2010 Prius - "STOP" FUSE keeps blowing !

Downstream of the STOP fuse are both the stop light switch and the stop light "relay". If a newly replaced fuse doesn't blow until you press the brake, there's either a problem in the short length of wiring from the switch to the "relay", or a problem in the relay or the circuits downstream of it.

This diagram only shows the downstream brake light circuits. Toyota's diagrams are like that. That junction connector A50/A51 also has a beige wire leaving from A51 pin 3 carrying the OUT signal to the skid control ECU so it knows when the stop lights are on. That's shown in a different diagram ("ABS, VSC, TRAC") but should be included in your testing also.

 

@ChapmanF NICE

Buy more fuses, you've got a dead short somewhere. If the car has ever been in a fender-bender, I'd start inspecting the wiring harness there. Look for any exposed wires (no insulation). Also check for rodent chew marks too. I've seen shorts in lamp sockets too, usually caused by water leaking into the brake lamp assembly.
If you can't find anything, pull those left/right brake lamp relays and test again. If the fuse doesn't blow, issue is downstream of those relays, if it blows, problem is between relay and brake switch. From there, isolate right or left side that's shorting. Install one of the relay and test again. If you pop the fuse, there's a short on that side of the wiring going back to that lamp. Lastly test the other side by moving the relay to the other side or use the other rely to test the relay itself and test the other pathway.

Hope this helps....

Note: looks like that left relay also lights up that middle rear brake lamp, someplace else to check - if the short is on the left relay circuit. Please double check, my eyeballs aren't as sharp as they used to be.

 

It might get a bit bigger (depending on the browser you're using) if you right-click or long-touch it, "open in new tab", and view it there. Sometimes a second click or touch brings it to 1:1 scale.

You can always get it from the source, too: Toyota Service Information and Where To Find It | PriusChat

The stop light "relay" (there is only one, A28 in that diagram) has a black 10-pin connector you just unplug and plug onto the new one. I keep putting it in quotes because it's not a plain relay, it's more electronicky than that, and might be a little pricey. I would look for a problem elsewhere before assuming the relay is the issue.

If you do decide to replace it, it shouldn't be hard in theory, but in practice I've never checked how easy it is to get to. Looks here like it might not be too bad, kind of up on the left kick panel, maybe near the parking brake pedal.



For those diagnostic jobs that start with "buy more fuses", one option is to use something like a Fuse Buddy. You take the fuse out, put it in the Fuse Buddy, then put the Fuse Buddy where the fuse went. The leads from the Fuse Buddy you can run to a meter, or even just a 12V light bulb or buzzer. When you make the fault happen, you just get a light or a buzz (or a voltmeter reading), and don't blow the fuse.

A fuse can blow either because of an overload (somewhat above the fuse rating over a period of time) or a short (some usually intermittent issue where some bared wire can touch a ground and conduct very high current, blowing the fuse very quickly). Sometimes you can tell by looking at the fuse whether it melted peacefully, or splattered its insides with molten metal.

Yours look pretty splattered.

 

I might try to disconnect the noise filter assuming you have disconnected the three brake light assemblies.

Otherwise it is going to be more difficult requiring isolation of wiring sections while hoping you don't temporarily clear the wire to wire or wire to chassis short.




Unfortunately Prius chat downsizes the pics.

Here is a shared link which can be downloaded

 

To be clear, disconnecting the noise filter is just one of many steps to finding the short just like disconnecting each of the three brake light assemblies. After one of the "low lying fruits" does not fix it, then it's time for more advanced wire tracing and isolating techniques.

Careful study of the diagram shows a noise filter which "could" short to ground. It is located under the hatch panel you had to remove to disconnect the center brake light. I would disconnect the noise filter's ground bolt and isolate it from chassis.




Inspection of all of the relevant wiring is the easiest next step, although access is not always straightforward. Following that step, additional physical isolation of the circuit is necessary and requires the paid TIS access to be efficient and further wiring isolation requires more advanced skills.

Finally a word of caution:
Don't even think about bypassing the fuse with a direct battery wire or jumper! This will lead to burning wire harnesses or modules which might change a $300 dealer repair into a $3000 fiasco.

 

Using something like a Fuse Buddy is safe, though, if you just use something like a small 12-volt light bulb between its leads. The light bulb will light up when the short is present; the fuse won't blow and the wires won't burn.

You can set that up and put a brick on the brake pedal—from your story the light will probably light immediately because the short is present. Then you can disconnect different items or sections of the wiring and see when the light goes out. If it goes out while you're just handling a section of wire, there is probably insulation damage shorting to something near where you handled it.

I also suggested a 12-volt buzzer upthread—you could just listen for a buzz instead of watching a light while you move around the car—but I've changed my mind. That fuse also supplies the power to the "relay", which is kind of electronicky, and might not get adequate power through a buzzer. A plain 12-volt incandescent light bulb would pass enough current to power up the "relay"—while remaining dark—and still light up to identify the short. (Hmm, when it lights up, though, it will reduce supply power to the "relay", which could lead to an oscillation; this might not be the great way of proceeding that I was thinking it was at first.)

The noise filter mentioned upthread seems to be placed a little differently in the diagram I attached and the one rj attached. Notice in the one I posted, it is V14 and located between the VR1 and dV1 connectors. In rj's diagram, it's R32 and located upstream of VR1 (also seems to have changed to a combined noise filter for the center stop light and rear defogger). I'm not sure how to account for that; the diagram I posted is from the 2010 US Prius liftback diagram, but I don't know what exact diagram rj posted.

According to the diagrams where it's V14, anyway, looks like you'd find it by popping off the inside hatch trim.

 

The only 'bulbs' in that assembly are for other things (turn signal and backup). As the diagram shows, the stop and tail light are provided by LEDs built into the assembly (tail through an extra resistor to be less bright).

Sounds like that driver side light assembly is kaput (though if it were mine, I would still inspect it for some easily-fixable fault before buying a replacement).

 

Where did you get that idea? The amber 21W bulb is the turn signal, and the clear 21W bulb is the backup light.

The tail and brake light sources are not replaceable bulbs, but light-emitting diodes built into the combination light assembly. Refer back to the diagram, where you see the light-emitting diode symbol:

Those LEDs are built into the combination-light assembly, and not separate parts as far as Toyota's concerned. You'd replace the assembly. (But, as I said earlier, if it were mine I might check whether some easy disassembly could reveal an easily-fixed problem, before I would buy a replacement.)

If you don't believe me that that's how the lights work, I'm out of ways to help.

 

Good advice would be to look in the wiring diagram (Toyota Service Information and Where To Find It | PriusChat) and see if the left and right side use the same connector and with the same pin assignments (but for the turn signal being the left or right one, of course). If you didn't want to look in the diagram, you could probably answer the question by taking both units out and closely inspecting how they are wired.

It could be even easier to test on a bench. Take the driver's side unit out and take it to a bench and test the stop light function (using a fused or current-limited power supply). The wiring diagram excerpts already posted above show you which connector pins are involved.

As I said a couple times, if it were mine I would be tempted to look closely for any simply-repairable problem (accessible without taking the glued assembly apart) ... some visible damage to the external wiring or what not. Looking it over carefully on a well-lighted workbench could be a lot more conducive to that than trying to see while it's on the car.

 

If you unplug the taillight and the problem clears and it returns when it's plugged in, replace the assembly. Most likely you can get a used one locally at a LKQ salvage yard preferably. You buy on ebay means another salvage yard has to ship it.

A local yard will give you some return capability. I would take yours out to have in hand to compare.