4WD braking on its own?!?

[mSteep06T4R]

Had a scary moment on the icy highway this morning...

Turned 4WD on, got on the highway going about 65 and it suddenly slowed itself down about 5 mph (I didn't touch the brake pedal). This freaked me out so I pulled onto the shoulder to turn off 4WD and as I slowed and got to about 10 mph, the brakes slammed ON THEIR OWN stopping me dead in my tracks. Had a burning brake/maybe transmission smell come through the vents.

Flipped it back to 2WD, put it back in drive, and pulled away and it seemed fine the rest of the way.

Help?

 

[Seymore Butts]

Are you sure the brakes activated and it wasn't something in the drivetrain that slowed the vehicle?

 

[mSteep06T4R]

It's very possible. Just assumed brakes, but you might be right. Had a buddy today mention the same thing. Afraid I'm about to sink some serious $$$ into it...

 

[simple]

Have you reengaged 4wd since then? Did you feel any jolts or anything when you shifted back into 2wd?

 

[Doozi]

The slamming brakes when coming to a stop makes me think that it might be a speed sensor on one of the wheels that’s jacking with the ABS. I’ve seen that before on a couple of vehicles.


Sent from my iPad using Tapatalk

 

[DZNUTS]

Given the fact that once you disengaged 4wd, the problem seems to have gone away, I would almost guarantee it’s drivetrain related. I’d take a good look st you’re transfer case


Sent from my iPhone using Tapatalk

 

[catrains]

you say brakes came on .. so you heard the brake pump turn on ??

I have pushed down on the brake pedal gradually to do a stop on the hywy and it did cause the brake pump to turn on because I did push on the pedal to a point where it did this activation...

your vehicle slowed down on hyway 5MPH could be fuel related .

PCM control for some reason reduced fuel injection ..you run this with low fuel from time to time in winter that will do it..

 

[DanielMaat89]

Is DAC turned on? on mine, the switch is located on the center console. I was on a beach in 4Lo and my dog hit the switch and as soon as I let off the gas, it slammed on the brakes until about 5 mph and I turned it off. If it is, there is a green light the looks like a car on a hill.

 

[Gatafly]

Whats the speed limit for it to be in 4wd? Mine is a v8 so not sure, but 65 sounds too fast imo.

 

[uz214]

I wonder if you ever figured this out, MY Lexus GX460 had the exact same thing 3 times in the last 20k miles. Its not reproduce-able so very hard to address it,

Mine has PCS and everything and nothing gets engaged. Nothing is engaged hill asset or anything.
I would be going in 60 and its like the car would brake hard itself and let go in a split second.

I was searching google and found this post.

Had a scary moment on the icy highway this morning...

Turned 4WD on, got on the highway going about 65 and it suddenly slowed itself down about 5 mph (I didn't touch the brake pedal). This freaked me out so I pulled onto the shoulder to turn off 4WD and as I slowed and got to about 10 mph, the brakes slammed ON THEIR OWN stopping me dead in my tracks. Had a burning brake/maybe transmission smell come through the vents.

Flipped it back to 2WD, put it back in drive, and pulled away and it seemed fine the rest of the way.

Help?

 

[Spriter]

So reading the replies here. I have some theories, hopefully they lead you in the right direction.

First, you can be assured that it is NOT DAC. that only functions in 4LO for one thing and doesn't activate if you so much as touch the throttle or brake pedal.

You mentioned hearing the brakes engage harshly when in 4wd on icy terrain, but not in 2wd.

My theory is one of two things.

1. The car detected severe slippage on a wheel, applied brake to stop the slipping, and then something went wrong. My first thought is a partially seizing brake piston/caliper. Basically, the brake engaged as intended, and then got stuck in a "braking" position since its a somewhat known issue with our trucks (seized calipers). Keep in mind the TCS system is always applying tiny bits of braking here and there to keep your ride smooth around corners and maintain max traction, even if you dont hear the max pressure ABS pumping action.

2. A wheel speed sensor is near death (or one of the insanely fragile wires coming from the sensors on the wheel) and it either triggered faulty readings in such a way that made the traction control system panic and apply brakes incorrectly (If this was the case, you would almost certainly have the TCS and other drivetrain lights lit up on your dash)

If this was my car, i would inspect my brakes, the brake pistons especially(these tend to rust and then sieze the whole assembly), and the sensors to weed out any possible hardware issues at the wheels before looking into progressively more expensive fixes (transfer case, differentials, etc)

 

[Bumbo]

Whats the speed limit for it to be in 4wd? Mine is a v8 so not sure, but 65 sounds too fast imo.

You can go as fast as you want in AWD or 4WD... you just need to be under a certain speed when switching modes. It's either 55-60 mph, something along those lines.

