Brake warning light on with confusing symptoms

Hi folks,

For a couple weeks the brake warning light has been on in our 2002 with 180k. The brakes are strong as ever, the ABS light isn't on, and there are no DTCs present (except P0420, unrelated). The brake pedal isn't harder to press than usual. There is a definite pulse/shudder when braking, but that's not new and the frequency of the pulse slows with vehicle speed, making me think that is just rough rotors (which I resurfaced last winter when I had the same symptoms). It almost feels like ABS pulsing, but rougher. I pulled an ABS fuse with no change, so I don't think that's involved. If I really stomp on the brakes the pulsing turns into an audible thunking in the front end. It doesn't pull either way when the brakes are applied. I can also hear a very faint cricket chirp sort of sound which disappears when I apply the brakes (I would guess wheel bearings are starting to go).

The day the light turned on I had loaned the car to my mother, and my first thought was that she hit the parking brake accidentally and maybe drove around with it engaged. It was bitter cold that day and I wondered if the switch on the parking brake froze or something, but engaging and releasing repeatedly didn't help and we've had warmer weather since then. So unless shoving a boot in the wrong place could do it, she's off the hook. The switch is inaccessible, otherwise I'd pull the harness and bypass just to be sure.

Since brakes are sort of important, I want to be proactive on this. The manual suggests the culprit may be low brake fluid or a faulty brake booster power supply. Plenty of fluid. The test for the power supply says to check the battery voltage (charged it over night, light stayed on) and to check the pressure at the brakes. I'm not equipped to test the pressure, so I'm wondering if there's anything else I can do.

I had thought I'd check voltage going into the unit, but it'd be a lot of work just to poke my multi tester around. I would test voltage at the fuse, but can't find a brake fuse besides the many dedicated to the ABS.

So I have symptoms of a more mechanical brake issue (pulse relative to speed), but from what I understand the BRAKE light is pretty specific to brake booster issues.

Any input is appreciated. I just don't know if I'm dealing with multiple issues here or what. Is it likely the BRAKE light is an early warning of a developing booster problem? If so, can I expect progressing symptoms or sudden failure? Could a mechanical issue causing the pulsing also illuminate the BRAKE light? I can't figure out how what the computer is sensing that causes it to turn on the light.

Barring any other ideas, I'm going to pull the front wheels and inspect the rotors, calipers, and bearings. Hopefully the weather cooperates this weekend.

Sorry for the long post, thanks for your time.

 

Thanks chap.

We've had the car less than two years, and I've never really paid attention to the accumulator pump so I don't have a baseline. It doesn't sound obviously bad, but it's hard to isolate with the misc gizmos running under the hood.

I disconnected the parking brake and fluid level sensors, and disconnected the battery for half an hour just to be sure. BRAKE light is still on. At least I can rule out those two, assuming an open circuit is okay and closed indicates trouble.

 

Really helpful, thanks both.

The BRAKE light has been on constantly for a few weeks. Aside from using the parking brake, it never came on prior.

For codes I use an OBDII wifi adapter and EngineLink on my phone. It has retrieved DTCs from a variety of systems in the car and displays live data as well, though none of the PIDs are directly related to brakes. I will use the jumper method to see if that reveals anything my gizmo is missing.

You're right Avi, it would make more sense to open the circuit when trouble occurs. Between the shop manual, testing the sensor continuity, and Google I should be able to sort that out for certain. And yes, fluid is at the Max line.

It's a simple test to see if the parking brake switch is open or closed when the brake in engaged. So I'll sort that out too, rather than assume.

 

Okay, confusion continues.

Both the parking brake and fluid level are closed by default open for trouble. Both seem fine.

I jumpered tc to cg (fourth from top left to fifth from left bottom) and rather than the ABS light flashing I got a diagnostic screen on the MFD. I couldn't read the (presumably Japanese) symbols but they all said OK. Same thing when I tried it with the level sensor disconnected. I'll upload the pic when I can, for now it should be visible at Share.Pho.to | This photo set was shared via Share.Pho.to

The only flashing lights were door, airbag, and seatbelt. They were flashing rapidly with no discernible pattern.

