I’ve been spending the last month or so hacking away at TPMS. This has been an ongoing project for the Tesla Roadster that is finally coming to fruition.

About a year or so ago, we already made changes to:

• Modify the Tesla Roadster cable to connect the k-line pin on the diagnostic connector through to the DB9 connector on OVMS. We labelled this cable ‘OVT1’.
• Modify the OVMS module to connect that k-line pin to GEP7.

I’m now working on an optional expansion board that will include a little TJA1027T transceiver and map it to EXP1 and EXP2 (for async comms) and EGPIO8 (for enable/disable) on the ESP32 and MAX. This board will include a jumpered option for K-line master (one diode + one resistor), as well as jumpered powered selection (5v from usb, 5v regulated from car 12v, or direct car 12v) for the K-line bus.

The hand-soldered prototype, using our breakout boards, works well and the K-line async comms works. The screenshot shows OVMS sending a 19 byte request to the ECU (0x0f 0x04 .... 0xf0) and the ECU responding 19 bytes (0x0f 0x05 ... 0xf0) with the 4 tyre IDs. There is a similar command to re-program the ECU with new IDs.

On the firmware side, I am implementing a new vehicle independent ’tpms’ subsystem within OVMS (enabled via configureable option). This will allow sets of tyres to be maintained in the config and read/written to the car's TPMS ECU. The following commands will be available:

• tpms status - show status of the system
• tpms list - list tyre id sets in config
• tpms read <set> - read IDs from tpms ecu and store in config tyre set
• tpms write <set> - write IDs to toms ecu from config tyre set
• tpms set <set> {<id>} - config a set of IDs manually
• tpms delete <set> - delete the specified tyre set

The initial version will support Tesla Roadster, but the functionality is there for other vehicles to use if they need it (in particular for standard K-line implementations).

Comments/suggestions welcome.

Regards, Mark.

+++Attachments+++

Bench setup, showing (from left to right):

• Tesla Roadster TPMS ECU opened up
• Breakout board (power, k, Lin, and can)
• OVMS for can decode
• Usb logic analyser
• Power supply (12v)
• K-line analysis tool (9010)
• Windows laptop running k-line software
• Mac running logic analyser and terminal to OVMS.

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Hand-soldered prototyping board

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Async comms over k-line.

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