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ALDL Mode 2 Data Stream

  Last Updated 14 September 2000

The Mode 2 data stream allows a 64 byte portion of the ECU's CPU memory to be inspected. This is useful for 2 reasons.

Firstly, the memory region from 0x8000 to 0xFFFF is the EPROM. Therefore, it is possible to read the entire EPROM contents out of the ECU without opening the ECU's box. This enables you to tell if your car has  had it's EPROM changed (called chipping). If you want to, you could also disassemble the EPROM contents to see how the ECU uses its various sensors.

The second use for mode 2 is for looking at the current values of the CPU's internal variables. The CPU in the ECU is a GM specific version of the 68HC11 microprocessor from Motorola. It has internal RAM at addresses 0x0000 to 0x03FF. (My ECU only uses 0x0000 to 0x02FF). Within this 1KB of memory are stored the Exhaust and Turbo BLM values, plus lots of other interesting stuff. Once you know what you are looking at, this can reveal quite a lot about the state of your engine.

Request Mode 2 Data   - PC transmit to ECU

Data Byte (hex) Description Notes
F4 Frame Start 0xF4 indicates the start of a message
58 Length of Packet 0x58 (0x52 + Length of Packet)
02 Request Mode 2 0x02
00-FF AddrMSB The MSB of the desired Address
00-FF AddrLSB The LSB of the desired Address
?? Checksum Packet added together then MOD 255 = 00

Reply Mode 2 Data - ECU transmit to PC

Once the ECU has received the Request Mode 2 Data packet, it will start to respond (within a mili-second) with the following data stream.

Data Byte (hex) Description Notes
F4 Frame Start 0xF4 indicates the start of a message
96 Length of Packet 0x96 (0x52 + Length of Packet)
02 Request Mode 2 0x02
(??) * 64 64 data bytes The Data starting at the specified address
?? Checksum Packet added together then MOD 255 = 00