I recently created the world's first car assistant which is a low cost, diy car automation computer.
As part of the growing feature set for the assistant, I needed to read signals from the OEM car sensors in order for the system to be a bit more effective. Read on for how I achieved this using just ribbon cable and diodes.
Step 1: Interfacing to the Engine Bay Sensors.
For now I needed sensor values for the Throttle Position Sensor (TPS) and the Engine Coolant Temperature (ECT) sensor. Via the service manual for my car, I identified the relevant wiring at the Engine Control Module (ECM) and made my splices into the OEM harness.
To avoid corrupting the signals to the ECM, I used blocking diodes to send voltage only to the car assistant and have zero backfeed into the ECM.
Step 2: Interfacing to the Instrument Cluster.
I wanted to know when the door/trunk is open and the driver seatbelt is buckled. Knowing the correct wires on the Instrument Cluster harness, I made my splices. I also used blocking diodes here but they allow voltage from the car assistant to the instrument harness wires. When a door/trunk is open, the signal goes to 0V. Likewise with the seatbelt is unbuckled.
Now my Car Assistant knows if I'm buckled in to remind me to do so and also to software interlock to the lights. Also it will voice annunciate when a door or the trunk is open.
Step 3: The Updated Interface!
Now I have digital reads of seat belt buckle, tps, ect and fuel!