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Methods for tapping OE TPS and Wheel Speed sensors to feed data logger?

Data Analysis Fundamentals

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I'm not sure if this question should be here or under the Wiring forum, but since it's primarily about data acquisition I thought I would start here.

The vehicle in question is a BMW E34 with an S52 swap and it is used almost exclusively on the track for HPDE events.

I have have an AIM MXS system with some simple analog sensor input to start: water temp, oil temp, oil pressure, fuel pressure. I would like to add wheel speed and throttle position. I know that I have trigger rings and sensors on each hub already to feed the ABS, and of course a TPS feeding the DME. I have four dedicated wheel speed inputs in the AIM harness, and open 5V analog channels.

Can someone help me understand if it possible to tap these existing sensors to feed to the AIM, or if that would degrade the signal to the OE systems and cause a malfunction. My vague instinct is that if you tap off some of the voltage coming from them that would distort the reading at the ABS controller or the DME system relative to sensor ground, but I'm not confident in this. If that were the case would it be practical to use an Arduino or similar to read the signal and then regenerate it appropriately to the OE system?

Or is this the sort of thing that is a dead end and I should just wait until I put an aftermarket ECU in the car, which is likely to happen eventually, and read this data over CAN?

OBDII?

Many ABS systems are designed to provide the wheel speed information either as digital output signals, or via communications (ie, CAN bus). This info is often used by the factory ECU (or other modules) for things like traction control, speedometer display, cruise control. Look at the factory wiring information for your vehicle, and see if you can figure out which of these might be available for your car. Splicing into a digital output is not likely to cause a problem, but test carefully.

Greetings,

It sounds like some additional contextual detail is in order. The ECU is a MS41.1 DME (Siemens) that came with the S52 from a 1998 Z3M, so I believe it is technically OBD2, but no CAN. The chassis is a 1991 that started out with an M50 and a M3.1 DME (Bosch), so I believe this means that the chassis is OBD1, The ABS unit accordingly doesn't have some of the fancier features, like being able to cycle it through INPA. The throttle position sensor is from the M50 throttle body paired with the M50 intake manifold, a typical mod for the S52 as it's a bit freer flowing that the newer one.

With that out of the way, it sounds like what you are saying is that I need to spend some more time with the wiring diagrams clarifying where exactly the wheel speed sensors go, and whether or not there is an opportunity to get the data as digital output from the ABS unit. I know that it might be possible to request that data via some data link to the ECU, but I suspect the sample rate wouldn't be fast enough to make the data usable.

Any thoughts about the TPS given the extra info above?

Tapping into the wheel speed sensors can impact the signal in a manner that can disrupt ABS operation, but I've seen devices designed for this.

Here's an example:

https://ecumasterusa.com/products/wheel-speed-to-can-module?variant=13733653839933

Thanks Mike, that seems pretty definitive. That ECU Master product looks like what I'm looking for. The documentation doesn't really explain how it works to run in parallel with an ABS computer, but that's likely a problem for ECUMaster support rather than here.

Nobody really spoke to the TPS aspect of the thread, but I'm guessing that the same issue presents of getting the ECU confused by distorting the sensor value with a tap.

Thanks everyone

Yup you're spot on. Simply tapping it can skew the voltage, but inputting it to a device that can replicate outputs avoids the issue.

As you mentioned with an aftermarket ECU, you'll have TPS wired to the ECU and can send it to the dash via CAN.

In the meantime since the S52 came in OBD2 vehicles, I would consider getting TPS to the dash via OBD2 communcation

There's a circuit called a "Voltage Follower" which prevent the signal to be affected by tapping into it. It's an OP AMP connected in unity gain configuration. Having almost no current draw, it isolates the "main" circuit from the one you are connecting to. I built one for my R32 GTR as the ATTESA module needs a TPS signal in order to work.

Here's a link explaining how it works.

http://www.learningaboutelectronics.com/Articles/Voltage-follower

Hope it helps,

Frank

Hi Mike and Frank,

Thanks for the comments. Sorry for the slow follow up, I got distracted with holidays and then other track-day prep on the car.

I had thought about trying to figure out getting an OBD stream to the AIM but was concerned about getting the sampling rate high enough to have good data fidelity. I have to admit also that I while I have a baseline understanding of CAN my knowledge of OBDII is very limited, beyond the belief that it was originaly intended primarily for emissions monitoring. Is there any good reference material you would recommend for getting started understanding how I would go about getting data to the AIM from an MS41.1?

Cheers,

Stephen

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