AM2 Fuse Blowing, NOT Inverter Coolant Pump, Prius Died While Driving, Won't Start

Have you just unplugged the inverter pump left it unplugged replace the AM2 fuse and boot the car up and the fuse still blows? Is this correct

 

Yes. Sounds like that is correct.

 

So something in that circuit is running straight to ground or something very similar for it to be blowing like that I mean if you put a fuse in and you watch it while somebody pushes the start button and it blows the second you push the start button or the power button whatever there is something running straight to ground. Best thing I can say is you got to think what the last thing was that might have happened . Something was taken apart the dash was electrically worked on something plug got wire got squashed reassembling something.

 

When the going gets tough, the tough get wiring diagrams. Toyota Service Information and Where To Find It | PriusChat

The heated oxygen sensors, it turns out, are not under the AM2 fuse. They are under the EFI fuse.

Besides the inverter cooling pump, the fuel injectors, spark igniters, and everything under the 7.5 amp IGN fuse, come through AM2.

Does AM2 blow as soon as you install it, or not until you make the car IG-ON?

Does it blow if you have unplugged all four injectors, all four igniters, and the IGN fuse under the dash?

 

Hook a 12 volt headlight in place of the AM2 fuse. It will go bright and not damage anything. Start disconnecting all devices on that buss one by one till the light goes out.

 

That's important information, because the injectors, igniters, and stuff on the IGN fuse are all unconnected until the IG2 relay is on, which only happens when the car is IG-ON.

So I normally would not suspect any of those circuits, if the fuse is blowing immediately.

On the other hand, the Prius power source control ECU can behave a little surprisingly: it may remember it was IG-ON from before (even after disconnecting and reconnecting the battery!), so that can complicate an observation like this. Are there any other signs of the car being IG-ON? Any lights on the dash, will the turn signals flash, etc?

 

The hazard button works any time, so that doesn't show anything. I think the door ajar light and security light also don't show much. Not sure what to make of the beeping.

I think if the turn signals or wipers aren't active. the power source control ECU must not think it is IG-ON.

 

If its blowing immediately you have a dead short to ground which makes me a little suspicious its not a device but something stupid like a wire chopped and touching ground or something 180 out of phase connected.

Just ohm out the load side of that fuse with the fuse out will probably show zero ohms and then start disconnecting whats hanging off that AM2 buss till the zero impedance lifts.

 

That would only be guaranteed to not damage something if the headlight also restricted current to less than or equal to the value of the AM2 fuse. That may be the case here, since I think the AM2 is a 30A fuse, and headlights usually only draw 5A or so. But people should always check these values for the circuit in question before trying a technique like that.

 

AM2 = 15 amp

 

Still safe then, assuming the 5A value for the light is roughly correct.

 

Very good work.

Yes, I had noticed that myself in some earlier post. Weirdly, those physical units are marked with letter B and C on the top, and the wiring diagram does show them as "Unit C" and "Unit B" respectively.

Look for the page in the wiring diagram that shows the Unit B "inner circuit". You'll see that 3I pin 1 and pin 4 are the ones you care about: they are the terminals supplying power from the AM2 fuse. Pin 1 supplies power at all times, and pin 4 only when the IG2 relay closes.

Pins 3I-2 and 3I-3 are just used to control when that relay closes. Pin 3 should measure as connected straight to ground, and pin 2 gets 12 volts from the power source control ECU when it wants to turn the car on.

As far as I can see, nothing hangs off pin 1 (the always-powered one) except the inverter water pump wiring and the power source control ECU itself. (It stands to reason that ECU needs power for itself even when it isn't turning the relays for the rest of the car on.)

So you can probably see with your meter or test light whether 3I-1 or 3I-4 shows as shorted to ground.

Because the fuse has been blowing immediately, my first guess would be 3I-1. With a short downstream of 3I-4, you would not expect the fuse to blow until the IG2 relay is on.

But it would be worth checking anyway. You could see the immediate blowing anyway, if IG2 is stuck on, or if it is getting 12 V supplied on 3I-2. You could check for a stuck-on relay easily enough, with an ohmmeter on the removed Unit B between the AM2 test points and 3I-4.