I have an issue sensing the throttle position sensor voltage if the sensor is already being used by the vehicle's own ECM. Namely, the vehicle ECM stops responding to the throttle input after a period of time from 20 seconds to 5 minutes later.
When I probe with my multimeter (positive lead on the TPS #1 Signal and negative lead on the TPS #1 Return), the voltage at the pedal goes from 0.5V to 4.5V. This doesn't affect the vehicle's ECM at all.
When I use my ATMEGA32u4, connecting the TPS #1 Signal to PF0 and the TPS #1 Return to my AVR's GND network, the vehicle ECM complains about not having a throttle connected.
What should I consider adding between the TPS #1 Signal and the AVR to make it 'disappear' in the circuit and not affect the vehicle ECM? Or is this a function of the internal pull-up resistor?