RX8 Instrument Cluster - CAN Bus Protocol

The Mazda RX8 utilises a high-speed CAN bus for communicating between the various modules found throughout the vehicle, with each module constantly transmitting and receiving packets of data containing variables relevant to its operation. The following table contains a list of CAN bus messages that are required in the operation of the RX8s instrument cluster, each being of standard frame size and transmitted onto the bus at a speed of 500 kBit per second.

ID Size (Bytes) Module Contents
0x420 7 PCM Coolant Temperature & Warning Lights
0x201 8 PCM Vehicle Speed & Engine RPM
0x650 1 PCM Cruise Control lights
0x231 2 TCM Gear display, AT warning light
0x300 1 EPS Power steering warning light
0x212 6 ABS/DSC Brake warning light, DSC OFF light, DSC light, ABS light

Module Acronyms

PCM - Powertrain Control Module
TCM - Transmission Control Module
EPS - Electronic Power Steering
ABS - Anti-lock Braking System
DSC - Dynamic Stability Control

CAN Bus Message Layouts

Message 0x420 - Coolant Temperature & Warning Lights
Coolant Temperature & Warning Lights

This message, originating from the PCM, is responsible for initialising the cluster, allowing for the engine-related warning lights to be cleared and the engine coolant temperature to be displayed on the gauge. If the cluster doesn't receive this message at least once every couple of seconds, then all of the warning lights will default to being illuminated and the needles of the oil pressure/coolant temperature gauges will sit far to the left, off the face of the gauge itself.

Message Layout

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 Coolant Temperature
1 N/A
2 N/A
3 N/A
4 - - - - - - - OilP
5 - CEL - - - - - -
6 OilL Batt - - - ECL - -

Flag Descriptions

Light Flag Name Description
OilP Oil Pressure The RX8 is equipped with an engine oil pressure gauge, however the resolution is only a single bit. This makes it a dummy gauge, with the needle pointing to L (low) when the bit is set to 0, and to the centre of the gauge when it is set to 1
CEL Check Engine Light Illuminates the Check Engine Light when set to 1
OilL Oil Level Light Illuminates the oil level warning light when set to 1
Bat Battery Voltage Light Illuminates the battery/electrical system warning light when set to 1
ECL Engine Coolant Level Illuminates the battery/electrical system warning light when set to 1
Message 0x201 - Vehicle Speed & Engine RPM
Vehicle Speed & Engine RPM

This message, also originating from the PCB, broadcasts both the engine speed (RPM) and the vehicle speed, which is likely being fed to the PCM from the sensor on the extension housing of the transmission. The values accepted by the cluster for the vehicle speed and RPM aren't 1:1, and require scaling and sometimes an offset.

For example, the expected vehicle speed is in km/h, but with an offset value of 100.0 and a scale of 100.0
(kmh + 100.0) * 100.0

Message Layout

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 Engine RPM
rpm * 3.85
1
2 N/A
3 N/A
4 Vehicle Speed
(km/h + 100.0) * 100.0
5
6 N/A
7 N/A
Message 0x650 - Cruise Control
Cruise Control

Once again originating from the PCM, this message simply sets the state of the Cruise Control status indicator lights

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 CCM CC - - - - - -

Flag Descriptions

Light Flag Name Description
CCM Cruise Control Main Light Illuminates the green Cruise Main light when set to 1.
In the OEM application, this light indicates that the cruise control system has been activated and a cruising speed has been set
CC Cruise Control Light Illuminates the Cruise Main light when set to 1.
In the OEM application, this light indicates the cruise control system is active
Message 0x231 - Shift Position Indicator
Shift Position Indicator

This message originates from the TCM and is used by the instrument cluster on automatic transmission models to display the current shifter position. The gear position can be displayed in either regular or manual mode, selected by setting the Man flag in byte 1. In regular mode, the cluster will display P, R, N or D. In manual mode, it will display a numeric value on the 7-segement display representing the gear the transmission is currently in. For the specifics of each mode, refer to the entry for the Man flag in the flag descriptions table.

Message Layout

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 Manual Mode Gear
First (1) to Sixth (6)
Shifter Position
P (1) | R (2) | N (3) | D (4)
1 Man AT - - - - - -

Flag Descriptions

Light Flag Name Description
Man Manual Shift Mode When this flag is set to 0, the position indicator will display the position defined in the lower 4 bits of byte 0. Valid position values are:
  • 0x1 for Park
  • 0x2 for Reverse
  • 0x3 for Neutral
  • 0x4 for Drive
When this flag is set to 1, the position indicator will display the numeric value defined in the upper 4 bits of byte 0, using the 7-segement display. Values 1 through 6 have been tested on a cluster from a 6 speed automatic, however the behaviour of one removed from a 4 speed automatic model is uncertain
AT Automatic Transmission Malfunction Light Illuminates the AT malfunction light
Message 0x300 - Electronic Power Steering
Electronic Power Steering

This message, originating from the EPS Control Module, sets the state of the EPS malfunction light.

Message Layout

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 EPS - - - - - - -

Flag Descriptions

Light Flag Name Description
EPS Electronic Power Steering Malfunction Light Illuminates the EPS malfunction light when set to 1
Message 0x212 - ABS, Traction Control and Stability Control
ABS, Traction Control and Stability Control

This message is broadcast by the ABS/DSC module, it is used to set the various malfunction indication lights relating to ABS/DSC/TCS. It appears the warning light for the TCS can only be illuminated while the DSC OFF (DSC) flag is set to 1. As I'm not familiar with the RX8 and the operation of its TCS, and the owners manual only briefly glosses over the topic, I am unable to determine the significance of this.

Message Layout

Byte Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0 N/A
1 N/A
2 N/A
3 - - - - - DSC - -
4 - BSW - - ABS - - -
5 - - TCSA TCSO - - - -

Flag Descriptions

Light Flag Name Description
DSC Dynamic Stability Control Off Light Illuminates the brake system warning light when set to 1.
Curiously the cluster requires this light to be illuminated in order to switch on the TCS light via the TCSA or TCSO flags
BSW Brake System Warning Light Illuminates the brake system warning light when set to 1.
This light is used to indicate a malfunction in the braking system, as well as to indicate that the handbrake is engaged
ABS ABS Warning Light Illuminates the ABS warning light when set to 1
TCSA Traction Control System Active Flashes the TCS light when set to 1.
In the OEM application, this is used to indicate the TCS is on and is currently active
TSCO Traction Control System On Light Illuminates the TCS light when set to 1.
In the OEM application, this light is briefly illuminated upon switching the TCS on