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I'm currently working on a project to build a TFT dashboard for my motorcycle (based on an ESP32 as the main IC). I want to keep the original LCD dash and all its wires connected and just "split" into the harness for the dash and grab all the ones I need for my own dash.

So all the colored wires would be split into two, one goes to the original dash, one to the new dash.

Would that somehow affect the values that the original dashs IC reads? Would I need a sort of diode in certain wires so the original dash reads from the sensors/switches and doesn't "see" the new dash/ESP32?

I'm mostly concerned about the wires that read the gear switches and the fuel level resistor, for example. The ECU is a bus anyway and most of the other wires are not connected to the IC in the dash and just wired to the LEDs.

Thank you a ton for any possible help with this, I hope I didn't miss any important information and that the question is understandable enough; I'm still trying to wrap my head around all this.

I have the part of the motorcycle schematic for the original dash and the full schematic attached:

schematic for original motorcycle dashboard

Wiring diagram labels:

enter image description here

Full wiring diagram:

enter image description here

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  • \$\begingroup\$ I’d be connecting to the canbus - all the information you require is there. \$\endgroup\$
    – Kartman
    Jan 12, 2022 at 11:02
  • \$\begingroup\$ My OBDII Adapter will arrived today and I will check it myself but according to the schematic and what I read online the ECU doesn't know stuff like current gear, if highbeams are on, fuel level and other stuff I need on my dash. \$\endgroup\$ Jan 12, 2022 at 11:10
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    \$\begingroup\$ I’d be surprised if the ECU does not know what the gear position is. For the inputs you would need to measure the voltage and the current. For the switches that is easy, for the fuel level sensor you’ll need to disconnect the circuit and place the multimeter in series. Once you have some measurements, then you can make decisions based on numbers rather than guesswork. \$\endgroup\$
    – Kartman
    Jan 12, 2022 at 11:24

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I have experience with electrical design, but never worked on any motorcycles in that regard. So take this answer with a grain of salt.

Also, what more would you like to see on the dash? Your MT09 is nice and modern. If you don't like the way the dash looks they make aftermarket ones, which will be cheaper and faster to get than what you are trying here. You could even install some nice analog dials if you like that kind of thing (my CBF500 dash only displays two things, but those dials are timeless ;))

But that's not what your asking.

First of all you mention splitting cables: be very careful making modifications to cable harnesses. I would definitely not solder wires directly onto existing ones with some heatshrink over it and call it done. That has a likelihood to fail over time which you wouldn't want, especially if it happens while driving. Cable assemblies in industrial/automotive environments shouldn't be molested like that!

I recommend making Y splitters with connectors on all three ends, matching up to the connectors that are on the existing cable harness/dashboard. If you make it removable like that it would make it more easy to sell later too. I wouldn't ever touch an MT09 that has the complete cable assembly to the dash cut open. Either buy off-the-self cable assemblies and cut those open all you want, or make your own cable assemblies starting from the wire housings, pin terminals, cabling etc. Anyways make it removable and don't cut any existing wires.

For your actual question: no, your circuit won't affect the signals going off to the original dash, if your hardware is well designed, and if it did, diodes on data lines wouldn't help necessarily. As far as I can tell most of the things going to the dash are quite slow on/off-type signaling.

The diagram suggests that the bike electrical systems connected to the dash are only 'completing the loop' by means of a low-side switch, turning on a light. You want to measure this voltage, buffer it (to protect your hardware), and then convert it into (presumably) 0-3.3 V signals that work with your ESP32.

A well-designed digital buffer should only consume micro-amps of current from the signal that is being buffered (this 'measuring current' is often referred to as input bias current). This amount is absolutely negligible to the actual signal current, so you will be fine.

The analog measurement for the fuel level is a bit more difficult. The diagram suggests it is a resistive sensor. I'm reading that fuel senders are actually pressure measuring devices in some cases. So, not sure if it's really just a resistor from ground to the dash, but lets assume it is.

Resistance is measured by exiting it with a known current, and measuring the voltage that develops across it (or the other way around, basic Ohm's law stuff). I think the dash is doing that in this case. You can leave the resistive sensor attached to the dash and just make a voltage measurement again. This time your buffer should have an analog output so the ESP32 or whatever ADC you use can measure it.

The analog signal would be more susceptible to being loaded down by the input circuitry of your custom dashboard (more so then the digital signals), but careful analog design will do the trick. Just grab an analog voltage data logger, connect it across the resistive fuel level sensor and go for a ride. This will give you an indication of the voltage levels you need to buffer and level shift to accommodate the input range of your ADC.

The CAN signals could actually be messed up if you don't treat them properly. CAN is a differential serial bus. The cables should be of defined impedance and terminated in a certain way to keep the signal integrity as good as possible. But at the same time CAN is designed to be robust, so if you stick to the beaten path in terms of designing your receiver nothing should go wrong. Depending on the standard used, CAN is either terminated at each node on the bus, or just at both bus ends with 120 Ω. I'm not sure what kind is used on bikes. Plenty of CAN receiver ICs exist that will do all level conversion for you; just keep the stub-length short and terminate appropriately. The datasheet for the CAN IC will usually suggest protection schemes for the data lines.

Some other notes:

  • Be careful with the power coming in. Assume it is not well regulated (if at all). Also expect a lot of interference, noise, and large voltage spikes and dips. Take the necessary steps to make your supply robust so things don't die on you. If you are not familiar with power supply design then buy some kind of PCB module that has been tested to good standards, and put a lot of filtering and transient suppression before it. And, this goes without saying, don't short anything. The bike has fuses on the electrical system but that will still pass large current before opening, and potentially completely destroy faulty electronics. Include your own protection on the incoming supply rail.

  • Design your analog and digital buffers well. That's not only to minimize the extent to which the original signal is loaded down by your electronics, but also to protect your own devices. Transients picked up or generated by parasitic effects of long cables can be quite harsh.

  • Make a nice PCB for this. Some modules stuck onto a bit of perfboard will not stand the test of time; not on a bike anyway.

If you have any further questions let me know.

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  • \$\begingroup\$ Thank you for the super detailed write up, it will definitely help me in the right direction. I have bought the original MX34 connectors that my dash uses and the Y splitter will be my first task. How would I go about connecting a buffer for the normal digital inputs, do I just put it in parallel or in series? How could I contact you for further questions, I would like a chat for some help if that would be fine with you. \$\endgroup\$ Jan 13, 2022 at 19:19
  • \$\begingroup\$ Nice work finding the exact connector! The buffers would be used to 'probe' the voltages on the wire harnass. It would essentially be in parallel to the inputs of the original dash but you dont normally say it like that. Your ground should be at the same potential as the rest of the bike, but that is manageable. This platform has no means to send private messages afaik, ill contact you via the social channels you left on your page. \$\endgroup\$
    – Thijs
    Jan 15, 2022 at 16:56

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