Front VS rear turn signals

Kid stuff! You can find the real diagram here, it's more like 200-ish.

My dad said when I was leery of swimming in deep water: you're going to swim on the top, right?

40 pages, 200 pages ... you turn to the page with the circuit you're looking for. It's not like the other ones get in the way.

Could be some damage to the wiring up front. Any recent knocks to the left fender? There's a junction between wire harnesses behind there.

 

Not bad, I won't even open a "Where's Waldo?" book if it's more than 10 pages.

 

I’ve fixed several vehicles of various makes/models with a rapid flash (and four operational turn signals!) by replacing the bulbs. They can sometimes fail in a way where the filament isn’t broken but the resistance does change.

Try putting bulbs in it. They’re cheap and the odds it’ll fix it are over 50/50.

 

Yes, especially considering the potential difficulty of debugging, I'd start with this.

Back in the olden days, when cars did not have computers, blinkers were "controlled" with a bimetallic spring relay, and a bulb would burn out, the rate of flashing would slow down due to less current flowing through the single remaining bulb (and slower thermal change in the relay). Now that everything is computer controlled, the light controller will typically detect a failed bulb and in homage to yesteryear, indicate that to the driver by making the remaining bulb flash faster.

And these computers can be even smarter than we give them credit for. Such as when they detect that a bulb has failed in ways beyond what you and I measure as a cold filament resistance using our multimeters. So the computer may detect the bulb resistance (etc.) is out of spec, shut down the bulb, and cause the other bulb(s) to flash as a signal to the driver. I have had that exact thing happen on my 2001 BMW.

So I'd try a new bulb or two. Also consider that when one fails due to wearing out, it is likely that its twin is not far behind.

 

We won't know until we know, but my guess is that the computer is doing nothing nearly so clever, and continued dogged focus on why electrons aren't getting from point B to point A will give the shortest time to solution.

Sometimes thinking up amazing things computers could be doing (but probably aren't) is a tempting distraction from the simple focus it takes to solve an old-fashioned problem.

 

You guys just had an experienced mechanic tell you he’s solved this issue on multiple vehicles for several of his customers, on vehicles both foreign and domestic, in 20 minutes with $10 worth of bulbs.

If you want to blow past that advice and listen to guys on the internet who sit scratching their heads and discussing various ways to interpret the FSM? By all means, feel free.

 

I’d certainly start with a package of bulbs, amber wedge base, type 7440A, WY21W, or Toyota 90981-15021.

I think @ChapmanF's point was just that the circuit is simple, without computer controls, and if replacing the bulbs doesn’t solve the problem, traditional techniques of electrical troubleshooting—ideally, using the circuit diagram to save time—will be the fastest way to fix it. Keep in mind that answers here aren’t just for the original poster, but also for later readers with similar problems that might not have the same cause.

For example, the diagram shows that the ground wires (white with black stripe) from the two front turn signal lights are spliced together and then connected, via wire-to-wire connector CA1, to ground point A3. An open circuit along this path is the next most likely cause of a simultaneous failure of both front turn signal lights, if the rear turn signal lights work and all the bulbs are good.

 

And the guy (me) in post #6 even explained why it works this way, in case anybody wants to learn something. Not writing science fiction here.

No guarantee this is the fix for the OP's problem of course, but that explanation of how old cars (that flash slower with bulb failure due to reduced current) and new cars (that flash faster with bulb failure as a controlled signal to the driver) work with the blink rate changing is true, and might help understanding. And the probability of solution and simplicity of the bulb changing first step makes this a good place to start, as the esteemed professional mechanic has shared.

Some cars (like my relatively ancient but ahead of its time back then 2001 BMW) have a computer (light control module, LCM, which does a power-on test of every light in the car each time you turn the key ON) and some modern but simpler cars will have a solid state relay / controller that actively causes the faster flashing in a simpler way. I don't know where the Prius falls in the spectrum. But either way, the fast flashing is from the car being designed to do that as an indication of a problem. Unlike the old cars, where the slow flashing was a consequence of reduced current, the car could be designed to do whatever it wants, and flashing faster is what it does, which makes sense, as a fast flashing light does its job better than a slow flashing light, while also alerting the driver.

One slightly odd feature of the OPs problem is that it happens on both sides, but still I would proceed as stated before searching through circuit diagrams. On any car built since 1995.

And here's another fact that some people may want to learn (or not) - the filament in a bulb behaves very differently when it is cold vs. when it is lit up carrying current. Resistance can change significantly. Some would exaggerate and say that a cold resistance measurement is meaningless other than to see if the filament has burned out completely.

So when you do your resistance measurement with the bulb in your hand with the multimeter, that is not as meaningful a test as the car's lighting control module may do, measuring voltage drop transient response, current flow, or whatever else it may be able to easily do to decide a bulb has failed.

Repeating from above, I have encountered this exact situation - a bulb measuring cold resistance that appears good, car not lighting it up, making its partner blink faster, puzzled, then switching in a new bulb which fixed everything. And I researched, understood, and learned from it. Highly recommended.

 

To be honest, looking back over the original post #1, seems like I skimmed it the first time and thought the OP had checked the front bulbs.

Reading more closely, it sounds like the only "checking" done was to look at them and not see filament breaks or dark spots. Doesn't even sound like any resistance measurement was made, cold or otherwise.

Under those circumstances, I would certainly be on board with trying new bulbs before anything else. Not for any fancy computerese reason, but just because it wasn't tried in the first place.