The disadvantage is on a factory memcal it also locks the spark advance to 10deg BTDC until 2000RPM, this can be disabled by editing the memcal binary. This tells the ECM to start outputting data on the CEL wire. VN/VP – This is a uni-directional data link so to enable ALDL data a resistor must be placed from the diagnostic enable wire to ground. How It Works: Due to the different types of ALDL there are different ways to get data out of the ECM… The PCM is linked to the BCM, ALDL is used for the linking process. VX and VY – The NA V6 uses a flash based PCM, this also added some new ALDL modes for the security system. A regular message containing all the dash information is sent at the request of the BCM. The VT took the data bus further by also including the instrument cluster. It is also 8192 baud but the major difference is the bus master is no longer the PCM but instead the BCM. VT – This is a very similar implementation to the VR and VS ALDL. PCM Device ID - $F4 for VR V8, VR V6 and VS V8. This way only the device that is addressed process’s and responds to the packet, all the others ignore the data. Every device on the bus has a device ID, so when data packets are put on the bus it is ‘addressed’ to a particular device. In VR and VS applications the bus master is the ECM/PCM. This data bus allows for only one device to communicate at a time and connects all the major modules – BCM, ABS, SRS, ECM etc. The purpose of the transceiver is to convert the transmit and receive lines of the processor UART into a single wire half-duplex data bus to allow the ECM to communicate on the vehicle bus. It is a bi-directional half-duplex data link using a hardware UART in the processor and a Delco/Delphi transceiver IC. VR/VS – The second and probably the most widely used type is 8192 baud, this is also used in many GM built US vehicles from the mid/late 80s to the late 90s and often referred to as OBD-I. When ALDL is enabled the CEL will flash with the data. On every interrupt a bit is applied to the CEL wire, 6.25ms = 160Hz or 160baud. The 160 baud rate is due to the ECM operating with 6.25ms minor loop interrupts. It is ok for slow data speeds but puts too much load on the microprocessor for high baud rates.
Bit banging is a method to transmit serial data using only software rather than a hardware UART. The ECM uses a ‘bit-banging’ technique on the CEL (Check Engine Light) wire to flash data out bit by bit. It operates at 160 baud and is a uni-directional (read only) data link. VN/VP – This was the first implementation, was also the most basic and is the only implementation that uses ALDL for purely diagnostic purposes.
There are a few different implementations of ALDL…
It is used for a variety of things from, as the name suggests, initial system testing when the vehicle is manufactured to vehicle module communications to diagnostics at the dealer/home to real-time tuning. What It is and What Can It Do: ALDL stands for Assembly Line Diagnostic Link. About: This information will be updated from time to time and serves as a basic introduction to GM ALDL, specifically aimed at GM Holden Commodores.