Diy Electronics

Overview

Subaru Select MonitorThis project looks at building a display that will connect to a Subaru's OBD style connecter in order to display engine parameters to the driver. Almost anything can be diplayed including things like battery voltage, manifold air pressure, injector duty cycle, wastegate duty cycle and many more dependant on car. This would allow you to check vital engine parameters on highly modified and tuned engines as well as completely standard engines. Instantaneous fuel consumption can be calculated from injector duty cycle, knowing the injector size and the speed of the vehicle.

Subaru Select Monitor (SSM)

Those people with a Subaru post 98 in the UK (and probably elsewhere in the world) will most likely have an OBD style port somewhere in their car. On 99/00 cars this will probably only support Subaru's own protocol SSM but on later cars it will also support OBDII. Both standards are used for obtaining engine parameters such as injector duty cycle, or oxygen sensor readings, but SSM is far superior in terms of its update rate and more detailed engine parameters.

Before reading any further you may want to read up on the SSM protocol. For early cars like the Impreza MY99, the SSM is a protocol using a single line for sending and receiving data. It uses the OBDII K-line and a similiar interface to that of ISO-9141-2 but uses a 4800 baud rate instead of 10.4k baud.

Circuit Design

The circuit is based around a PIC 16F628A. I widely used MCU with built in UART. The built in UART uses TTL signals which are converted to correct levels using two transistors. This also combines the read and the write data - the MCU sends a command (to obtain engine parameters) and then waits for a response before sending another command. This is because all communication takes place over a signal wire effectively rendering it half-duplex - (it is not possible to have a full duplex communication).

When I get a moment I'll upload some schematics but currently its on board in the car!

I plan on using a larger PIC MCU so that the software is more flexible and can obtain more engine parameters than just those required to calculate fuel consumption.

Current Update

A prototype is running in the car and I adjusted the calculations to make the average consumption a little more accurate. The fuel consumption meter shows just how much a little gradient can affect consumption as well as how much more economical 5th gear is than 4th etc...