 

[jbtvt]

Damn it's annoying when you get a unique problem like this and OP never responds with the fix. Don't leave us hanging UZ. Kinda sounds like a booster issue. Early 4Rs had vacuum 4x4 and a vacuum booster, possible line misrouting combined with a bad booster, driving with no boost and then pushes 4x4 and gets vacuum to booster for the first time in a while maybe? Believe they're different line diameters so not sure how easy that would be to do, but seems possible. I don't see how a wheel speed sensor would cause this, as they will drive with open circuit (disconnected), just triggers the light, and have to believe that Toyota would design a failsafe if there was a short as well.

GX460, and even the Lexus back into the late 90's use a different, electronic booster than these. Haven't had cause to look into it but they don't last forever. Since you say PCS wasn't engaged I'd probably start there as well and see what shakes loose. More vehicle info? Were you braking, even slightly, at the time of the malfunctions, accelerating, cruise control?

 

[uz214]

My Gx is 2015 luxury / preferred package/ driver assist etc basically top of the line. No one in my clublexus forums was much helpful seems like they have not seen the issue.
Brakes pistons everything are ok, I just did a whole bleed, full brake job at 30k everything checked out by Lexus.
I actually called lexus corporate because couple of days ago the suv braked so hard on highway that my wife hit her on the wheel. So now I am seriously thinking about getting rid of the car. They want me to bring the car in for full diagnostic.

I manually turned off the PCS and it happened again, so its not a PCS issue.
PCS there are lights on dash and beeps that flash in my case no lights no beeps nothing, I am also thinking it might be a brake assist or brake booster sort of issue. The thing is it happened like 3 times last 20 k miles.
every time it has happened the road conditions were clear, no rain no snow. Dry sunny beautiful weather. No cruise control, I think my Foot was on the brake but I wasn't braking.

Damn it's annoying when you get a unique problem like this and OP never responds with the fix. Don't leave us hanging UZ. Kinda sounds like a booster issue. Early 4Rs had vacuum 4x4 and a vacuum booster, possible line misrouting combined with a bad booster, driving with no boost and then pushes 4x4 and gets vacuum to booster for the first time in a while maybe? Believe they're different line diameters so not sure how easy that would be to do, but seems possible. I don't see how a wheel speed sensor would cause this, as they will drive with open circuit (disconnected), just triggers the light, and have to believe that Toyota would design a failsafe if there was a short as well.

GX460, and even the Lexus back into the late 90's use a different, electronic booster than these. Haven't had cause to look into it but they don't last forever. Since you say PCS wasn't engaged I'd probably start there as well and see what shakes loose. More vehicle info? Were you braking, even slightly, at the time of the malfunctions, accelerating, cruise control?

 

[jbtvt]

In a vac system any pedal pressure releases vacuum on one side of a diaphram and makes the pedal easier to depress. In an electronic system they might use an on/off switch to mimic vacuum release, I don't know, but they could also use a resistance strip to gauge how much pressure the driver is trying to apply and ramp up pump pressure based on that. In that case, it seems possible that if you were getting too much (or too little depending on setup) voltage based on what the computer expects for that pedal position, that you could get a pump going wild and causing this. I'd also check the plugs for corrosion, esp given how corrosive brake fluid is.

My GS has the same type of system but a brief look in the FSM right now didn't turn up any pinout tests that could be done. I would absolutely let Lexus take a look at things. Those booster/cylinder units are very expensive, might as well get it taken care of under warranty, if that's the case. Trac/ABS as others suggested does make logical sense as well, I just find that harder to envision the mechanics of, personally. A competent tech with better info and wiring diagrams than we have available should be able to figure it out. Do post an update when you get it solved.

 

[uz214]

initial dianostics didn't find anything, they wanted to keep the car for few days.
being that my wife uses it everyday for work we just traded it in. Its unfortunate

In a vac system any pedal pressure releases vacuum on one side of a diaphram and makes the pedal easier to depress. In an electronic system they might use an on/off switch to mimic vacuum release, I don't know, but they could also use a resistance strip to gauge how much pressure the driver is trying to apply and ramp up pump pressure based on that. In that case, it seems possible that if you were getting too much (or too little depending on setup) voltage based on what the computer expects for that pedal position, that you could get a pump going wild and causing this. I'd also check the plugs for corrosion, esp given how corrosive brake fluid is.

My GS has the same type of system but a brief look in the FSM right now didn't turn up any pinout tests that could be done. I would absolutely let Lexus take a look at things. Those booster/cylinder units are very expensive, might as well get it taken care of under warranty, if that's the case. Trac/ABS as others suggested does make logical sense as well, I just find that harder to envision the mechanics of, personally. A competent tech with better info and wiring diagrams than we have available should be able to figure it out. Do post an update when you get it solved.