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.