Missing this wire. Is it important?

mine doesn't have one neither, so if Toyota didn't put it in from the factory it can't be sooo important /

 

That's not even shown in my wiring diagram. I can see you have the two ground points next to each other on the front of the valve cover. Those are in the diagram.

 

I have one of those ground wires. You can easily make one out of 10 gauge wire and some spade terminals.

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Where is the other end connected? (I would look in the wiring diagram, but can find no such wire there.)

 

Just connected to the body.

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Some back story, in a car with a engine belt driven alternator, good grounding of the engine is crucial. The same basic engine block is used in such cars, so Toyota includes the connection point in a Prius even though it has no engine belt driven accessories at all. 12 volt power is provided by a DC to DC converter in the Inverter assembly. I bet the Inverter has ground straps, like most cars have them for the engine.

 

The inverter/converter brackets are bolted directly to the body. Engines generally have rubber mounts allowing them to move and shake, and need ground straps to conduct around those.

The Prius engine still needs good grounding; there are electrical devices on it. On Gen 3 you will see two big ground connections right next to each other on the front of the valve cover. Those are the ones the Gen 3 wiring diagram shows in the engine compartment ground points diagram.

Weirdly, while I don't find this other wire in the wiring diagram, I do find it in the parts catalog. About $20 if you want the official one. It has a whole diagram all to itself.

 

Our 2010 has it. You can just see other end, bolted to body, peeking through between coolant hose and AC line, near corner of coolant expansion tank.