 

Fun twist: the ABS light doesn't come on at IG:On with the rest.

And the manual says if the ABS doesn't come on when tc and cg are connected, the ecu may be faulty. However I believe there is a DTC for communications failure with the brake ecu, so that seems unlikely. There are 11 volts between the pins.

I'd love it to just be a burnt out bulb. DI-363 says to check the indicator by IG: on. It isn't clear if that is THE test, but the light certainly doesn't come on.

There's also a test on DI-446 of connecting pin WA of the brake ECU harness to ground. However, the light goes off if the test is okay. So that seems to be unhelpful.

If I just disconnect the brake ECU and the light still doesn't come on, would that be definitive proof the light (or associated wiring) is bad?

Sorry for the rambling as I stumble through! I thought I'd grabbed the whole service manual from tech.info, but I'm missing BE-2 to 11, which seem related to this system.

 

Yup, burnt out ABS bulb.

Now that it's working, the car is much more communicative. ABS stays on while driving, and in diagnostic mode gave these codes:

11 Open circuit in ABS solenoid circuit
46 Malfunction in regulator pressure sensor
63 Malfunction in HV ECU communication circuit
64 Malfunction in hydraulic system

And the door light flashed out:

41 Low or abnormally high battery positive voltage in IG1 circuit

Given the long list, my first stop will be the brake ECU harness. Otherwise all these have diagnostics in the manual so I will just go down the list.

It's a bit daunting to see so many codes (and have no idea how long they've been present -maybe we never had functional ABS-) but I'm back in familiar waters now that I can get the codes.

 

41 is a relief, and I did get that from the brake list. I wondered if there was a separate list, but since the list was labeled something generic and I assumed they wouldn't use duplicate numbers.. I didn't think much of it on my first pass.

Tomorrow will be a car day. If the ECU connections look okay I'll head for the relay. I'd love to get the ABS working again, but especially want to identify anything that might effect the general brake functionality.

Thanks yet again Chap

 

I was really hoping that relay would be bad, but it passed all the tests perfectly including connection to the brake ECU. And the brake actuator in between had the appropriate 30 Ohms of resistance.

The manual says 'if it passes all the tests, put the burnt-out bulb back in and pretend you never knew'. (I was surprised too). Or the brake ECU may be defective. Although a bad computer is a convenient scapegoat, they don't go bad very often do they?

The diagnostics for 46 and 64 involve pressure testing equipment I don't have, so unless the readings were way out of range I wouldn't know. I can check the various connections, but that's about it.

63 is probably the one that lit up the BRAKE light and started me down this rabbit hole. I haven't met my HV ECU yet, but checking the connections between the two wouldn't be too hard. This seems like a reasonable next step, since I'm equipped to do it. Passing that test would also suggest ECU problems.

DI-444 has a brake ECU test, but it hinges on having no trouble codes. The only step that seems relevant is (6) confirming the ABS light goes out when terminal WA of B11 is connected to ground. And since the ECU believes there are faults, I can't blame it for indicating that.

When I was poking around in there, I noticed that terminal PRR (B13-13) was a bare wire, which seemed odd. It tested fine though (less than 1v to ground w/IG: on)

 

Yup, pin 3 to pin 6 of B13 with a few feet of wire to complete the loop.

You're right, I can squeeze some more info out this. Thanks for the roadmap, I'll get back in there with my multi tester. Since I know the relay works, I just have to figure out what else isn't doing its job.

 

Believe it or not, I'm still working on this. I've confirmed the ABS isn't working, the brakes have locked up a couple times on very hard stops. I resurfaced the rotors (seems like they need it every spring) and all the shuddering and noise is gone when braking.

The car is losing oil and has P0420 (cat converter - probably fouled O2 sensors from burning oil). I'm focused on finding out where the oil is going. I found a puddle in the intake manifold, but everybody says that's normal for a Prius.

It also had a bout of 'big hand syndrome' recently. I think I cleared that up (all hail bwilson).

But the car did get us and our stuff safely to our new home in Minneapolis, Minnesota!