I am currently testing my fuel system (Toyota, 5VZ) to diagnose a P0171. The Service Manual says this:
- Connect a TOYOTA hand-held tester to the DLC3
- Turn the ignition switch ON and push TOYOTA hand-held tester main switch ON.
- Select the ACTIVE TEST mode on the TOYOTA handheld tester.
- If you have no TOYOTA hand-held tester, connect the positive (+) and negative (-) leads from the battery to the fuel pump connector.
I am curious, why steps 3 or 4 are necessary. The ignition is normally in the ON position when driving, so I had expected power to be applied to the pump, whenever the key was at ON.
It seems like that might only actually be true if you've actually started the engine, first?
UPDATE: After asking my question, I noticed I had overlooked this text elsewhere in the service manual:
CIRCUIT DESCRIPTION In the diagram below, when the engine is cranked, current flows from terminal ST of the ignition switch to the starter relay coil, the starter relay switches on and current flows to coil L1 of the circuit opening relay. Thus the circuit opening relay switches on, power is supplied to the fuel pump and the fuel pump oparates. When the STA signal and NE signal are input to the ECM, Tr is turned ON, current flows to coil L2 of the circuit opening relay, the relay switches on, and the fuel pump operates. While the NE signal is generated (engine running), the ECM keeps Tr ON (circuit opening relay ON) and the fuel pump also keeps operating.
From a circuit diagram, it is explained that the NE signal comes from the Crankshaft Position Sensor.
My question -- borne of curiosity -- still stands, however: why wouldn't the fuel pump always be powered, when the ignition is in the ON position?