The sender unit on the engine block is a switch to ground.
The power comes from the 12V supply line (Gauge Fuse) which is connected to one side of the dashboard gauge. The other side of the oil pres. gauge connects to the sender unit.
(http://www.cygnusx1.net/supra/library/Wiring/pages/Page_137.html)
When the sender switch is open, the line is pulled up to 12V by the 12V supply. When the sender is closed current flows through the gauge (42ohms remember) and the 12 V is dropped across the gauge terminals and the line is pulled to 0V.
The ECU is a high resistance input (just like a voltmeter) and it senses the 12V square wave on the sender wire. It does not need to supply or sink measurable current.
If the ECU was disconnected from the circuit, the gauge would still read just fine.
The power comes from the 12V supply line (Gauge Fuse) which is connected to one side of the dashboard gauge. The other side of the oil pres. gauge connects to the sender unit.
(http://www.cygnusx1.net/supra/library/Wiring/pages/Page_137.html)
When the sender switch is open, the line is pulled up to 12V by the 12V supply. When the sender is closed current flows through the gauge (42ohms remember) and the 12 V is dropped across the gauge terminals and the line is pulled to 0V.
The ECU is a high resistance input (just like a voltmeter) and it senses the 12V square wave on the sender wire. It does not need to supply or sink measurable current.
If the ECU was disconnected from the circuit, the gauge would still read just fine.
