Constant audible tone that increases with speed

Problems with the ABS system. This is very serious for safe driving. By turning off the indicator on the dashboard, you did not solve the problem, but only closed your eyes to it. But you still have a sense of hearing, so the hybrid beeps at you - repair the "brake system or don't drive the car"

 

Snipping and cutting only makes things worse!

Get a code reader that works for the Prius and do a proper diagnoises to find out WHY you are
getting the beeping.

Hunting and pecking and hacking will only make things worse....

 

In the case of the "hazards flash at road-speed-proportional rate" known glitch, the problem really is that two adjacent pins at the brake ECU have become shorted together, SP1 and HZRI.



SP1 is an output that gets pulsed to ground at a speed-proportional rate, for use by other things in the car as a signal of road speed. HZRI is meant as a brake ECU input (so the brake ECU knows when you have turned the hazards on, though I've never been sure why it cares), but if you short HZRI to ground it will sho'nuff turn the hazards on, so when that "input" gets shorted to the SP1 output, you end up with your hazards flashing at a rate that gets faster with road speed. (Not just the dash indicator ... the actual hazard lights all around.)

Cutting HZRI at that point doesn't solve the underlying problem, the short between those two pins that has somehow developed inside the brake ECU. But it does solve the annoying part of the problem, having the ding-binged hazards flash all the time at your road speed. The only problem it creates is that now the brake ECU won't know when you have turned the flashers on (as if anybody ever figured out why it would care). Nobody has yet reported any ill effect from the brake ECU not knowing when the hazards are on.

This is not the kind of problem that the brake ECU is going to give you trouble codes about. It is an electrical short that has arisen between two of its circuits that are not supposed to be related to each other, a hardware failure that was never anticipated or covered by any self-test. The ECU is just broken, it doesn't know it's broken, and the fix is to swap in an ECU that isn't broken.

(Therefore, what I would likely do if it happened to me is to just back the HZRI pin out of the connector instead of cutting the wire, so I could just pop it back in later after getting around to replacing the actuator).

To my knowledge, no one has yet examined an actuator with this problem to determine exactly where the short arises. I think @LeftyLucy checked for obvious bridging corrosion between those two adjacent pins, outwardly visible in the connector, and it wasn't there, but no one later opened the housing further to inspect the inside. I'll do it, if anybody sends me an actuator that was taken out for this problem.

If it turns out to be something like corrosion between those two pins inside the housing or on the circuit board, there might be an easy fix. But it might turn out to be a short that develops in silicon, and not really fixable.

The beeping behavior described in this post is new to me. The pulse rate proportional to road speed seems again a dead giveaway for something getting bridged to the SP1 signal. The next question might be: does anything connected to the brake ECU make a beeping noise when grounded?

Might as well start with the obvious guess: the Skid Control Buzzer Assembly connects to the ECU via the brown wire at pin 13 (BZ), and beeps when the ECU grounds that wire—that's what it's for. BZ (pin 13) is not right next to SP1 (pin 11), but it's pretty close—keeping in mind that no one has looked yet to see just how those circuits are routed inside the ECU.

It could be that some progressive failure inside the ECU first shorted SP1 and HZRI (as seems to commonly happen), and may now have also shorted SP1 and BZ.

Pin 12, by the way, is IG2, where the ECU receives power from the IGN fuse. Whatever it is that causes shorting of pins in this area, if it ever ends up shorting SP1 and IG2, either that fuse is going to blow or some more magic smoke will escape the ECU.

Again, this is the kind of electronic hardware failure that happens below the level of any trouble codes the ECU would give you. It's just broken. The real fix is an ECU that isn't broken. But it would be of definite interest to do a post-mortem on one of these and try to determine what exactly goes.

 

I had plans to open up the ECU the next time it generated diagnostic codes (which were always related to speed too low on the right rear as I recall). However, I think I've had only one code in the last two years and it was brief. This is pure speculation, but I've been making an effort to keep the wiper cowling clear of detritus since I cut the wire and this may have helped. That first time, the drain ports of the cowling were clogged, so water may have accumulated and dripped on the ECU housing which shows a lot of evidence of corrosion. Alternatively, the last two winters in the Boston area have been mild and much less salt has been put down on the road.

 

If there's a lot of corrosion on the ECU housing, it may be prudent to place a coat of flex seal or snow roof over it. Just to keep out the moisture. But if that ECU housing is also used as a heat-sink, you run the risk of over heating the circuits within.
Pick your poison...