness back to a single location vs creating breakouts to ground to chassis.

What would guys recommend that have built a couple entire chassis harnesses?

Thanks, Aaron

I'm also not sure what to do in this situation, would we have any problems running the grounds in the harness rather than a branch out from most of the devices? Like coil packs for example.

I worry about star earthing when I need devices like sensors/controllers to all see the same voltage, and return a voltage relative to a reference ground. For things like pumps, fans, headlights, then using the chassis as the ground path back to your star point / battery negative is perfectly acceptable (and preferrable, especially if you would need long runs of heavy gauge wire).

I run mine like this,

Star point in the cabin: ecu, fuel pump. dash tail light and anything else in the cabin,

Engine block: coils. ecu,

Chassis Ground: front lights water pump, fans,

I may have missed something and it may be not the correct way in many people's eyes but it certainly works just fine, I also run good chassis to engine earths most often two of these,

Ross, so you run ECU grounds to two different points, in cabin and eng block? Although highly unlikely what happens if your engine to chassis ground fails wouldn't that fry your ECU as the starter will now have ground path thru the ECU?

to add to your question, and maybe a solution would running 2 -terminal studs along the chassis, let's say.. battery in the trunk. grounding stub (one one trans tunnel to battery and one on fire wall to battery) so all accesories that need grounding in seperate locations can still respect start point earthing. if i am wrong on any of this set up please correct me.

best regaurds


The nexus is kind of an interesting PDM in regards of grounding...

The Ecumaster PMU has 1 ground, and its pretty much for powering the unit only.

Motec is the same.

Aim's PDM's have only a few grounds and per the documentation they are mostly for the H bridge's.

Haltech's non Nexus PDM's have 4 ground pins, which seems like a lot but they do have 2 H bridges and the high amp outputs appear capable of low side outputs.

Now the Nexus has a Surlock that connects it to ground, which is way more wire then any of the others. It does have 4 H bridge outputs and the high outputs can drive low side, but it also has 2 ground pins on connector A that according to Haltech "Ground out for shields, low current CAN devices and digital sensors".

What I am not sure about is what the ECU side of the Nexus is connected to, but connector A seems like it holds mostly ECU related inputs/outputs.

I generally treat the chassis as my main star point. The only exceptions I'll make is the ECU gets grounded to the engine and the ignition coils to the cylinder head, but otherwise the battery ground, block, fans, lights etc get grounded to the chassis. When I do full chassis harnesses I try and consolidate the grounding down to 3 or 4 points; one under the dash, one in the trunk, one in the engine bay and sometimes a second one in the cabin somewhere, depends on run lengths.

I could rant on about how the steel body of the car is a larger conductor then running any size wire from the battery location to the block and running an additional ground is pointless, just do proper grounding connections from the battery to the chassis and the engine block to the chassis. Keeping device run lengths shorter is good for preventing voltage drop, I'd rather see the grounding path much shorter by going to the chassis vs running it back to the battery or a bus bar off the battery.

I haven't done a Nexus R5 yet, but I would be leaning to grounding the 2 ground pins off connector A to the engine block and the Surlock to chassis ground. Myself, I would try contacting Haltech to confirm details before starting construction.

What about ECU's that have separated grounds? Like FuelTech FT550 and FT600 that have POWER GROUNDS and BATTERY GROUNDS. If I star ground all the grounds to the same point, this could cause some issue in this kind of ECU?

Other ECUs like MoTec have just on kind of ground which is called Battery Negative on its datasheet...

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