There are a number of programs that enable you to log data about how your engine is working by reading information out of the ECU. For my OBD1 ECUs, I use CROME Pro or Freelog software on my PC to log what is going on. These software packages use a serial port on the PC (sometimes implemented with a USB adapter) to send commands to and receive data from the ECU.

There is a ton of information on this wiki about how to set up your ECU to be able to communicate with the PC. Basically, you need to hook the ECU's transmitter to the PC's receiver and you need to hook the ECU's receiver to the PC's transmitter. The ECU serial works at TTL voltage levels and most serial ports work with RS232 voltage levels so some sort of signal translator must be used.

Once the serial port hardware is operational, you need to replace the software that controls the ECU's serial port with software that controls the serial port in such a way that it can talk with the software on the PC. I use John Cui's data logging plugin in CROME to install this software patch into the ECU binary (program) that I want to log. That binary is programmed into a flash or EPROM memory chip and installed in the ECU in the car.

Once the engine is running with the modified ECU and the PC logging software is running and the two are communicating with each other, you can start data logging. When you tell the PC software to start datalogging, the PC software starts sending commands to the ECU. The datalogging software added to the ECU binary receives each command, interprets it and returns the results, which the PC software then adds to a log file. Typically one command is used to get one piece of data from the ECU. With John Cui's data logger patch, for example, there are standard commands for getting data that most people want to get such as RPM, O2 sensor readings, MAP sensor readings, etc. There are also commands that allow you to read any RAM location for logging less frequently logged data. The data logging software on the PC just steps through all of the commands to get the data you want, and then repeats the same sequence over and over as fast as the PC can run.

After logging the data from your car while it is running under different loads, you can use that data to figure out how to change many operational characteristics of the ECU software, such as the fuel and ignition maps. This is done with Rom editor software such as CROME for OBD1 ECUs.

-- markolson - 24 Oct 2005