ECU Signal Input and Output

Hello everyone,

Going through the fundamentals course and I love it!

Currently in the "EFI Components" section

I have a general question about voltage though...actually the completion of a circuit.

Specific to an ECU; So say a signal from a device goes to the ECU (i.e. sync sensor, TPS, etc...) Does the ECU "switch" the signal to ground to complete the circuit?

Better stated, what happens when a signal is rerouted to ground? I always through current flows out through positive and returns through negative to complete a circuit? But I'm starting to get the feeling that the ECU only acts upon signals that it switches to ground.

Can someone offer up some clarification of this process?

Most ECU's just measure the voltage from the sensor signal (relative to the sensor ground). The sensor itself it what uses the current by internally connecting the Supply Voltage and Ground.

When a signal is grounded, the ECU will just measure 0V. If it's a digital input with the signal pulled high with a pullup resistor (either in the ECU, or in the sensor), then the ECU might think this means "on" or "active". If it's an analog input, the ECU will just use the sensor calibration to translate that voltage into some useful units (like a pressure sensor, might read -12psi or 0 psi for a pressure sensor).