Discovery 3 CAN bus sniffing series
- Part 1: OBD pins, incorrect wire colours, and the DIY cable trap
- Part 2: Bringing up the LilyGO ESP32 CAN board
- Part 3: Debugging when CAN frames stay at zero
- Part 4: Corrected wiring and the first real CAN frames
- Part 5: Finding the first likely suspension height values
Important correction: the supplied cable colour chart was inaccurate. On this cable, continuity testing showed OBD pin 6 = Green, OBD pin 14 = Green/White, and OBD pin 5 = Light Blue.
I started this Discovery 3 CAN bus test with a LilyGO CANbus ESP32 board, a DIY OBD-II pigtail, and a simple goal: read packets from the vehicle without disturbing the network.
The first assumption was that the pigtail colour chart could be trusted. That turned out to be the wrong assumption. The standard OBD-II pin functions were fine, but the colours on this particular cable did not match the supplied diagram.
The standard pins we cared about
| Function | OBD-II pin |
|---|---|
| CAN-H | 6 |
| CAN-L | 14 |
| Signal ground | 5 |
| Battery +12V | 16 |
The seller chart claimed CAN-H was green, CAN-L was brown/white, ground was yellow, and +12V was green/white. Only one of those mattered after testing: pin 6 really was green.
The actual cable mapping
Continuity testing from the OBD plug pins to the wire ends gave this mapping:
OBD pin 6 = Green = CAN-H
OBD pin 14 = Green/White = CAN-L
OBD pin 5 = Light Blue = GroundThat is the wiring that should go to the LilyGO board:
Green -> CAN-H
Green/White -> CAN-L
Light Blue -> GNDThe dangerous part is that the seller chart marked green/white as OBD pin 16 battery positive. On this cable, green/white was actually CAN-L. That is exactly why the cable needs to be mapped by pin number before connecting electronics.
The voltage clue
A first measurement using the assumed wire colours gave a low reading of about 0.25 V from the assumed ground wire to the assumed CAN-H wire. That made the bus look dead. Measuring at the actual OBD port showed the expected CAN voltage, so the vehicle side was alive and the pigtail mapping became the suspect.
The useful tests are pin-based, not colour-based:
Pin 5 to pin 6 - CAN-H to ground, typically around 2.5 V when awake
Pin 5 to pin 14 - CAN-L to ground, typically around 2.5 V when awake
Pin 6 to pin 14 - about 60 ohms with the vehicle off/asleep, depending on topologyThe takeaway is simple: the OBD-II pinout is the reference. The wire colours on cheap pigtails are only a suggestion until proven with a multimeter.
Leave a Reply