2010 Prius Solar Option - Horn No Longer Working...help?

The S-Horn is a dedicated alarm-system horn ("security horn") and I think US-market cars just don't have it, hence the missing relay and fuse.

The regular vanilla hey-watch-where-you're-going horns don't have a separate fuse or relay; they are controlled by the big white integration relay (which controls lots of other things too).

The input to trigger the relay (HNSW) comes through the spiral cable behind the steering wheel, when you push on the airbag.

Those spiral cables are known to wear out (except in a car that's only ever driven straight). If you try pressing and holding the airbag down and turning the steering wheel, and the horn beeps in some wheel positions and not others, that's a smoking gun for a spiral-cable problem.

If you don't get the smoking gun, it could still be the spiral cable, or could be something else. Then you just grab a multimeter and the diagram and figure out what it is, and fix whatever that turns out to be.

 

But does it sound when you push on that spot, regardless of how the wheel is turned?

If so, the spiral cable is probably ok, and you've likely got something simple going on behind the airbag/pad.

If I remember right, the horn contacts are open and exposed behind there. A set of them might be kind of oxidized or dirty, or there might even be something in the way back there, like a wire that happened to fold the wrong way when the pad was last installed.

Should be pretty simple to find, but will mean taking the airbag out. Be sure to bone up on all the safety rules for that.

 

So these cars have no horn pin ? It's all done in clock spring connector?

 

All will likely become self-evident as soon as the airbag is removed for a look-see behind it.

The airbag itself doesn't really know or care that you use it as the horn switch. You're just pushing the whole thing (or one or another corner of it) back into the wheel, and the horn contacts are behind it.

 

Toyota Service Information and Where To Find It | PriusChat

 

The video's not bad. For your own purposes, you're not likely to need any more of it than the part for getting the airbag off.

The 12 volt battery is disconnected as soon as either terminal is. No need to do both. No need to fuss with the orange traction battery plug at all. You're not going to be doing any high-voltage system work here. A quicker way to disconnect the 12 volt battery is just to unplug the fat white cable in the underhood fuse box. (In my car, that would leave some of my added toys still powered that are connected right back at the battery. But it depowers all the stuff that came with the car, which is what matters here.)

The more willing you are to work on those plastic cover panels without metal picks and such, the fewer unsightly gouges and scratches they'll have when you're done.

Once he has the airbag out, he puts it out of the way in the back seat, off camera. That makes this a good place for an open-book safety quiz: whenever you set down an airbag, you set it down which way up?

Once you've done that, you should have no problem seeing what's up with the horn contacts. The horn beeps when that blue wire attached to the airbag gets grounded (to the metal frame of the steering wheel itself, mounted solidly on the steering shaft). The structure that the airbag pops into is normally insulated from the grounded parts, except when you are pushing on it, and you should be able to find the contacts that touch when you do that.



Only for future readers who would want to follow that video further, say to replace the spiral cable:

Mating marks are good for the wheel and the steering shaft. A mating mark on the nut won't necessarily tell you when it's properly retightened; a torque wrench is better. Toyota does not specify Loctite for the nut, as long as it's correctly torqued.

Avoid pulling a lock pin from a new spiral cable before the spiral cable is installed on the properly-centered steering shaft with the wheels straight ahead. If you're transferring over the steering angle sensor from an old cable like the guy in the video, and you have his problem with the sensor tabs not lining up with the spiral cable notches, don't forget the sensor also can be turned. (You don't run into that problem if you buy a Toyota spiral cable, which comes with the sensor already.)