This document is designed around connecting to a 2005 Volkswagen Passat TDi (B5.5) but with slight modification should work for most OBD2 compliant cars.
The EML chipsets can be controlled using AT commands. These set configuration options on the OBD2 connector, such as which protocol to use communicating with the vehicle.
I will be using a generic bluetooth module with an ELM327 v2.1 chipset, and an ELM323 v2.0 USB cable.
- [todo]
Connect to the OBD2 adapter from a serial terminal app and start issuing the following commands.
AT Z
This command resets the ELM327 interface so we are starting from a known good point. Once reset the interface should respond with ELM327 v2.1.
ELM327 Only: If this is the first time you have connected the interface to the vehicle you may want to set the protocol (for the Passat this is ISO-14230-4 KWP):
AT TP 5
Before we start issuing commands to the ECU of the vehicle we can check that we are talking to the ELM327 correctly by asking it check the voltage of the vehicle battery by sending the following command:
AT RV
This will respond with a value such as 12.6V. When the engine is not running a value between 12v and 13v is normal, whereas when the engine is running this is typically above 13v and maybe as high as 15v. Because of the voltage difference between a running and not running engine, we can use this check in any code we write to see if the engine is running or not.
To check if we can communicate with the vehicle we will send an OBD2 command to get the engine RPM.
01 0C
If the command is successful it should return a value similar to:
41 0C 1A F8
The first two bytes 41 0C represents this is a response to our 01 0C command, and the following 2 bytes are the RPM value 1A F8 which is 6904 in decimal. OBD2 Engine RPM values are sent as 1/4rpm intervals, so we divide 6904 by 4 to get 1736.
Request:
01 0D
Response:
41 0D 37
Vehicle speeds are returned in hexadecimal format and represent the speed in km/h. So in this case 0x37 represents 55km/h.
There is no way to get the currently selected gear directly from the ODB2 interface, however by using the ratio of engine RPM to vehicle speed we can determine which gear we are in.
Example: I know that in my Volkswagen Golf at 110km/h (68mph) I will be doing 2000rpm in 6th gear. So if we take 2000 divided by 110 we get a ratio of 18.181818182. So now if we read the engine rpm and engine speed over OBD2 and the ratio is around 18, we can infer that we are in 6th gear.
The ELM323 chipset has a set baud rate of 9600, so it could be possible to max out the connection if we are making a lot of requests. The ELM323 supports a number of settings that can tweak the efficiency that data is passed over the line.
By default any commands sent to the ELM chipsets repeated back to the host computer. To save bandwidth, we can turn this off by issuing the command:
AT E0
Which ELM will be compatible with Suzuki Cars. 327 or 323?