AM2 fuse keeps blowing, unable to find cause

One trick that helps, for pinning down an issue like that without going through a heap of spare fuses, is to plug in something like a "fuse buddy" where the fuse goes, and wire it through a 12 volt light bulb.



That way, the short (wherever it is) doesn't keep blowing your fuses, it just lights up the bulb. Then you work your way through the wiring diagram (more info) to see everything that branches off downstream of that fuse, and disconnect one branch at a time until the light goes out. Then reconnect that branch and repeat with the specific end components on that branch. Before long, you've found the culprit.

 

Nice... Gonna have to research fuse buddy more. Got any tips for best one to buy that's also the best price?

 

Well, you can see it comes in two flavors, one with its own built-in ammeter, one with ordinary test leads you can plug into your own DVM (or connect to a headlight, or something, if you're using it to trace a short). So the plain-leads flavor is less money, of course, and also more versatile.

There's no rocket science to it, it's got a pair of blades the right shape to fit in a fuse socket, and two wires. It also has its own fuse socket on the side. so you can put in the fuse that came out of the socket, and the circuit is still protected while you're testing.

The plastic case has squarish shoulders that prevent it from fitting in some locations in Toyota's fuseboxes. I took mine apart to see the shape of the actual metal bits inside, then cut down the corners of the plastic case to match, and put it back together with Plasti Dip for insulation there, so now it fits in more of the fusebox locations, anyway.

 

The AM2 fuse is a difficult one since it is essentially a switched ignition power source that feeds many systems.


If you have a hard fail, meaning a permanent short, a test light check looking for a ground or special tool like the fuse buddy gets you a test instrument. Then you have to start disconnecting wires usually at the connectors.

Often techs might pick one circuit and work backwards.

The real problem is the multiple circuits involved and the fact that Toyota schematics sometimes show black boxes such as the power source control ecu and assume you have been trained to know how and when the outputs are connected. Often one system is illustrated on a drawing, yet there are many other systems being fed by that fuse. For example the fuel injectors are powered by the fuse but you would not know that from this drawing.



So you look for another drawing



To add confusion, a fuse like this feeds different paths depending on your trim and options. Sometimes Toyota will show both circuits on the same drawing using * to differentiate.

The online paid site has some advantages here since it will highlight the circuit better and link to explanations.

https://techinfo.toyota.com/techInfoPortal/appmanager/t3/ti?_nfpb=true&_pageLabel=ti_home_search_docs_page

 

Here is a similar issue. The root cause and solution is pretty common even though the car is not the same.

 

I doubt it has anything to do with the manual parking brake. Perhaps the Park position shift actuator was not working meaning the car could roll when off instead of being in Park. But that could just be a side effect of 12v switched power not properly operating hybrid controls or the Park mechanism itself.

It would be interesting to know if the fuse continues to blow or will it take an intermittent short or device failure to overload it?

The last post from March was pointing at the dash again. Was a problem found there?

One troubleshooting fallacy is to assume something you already checked can't be the problem. The dash and inverter pump come to mind.

There are auto electric specialists who have experience in these kinds of difficult shorts. Isolation of downstream parts is often the key strategy. Having an excellent wiring document for your vehicle year and trim is essential. Sometimes they have to de-pin connectors, insert probes through wire insulation, bypass power to certain devices through a different path, it gets tricky. Moving wiring and connectors may temporarily resolve the issue making a permanent solution difficult. Even dealers are not guaranteed solutions in cases with something intermittent. They and others may resort to wholesale part and harness changes.

So if diy is still the plan, I would revisit the assumptions and verify if the short is continuous.

 

I might wonder if the warning message is a result of the AM2 circuit voltage going low as the high draw is overloading the fuse. I would scan all ecu's for codes.

One way to isolate a fault is to add smaller fuse in part of the circuit. In this case, you can put a fuse holder in the ground side of the inverter coolant pump with a 7.5A fuse. If the pump has a short, it will pop the small fuse, not the AM2 fuse.

Posted via the PriusChat mobile app.

