I'm interested in data logging the suspension ride height sensors on recent model PASM equipped cars.

I've traced the left front ride height sensor wires out on a 2021 718 GT4, wires #5 (LF Height Sensor GND) and #6 (LF Height Sensor SIG). Those wires match the suspension circuit diagram for the 981 series <attached>


I hooked up a logic analyzer to #5 and #6 and powered the car on, then used a jack to raise the front left of the car while it was running.

What I expected to see was the SIG voltage change as I raised the car. I didn't really see that. Voltages on #5 and #6 were similar with the car raised as they were with the car at ground level.


I'm attaching some photos from the sampling I took. The 4 channels being logged here are #5 and #6 but with an digital (first 2 lines top to bottom) and analogue (lines 3 and 4, top to bottom). I'm only focused on lines 3 and 4 right now as I have no idea if there is a digital channel present on this circuit (I sorta doubt it using single wire CAN but who knows)



After PASM/DSC startup, I see the channel 6 (SIG) voltage 3.7~V fall to a 2.4~V signal that remains the rest of the time the vehicle is on


This is the V captured for the remainder of the run, 2.486V on channel 6 (SIG)


This is a zoomed out view of the voltage logged, roughly half a second. You can see a pretty regular pattern which persists the remainder of the 120second run

This is the registered voltage on channel 5 (GND) AFTER raising the front left of the car up

This is the registered voltage on channel 6 (SIG) AFTER raising the front left of the car up

I was able to data log with the DSC when I made the ride height changes, so I know that the ride height sensor was working and being logged. I saw a change when I jacked the car up. (line 100 is when the ride height changes https://docs.google.com/spreadsheets...it?usp=sharing)


So, what do you think I am seeing here?


Could it be SW-CAN (Single-Wire); J1850? SW CAN uses Pulse Width Modulated (PWM) to generate a value. CAN might be too slow anyway for this application (pretty sure I need at least 12bit +200Hz sampling to get any value from a wheel height sensor). This sensor also doesn't need to listen, just report...so why add complexity and cost with a duplex capable protocol and microcontrollers?

Could it be a current loop circuit? I ordered a resistor to put between the GND and SIG wires to try and register a voltage. Current loops are almost perfect for this situation, where a remote sensor only needs to send signal and can be powered by the wire pair.

I can't possibly be the first person doing this research, but I haven't found much online yet about this vehicle or people using OEM ride height sensors to monitor suspension movement.


Any thoughts or direction you can provide would me much appreciated!

 

I think there is something wrong with the setup you are using. I can't believe the sus pots on that car are anything other than analog potentiometers. So just probe the signal pins with a multimeter and create a calibration equation as you record damper stroke as a function of voltage (with the spring removed) and use that as an analogue voltage input to whatever logger you are using.

The suspension position is almost certainly also converted to a CAN message that will be available on the CAN lines seen in that pinout, and also bused by the gateway ECU to any other CAN buses on the car where that info may be needed by other ECU's. So if you have access to a CANalyzer or some other SW like Busmaster you could probe those CAN lines and reverse engineer the message format to then configure your logger to record via CAN. You'll be limited to the speed at which the PASM module's CAN transceiver sends that info at though, which is likely 100Hz.

 

Thanks for taking time to respond.

I agree with you, my guess also is this is a simple potentiometer. But then I should have registered a voltage change in the highly accurate log of voltage signal sent from that sensor, which I didn't. The varying voltage change within a given spike is also inconsistent enough that I question how reliable the signal would be to report suspension position.

There is one wire I didn't trace or log on the wheel sensor but it has to be the ground wire...i should double check that it is indeed a ground and nothing more.

When I swap the struts for aftermarket ones I will use that opportunity to measure raw voltage off the sensor like you described. Hopefully I make more progress before that happens though (couple months out likely)

I also agree with the CAN description; CAN will be too slow for this application and I wasn't planning to tap suspension data from CAN.

 

Agree that the log data seems off. Maybe try probing a different pair to see if you get the same results.

What us your plan with logging this data? If it is for setup purposes, you'll need to log at a very high frequency to get much use (~1000Hz).

 

I just want to find a way to utilize the existing hardware to log the suspension movement to a data logger. This will provide performance data for suspension changes I make to the strut valving. I would like to know when the changes I make to the DSC negatively or positively affect the mechanical grip of the car.

From my understanding of 250-500Hz should be sufficient for strut movement logging, but some https://www.bmotorsports.com/shop/product_info.php/cPath/179_660/products_id/5548 loggers managed 1000Hz on their inputs which is awesome.

 

I've since learned that the wheel height sensors are utilizing PWM (pulse width modulation) which uses a current loop to power, and send data from the sensor to the PASM controller.

Whats difficult about PWM is that in order to convert it into a voltage value, you have to slap resistors inline of the current loop and that terminates the signal effectively. So this means unless I can tap the signal on the PASM controller, I will have a difficult time measuring the OEM wheel height sensors.

My logic analyzer was properly identifying the signals it saw, but it is unable or I lack the knowledge to convert the signal into a voltage I can use on a traditional data logger.

 

I forgot but awhile ago I took another look at the resolution the wheel position sensor could provide. Resolution of voltage, the voltage range and how much of the physical movement through the sensors range all have an impact on how accurate the sensor can determine wheel position, movement rate and ultimately, chassis angle and velocity.

At the physical full extension the wheel sensor outputs 2.08v, and maybe the max it can be compressed while attached to the suspension registered 2.8v

The sensor is capable of 5v, I was able to register voltages of .78v through 4v by disconnecting the sensor and swinging the arm through its arc.

I very crudely measured how much front and rear travel the strut has with a strut off the car. This measurement isn't helpful since we don't know the installed struts bumpstop and travel limits, or the travel limits as installed...but assuming the numbers match there is some concern about the ultimate resolution and rate I can capture movement at. This assumes a resolution of 8 bits:

0.72v wheel height voltage range. 720mv / 256 steps (8 bit) = 2.8125mv

2.8125mv smallest measurable voltage we can register for movement in the sensor.

Assume the front strut has a movement range of 64mm,
64mm / 256 = 0.25mm smallest measurable front travel movement

Assume rear has a possible movement range of 77mm,
77mm / 256 = 0.30078125mm smallest measurable rear travel movement

So if we sample at 100Hz we can see strut movement rates of 30.078125mm/s (rear) and 25mm/s (front)

Not great