This is not a request for help, but more a case study. Also, first time posting, so hopefully I get this right.

The important part:

I manage a fleet that includes six 2010 Toyota Prius. Surprisingly, this has happened to two of them within the last 2 months. The problem is that the hazards would not stop flashing and eventually the 12v battery would die. The only way to stop them was to remove the 10A hazard fuse from the under hood fuse box. Of course, you lose the turn signals also, but at least the battery won't die. The solution ends up being to replace the Skid Control Unit (SKU), otherwise known as the ABS module, which is part of the valve body.

I've searched for similar cases, and had discussion with two dealer service writers. One other case can be found on an internet forum, and another on this site. Neither dealers had ever heard of such a problem - figures. That said, it does not appear to be a common problem, or isolated to just my fleet. Unfortunately, I've had to replace two SKU with no idea why they failed, or if others will also.

Attached are both OE & Non-OE diagrams (AllData) for the SKU with the hazard circuit highlighted. Also attached are some image files from the UScope - I just started using it, so not sure how helpful they are.

The story time part:

Prius #1:

Driver reports hazards do not turn, occasionally flash very rapidly, and sometimes faster at higher road speeds. Of course I chalk this up as impossible; the driver must be confused. When we got the car, the driver had removed the hazard fuse. Upon replacing the fuse the hazards flash very rapidly in pulses for a few seconds, then continue to flash as normal. Turning the hazard switch on/off has no affect. Key on/off no change. Did not test drive. No DTCs other than battery voltage low.

A quick check with a DVOM showed the circuit was not shorted to ground. This made for what seemed like a quick and easy diagnosis of a malfunctioning hazard flasher. Part ordered, dash removed to access the flasher, flasher replaced, problem remains, mechanic spun.

I'll pause here to note that the hazard flasher module is ground switched on pin 8 yellow of L55. It is under the dash at the left front. It is an 8 pin component mounted to the right side of a white junction assembly.

So, I consulted AllData, and after way too much effort, I found that the hazard flasher input circuit is connected, for no known good reason, to the SKU (connector A58, pin 10 sky blue). It is also wired into the BCU (connector L7, pin 3 yellow) as part of the keyless entry answer-back function. All these connect internally at junction box 3 on pins 14,38,63, and 4. This is not shown entirely on either OE or non-OE diagrams for the hazard warning lamps, SKU, or BCM:

- The hazard switch is shown on the ABS diagrams; - The flasher is shown on the wireless door lock/antitheft alarm diagrams; - Neither ABS or BCM modules/connections are shown on the hazard lamp diagram; - Junction box 3 is shown on all diagrams though; - The J/B 3 inner circuit diagram shows the connection of pins 14-38-63-4, but without labels or other means of identifying circuits or components.

This is not the exact order in which testing was performed:

•Service literature lists 10 Kohm or higher to ground as the spec for this circuit with the hazard switch off.

•Testing the hazard input circuit with components connected results in a reading that varies between 10 and 14 Kohms to ground. Hazards flash continuously.

•Testing the circuit with the ABS module disconnected results in a steady reading of approximately 34 Kohms to ground. Hazards do not flash.

•Testing the circuit after reconnecting the ABS module results in a reading that varies between 1 and 6 Kohms for 1 to 2 minutes, then 6 to 14 Kohms for several more minutes before eventually remaining steady at 10 -- 14 Kohms to ground. Hazards flash/flutter very rapidly for shorts bursts before flashing continuously at a normal speed. Suspect this may be comms...

•Testing the circuit with all components disconnected (flasher, ABS, and BCM) results in no continuity to ground.

•Testing with only the ABS connected results in 6 Mohm & .45 volts to ground. Hazards do not flash

•Voltage on the circuit with the ABS disconnected is .5 to 160 mV to ground.

•No scope was available at this time.

•Since it appears the hazard input circuit is being grounded we back probe the circuit with a test light connected to B+. This results in the hazards turning off and the test light not lighting. It appears the circuit is commanded/grounded on, but will not carry current for the test light (as it does if the hazard switch is turned on).

• Also worth noting is that attempting to check resistance of the circuit with a DVOM will cause the flashers to operate/flutter very quickly. This will occur with or without the other components connected. This is likely due to the small voltage applied to the circuit from the DVOM interacting with the flasher internal circuitry.

At this point, with no idea why the SKU is wired to the flasher, we disconnected the flasher from the SKU. Subsequent test driving though resulted in intermittent invalid data from the R/R WSS. At this point we caved and replaced the SKU, which solved all the problems. Used an Autel MS906TS to do the activation/reset and bleed.

Prius #2, about 6 weeks after #1

Driver calls and says the battery is dead. Had a tow company jump the car and the driver calls and says the ABS warning lamp came on and the hazards started flashing briefly, but did stop. He calls the next day; the battery is dead again. We have the car towed to Toyota (it's over an hour away from our shop).

Toyota calls and says it needs a battery and a hazard flasher module, also to clean the HV fan. I eventually convinced the service writer to tell the tech to check the SKU. They call back and say it is the SKU. I won't identify the dealer or what they quoted, but it was excessive, so we let them warranty the battery and went to get the car.

Once in our shop we confirm the hazards flash key on, key off, switch on/off, etc.; they don't stop without removing the fuse or unplugging the SKU. Before replacing the SKU I hook the new Uscope to the hazard circuit at the SKU to see what it looks like - not obvious to me what it means (images and video somewhere below).

RR the SKU and problem solved. Check the HV fan - spotless (dealers, right?)

Best guess is some part of the SKU is shorted and somehow interacting with the circuitry in the hazard module causing the module to activate and flash the lamps.

Needless to say, a learning experience...

If anyone can explain why the SKU is wired to the hazard circuit, I'd love to know. Any other comments or advice that may help in the future is also appreciated.

Thanks for reading and I hope this helps others, or at least entertains.

CK

[2010 Toyota Prius, ABS/Inputs/Outputs Document] [2010 Toyota Prius, Video]

[Hazards Flashing, Malfunctioning SKU]

Chris Krause
Manager/Shop Foreman
Pizzo & Associates Ltd
Leland, Illinois, USA

Files Referenced: