Diagnostics
A while back my LPG system started playing up. Strangely enough while running on LPG the car would stall intermittently. To make matters worse, when it did stall I was unable to switch back over to petrol! This meant I had to pull over somehow, pull the ECU’s fuse to reset the ECU so that it would start up in petrol mode.
To start diagnosing things I figured I would need a serial cable to talk to the ECU. I looked out there and the cables ranged from USD$80 to over USD$200!
I thought to myself, after the Brewie UART sniffing project it wouldn’t be difficult to wire up a quick Serial interface to the laptop. This time however instead of logging I needed it to work with the OMVL Diagnostic Software.
The Interface
Simply put the OMVL has a 8 pin female Molex connector with only 4 pins fitted. With a multimeter I quickly figured out the following pin assignments:
Note: I couldnt work out just with a multimeter which was TX or RX. I just swapped them around until I got it working. They work at 5V levels.
I also immediately recognised that the molex connector looked a LOT like part of the plug on an ATX PSU connector. Since I collect these PSUs for spart parts like fans, mosfets, heatsinks and hookup wire, I identified the part of the ATX connector that fit the OMVL serial interface plug.
Below is the part you will need:
Basically cut the connector so that only the part in red remains. It will fit the OMVL serial interface perfectly.
However if you don’t have spare PSUs or don’t want to sacrifice one, you can buy this portion of the cable and butcher that. Just cut it at the red X.
You can buy one from here at no extra cost to you but it benefits me:
FTDI to USB Adapter
I bought a cheap FTDI to USB adapter to form the interface between the ECU and the laptop’s USB port.
You can buy one from here at no extra cost to you but it benefits me:
Connections
The FT232RL had the following pinout:
Basically follow the connection as shown below:
| OMVL Connector | FTDI Adapter |
|---|---|
| RX | TX |
| GND | GND |
| 12V | DO NOT CONNECT |
| TX | RX |
Testing
Few points:
- You may need the FTDI drivers for windows.
- You may need to swap RX and TX if it does not work at first
- Make sure you set the FT232RL jumper setting to 5V (NOT 3.3V)
The setup above successfully connected and I was able to diagnose my LPG ECU.
Hope this article was useful to you!