 

Wanted to add to the knowledge base on how our “AM2 fuse” fix went. TLDR: swapped out inverter cooling liquid fluid pump for DIY successful repair. Part number for 2008 Prius: OE 04000-32528/G9020-47031. After the vehicle unexpectedly died on the train tracks! it was towed home. It wouldn’t start and showed the key ignition icon on the dash only. After checking the AM2 fuse we disconnected the connector on the inverter cooling water pump and tried running the car. Since it finally ran without burning out the fuse, then we figured this was indeed the problem. (Note: run the car without the pump for testing purposes only (for a very short time)). Replaced the pump and verified it was working by seeing the water move in the coolant reservoir. The car has 250k miles on it.

 

Hi all, I've done quite a bit of researching on this issue to no avail. I has the inverter pump go bad, which blew the AM2 fuse. Replaced the AM2 fuse, and the dash lit up like it should, all the buzzing and whirling going on in under the hood. I looked and it appeared low on coolant assuming some was lost during the pump change. I ran to the local auto part store 5 min away leaving the car as it was (on but not running). When I got back everything was dead exactly like it was when the AM2 fuse was blown. However, this time The fuse was not blown. I checked every fuse under the hood and under the drivers console.

From what I've read, apparently the IG2 (?) relay could be bad. Could that have died during this swapout ordeal? Where is the IG relay located? Could it be the fuse block that holds the 4 main fuses?

There is 12v on both sides of the AM2 fuse but nothing is coming on at all except the door ajar light on the dash. The 12v battery is 12.6v and this is my daily driver that died 2 days ago.

 

I would be incredibly grateful for any suggestions as this is my daily driver and needs to be towed if it's not a hidden fuse or the IG relay. Does anybody know if the IG relay is inside the AM2 fuse holder?

 

The IG2 Relay is supplied by AM2 and feeds a lot of things, making your challenge more advanced. But it is located in Unit B module of the main fuse box.

Relays by themselves rarely blow fuses. Typically it’s a load or a wiring short to ground. It could be intermittent based on something occasionally touching ground or it could be another fuse downstream.

Knowing all the loads is key to isolating the issue. I have included a few screenshots from the wiring diagrams - which can be difficult to follow by themselves.


This is a case where an experienced tech with online TIS / Techstream and proper probes might be necessary.

A quick look at some of the loads based on what I could trace on a small phone.

 

There was a very recent thread created similar to yours. The solution to his problem was a blown 7.5 amp fuse in the fuse box under the hood. Read this thread:https://priuschat.com/index.php?posts/3481813

Recheck all the fuses.

 

How about the start button, or as it used to be called, ignition switch? I went through the am2 blown fuse nightmare once. The lesser fuses after am2 were all ok. One of the culprits is ignition switch. It can blow the am2, but it can also fail itself I think.

 

How would I go about testing it? Is there a wire that commonly burns up connecting the ignition switch? I know anything is a possiblility, but the car was on and working after I replaced the inverter pump. I left for 20 min and came back to it DOA. Is there a way to bypass it to verify that as the issue?

Can you verify the ignition relay is inside the larger block that holds the AM2 fuse? img attached

 

There's an IG1 relay in the under-dash fuse box, and an IG2 relay in Unit B as shown. (For reasons known only to Toyota, the module clearly called Unit B in the diagrams has a big letter C on it in the actual car, and the one called C in the diagrams has a big letter B on it. The ones I've seen, anyway. Maybe in certain years the letters matched.)

Hard for me to think of a way the Power Sw goes wrong and blows AM2. Its only connections are to the Power Source Control ECU and ground. It's effectively a little game-controller input to that ECU.

You may want to refer to a complete wiring diagram to get ideas about what could be going on.

Toyota Service Information and Where To Find It | PriusChat

 

Check the ignition coil black for 12v

 

TY. I agree and I'm extremely confident it's not the ignition switch, I'm going to evaluate using whatever methods and suggestions you prius guru suggest. Where would you begin knowing the current situation?