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Sensor ground location for analog sensor inputs to PDMs without sensor ground circuits

PDM Installation & Configuration

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I'm curious to hear how others are handling this situation. Multiple PDMs are now offering analog inputs. This is fantastic for IO expansion.

When using this feature for 0-5V sensor inputs rather than simple switch or rotary inputs, we have a dilemma.

The PDM has a dedicated 5V output for sensor power, analog input pins, but no sensor ground circuits.

Ground offsets are a known issue causing analog voltage error, so we want to reduce their impact as much as possible.

That said, we also want to avoid grounding the sensor in a location impacted by something noisy such as an ignition component.

In addition, we want to avoid grounding the sensor where something else is sending a large amount of flyback, ideally also not where something that suddenly draws a great deal is grounding, such as a fan.

For those reasons, grounding the sensor where the PDM grounds doesn't sound ideal, despite it seeming a good idea from a ground offset perspective.

Are folks running sensor ground wires to a chassis ground not populated with anything but sensor grounds?

Are you connecting that chassis ground to your star system directly, or not doing so in the hope that the chassis distributes and effectively damps any noise, flyback etc. from other devices?

That's an interesting observation Mike. I'm in the middle of wiring up 2 x MoTeC PDM32's for my FJ40 project and just reconfirmed that MoTeC provide a dedicated sensor 0V pin so that's plain sailing. I had a look at the ECU Master PMU16 pin out and that doesn't provide a sensor 0V but their manual recommends connecting AV inputs between the 5V and ground pins which would have been my fall back suggestion. I would expect this would provide the lowest chance of ground offsets. In saying that I'm a bit selective of what I'd wire up to a PDM and would have anything I consider mission critical wired directly to the ECU and then sent via a CAN message to the PMU if required. Unsure if any of this is super helpful though :)

Thanks Andre! I totally agree that sounds ideal from a ground offset perspective.

I also agree that if you have ECU inputs available, mission critical sensors should be wired there first. I look to Motec dash next for additional sensor inputs, but when those are both exhausted, and I've purchased a PDM which has 16 available analog inputs, I've been starting to take advantage of them rather than the next step always being purchasing additional hardware for the sole purpose of input and streaming over CAN.

On the phone ECUMaster USA suggested not using the ground point you connect the PDM ground to, due to analog offset error caused by flyback from all devices powered by the PMU going through that point, as well as potential noise. They mentioned they haven't seen it damage sensors, but another location would be prudent. In my case I did test grounding a sensor there and was not seeing noise beyond wiggles the size of the resolution of the analog to digital conversion of the PDM, but I'm running ignition and other dirty items through another PDM in the car. Still, I decided to ultimately ground the sensors elsewhere thinking it would be safer.

On my touring car which has been completely rewired from scratch, we're currently grounding the sensors wired to one of my PDMs to a chassis point with nothing else present. It is not starred to any other points and only reaches the negative battery terminal via the chassis, battery isolator, and its main ground strap to the negative battery terminal. We verified sensor readings reach the PDM within the accuracy of the ADC 0.019V per bit, when the sensor is grounded to this chassis point, or at the PDM ground pin location. In this case I'm happy with the lack of significant offset after careful selection of chassis ground point, testing it, and having the vehicle well grounded.

At this point I've only been using PDM inputs for important sensor data for about a year, so my sample size is small. By important I mean it's data I really want to have, but its accuracy doesn't directly impact engine operation. For example I'm using it for air shift system tank pressure and line pressure on this car. I've used it for aux temperature and pressure sensors on others which are solely for data gathering.

It's possible what I'm currently doing for sensor ground is the best option, but I believe we are never too old or experienced to avoid the situation of not knowing what we don't know, so I figured this was worth a post to see if anyone has found a fatal flaw in my current process. And if not, great. Then hopefully this will serve as an example of how people might want to ground their sensors wired to PDMs without sensor ground circuits. I haven't seen discussion of this before.

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