I am having an intermittent issue with my Gen2 Prius. I unlock the car using key fob and as soon as I press start button everything shuts down. Dash is completely dark. No light, nothing. The car is completely dead, there is no juice coming from 12v. I can not start the car, Can't use my key fob to lock the car or do anything. I'll leave it for a little while and come back and now I can see the 12 v is back to life. I can start the car and use it as usual but my clock, trip meter and radio is reset. It has happened at least 6-7 time with various intervals. 12v goes back to life almost always except one time when I detached the neutral battery and reattached and it worked. At that time I checked the battery and it showed up around 12.37. The 12v battery is almost 7 months old. The issue is that this is an intermittent problem and hard to diagnose. Any ideas, suggestions ?
Where are you measuring, when you measure 12.37, or say "there is no juice coming from the 12V", etc.? Sometimes being specific about that leads rather directly to a solution. I've seen cases, for example, where there'd be 12.37 when the test leads are pressed right on the battery posts, but not when pressed on the cable clamps on the posts. Or still present there, but not somewhere else. You get the idea.
12.37 was on Battery clamps (not the battery post). The clamps were also really tightly connected to the battery post. So no loose clamps.
Classic symptoms of a poor electrical connection in the 12v supply circuit. Assuming you have an internally good 12v battery with good posts. (I have seen 12v batteries with internal failures, but very very rare) The (-) post has one end of a black cable clamped to it, the other end is fastened to the car body by a 10mm hex head bolt. Verify those connections are clean and tight. The (+) post has a fuse block assembly clamped to it. Make sure this post and clamp are clean and securely fastened. Internal to the fuse block assembly is a large fuse (or fusible link, whichever you want to call it) that is fastened to the clamp on one side and the output busbar on the other. If I remember correctly, those are 8mm hex head fasteners. Make sure those connections are tight. The output busbar has a large red cable plugged into it. Make sure this plug is fully seated into the receptacle. Feel free to slide it out, inspect it for physical integrity and re-install it so you hear the "click" when it's fully engaged. The small plug is 12v feedback and should have no input as to whether the 12v supply is present or not. The opposite end of that red cable is fastened to a stud in the upper passenger side corner of the underhood fusebox. This is the 12v supply for the fuse box and has a 10mm hex nut clamping it to the stud. Make sure this nut is tightly secured. This stud is what supplies 12v power to the main fusible link, which is the portion of the fusebox that has the clear plastic window. Behind that clear plastic window, the 12v is then distributed to several circuits using various amp rated links.
Excellent. Just keep moving those test probes until you get from where the volts are to where they aren't. I saw an old Ford once where the battery clamps were tight on the posts, but the clamp and post surfaces were badly oxidized. Conduction between the post and the clamp was happening in one small clean area. That car would seem fine, all light light up and everything, until trying to crank the engine. Then everything would go dark. And stay dead for about five minutes. Then come back to life. The heating caused by starting current through the tiny conducting area caused the clamp to expand until conduction was lost. Then it would take five minutes or so to cool and contract again. It was acting just like an unintended self-resetting circuit breaker. The point of the example is just to observe carefully and stay open to unthought-of possibilities, because weird things do happen.
