VSC is Going Bananas & AHC is a Little Wonky...

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Jan 1, 2011
Breckenridge, CO
Long time 80 owner. I just acquired a free 2000 LX470 from my wife's uncle. I was driving it from Houston back home to CO. I had 3 instances on the highway, mostly going downhill at about 70mph just as my foot was grazing the brake pedal. The VSC light flashed, beeping, severe braking, engine de-throttle and swerved all over the highway. Was terrified the first time it went off and was more prepared and in control on the second and third time. Wife's uncle is the original owner, he's never experienced this....I have a few thoughts:

1. The check engine light was on when I picked it up. PO said it was an o2 sensor, so I didn't think anything of it. Upon bringing it home and checking the codes, I found two. Neither being an o2 sensor. P0715 O/D Direct clutch speed sensor and P0770 shift solenoid E. I cleared codes and have been driving around town at slow speeds and the P0715 came back on...could this sensor cause the dreaded VSC failure?

2. Upon getting home, I also noticed the passenger rear wheel well looked a bit higher. I tested the AHC when I picked up the car in Houston. It seemed to work well...but upon inspection in CO it appears the passenger rear corner did not lower to N. I fooled around with the AHC and it took 10-12 cycles to get that corner to lower from H to N...and it's still about an inch taller than the driver side? Could this suspension imbalance have caused the VSC failure? Do the height sensor's need to be adjusted from time to time?

3. Probably unrelated, but I do have a bit of a drivetrain clunk when accelerating. The harder the acceleration, the louder the clunk.

The wife's current DD is a '96 LX450 with 250k. The LX470 was supposed to be an upgrade, but I'm afraid to let her drive it with the kiddos in the car!

For some vehicle background...this LX has been used as a "farm truck" for the past 16 years. It has 255k miles. Engine and tranny rebuild done last year. This car has never been offroad, but it has been used pretty hard as a farm truck. I know they use Defender's for farm trucks in the UK, but LX470...who knew?!?! Lots of body dings, bumper dings and a bit of a drivetrain clunk.

On a brighter note, i was excited to see 17mpg from Houston to Breckenridge going 80mph with the AC blasting!

one week later and I've had one more instance, going mildly uphill at 50mph around a slight curve. I barely touched the brake and VSC went nuts. On a short offroad test up Peak 9 in Breck, I discovered the CDL light wasn't turning on...Found the CDL switch was unplugged and while under the car I removed the tranny's speed sensor to bench test. It was only pulling 120ohms between terminals not the 500-600 that the FSM dictates. I also removed and cleaned all the ABS wheel speed sensors, they all appeared fine...I re-installed thinking I would order the tranny speed sensor online...now my CEL light is off and it hasn't come back on in several small trips around town?!?! GEEZ! My DD is a '93 chevy 3500 with a 12v Cummins and the wife's DD is a '96 LX450...I've always wanted a newer vehicle, but I have been terrified of all the sensors...this problem seems to justify my fear of newer vehicles!
Any good :idea:'s?
Could be the yaw sensor or one of the wheel speed sensors. Also there is a calibration sequence for the yaw sensor that might fix it.
Steering rack perhaps? I had basically the same happen on my 2000 LX. After 295k miles, the inner rods were gone and rack was leaking. Replaced rack and everything is perfect.
One thing to check with the VSC braking randomly is your wheel bearings. I had the same problem and found the cause was the front wheel bearings needed to be repacked. Lift the front of your truck and see if you can shake the front wheels. Any movement due to loose wheel bearing can cause the speed sensor to read a speed difference between the front wheels. If this happens on a curve or a banking curve the yaw sensor will read also. The truck's computer then sees the yaw sensor reading a tilt and the front wheels reading different speeds and thinks the truck is starting to spin so brakes one of the front wheels to correct. It is very scary when it happens and could catch an inexpereinced driver by surprise.
Just had this issue with VSC. If you have an ODB2 reader w/ techstream you can check if you're getting the same wheel RPM/speed at all 4 corners, as when it engages one of the ABS circuits independently you won't get a stored code.
Mine was loose inner tie rod ends. Had them replaced, no issues since.
How did i diagnose loose tie rods? Jack up front, wiggle wheel. If there is play in the wheel at 3 and 9 (e.g. left to right) check the tie rods. The outers are easy, but the inners are under a boot.

I've seen same thing pop up in forum posts for steering rack (replaced/fixed), steering bushings (replaced/fixed) all the way to what you diagnosed, the ABS wheel sensors.
Thanks for the insight. Here's an update....

I have techstream up and working on a windows tablet. I had a slew of VSC/TRAC trouble codes marked current:
c0278 open or short circuit in ABS solenoid relay circuit
c1223 ABS control system malfunction
c1231 steering angle sensor
c1246 Master cylinder pressure sensor

Jacked up front end and discovered a LOT of play in the wheel bearings. I removed hubs and tightened both sides up to spec. I did not repack bearings, as it was evident they needed to be cleaned and re-greased. That'll be a project to do another day. I just wanted to test the loose wheel bearing theory so I tightened the adjusting and locking nuts, reset codes and took her for a drive. Nothing on first drive test. Second day out on the highway, I had the Slip Indicator light pop on along with a beep (sounded like VSC beep). The light and beeping lasted longer than the previous times, however there was NO flashing VSC light, sudden braking, or engine deceleration...I hooked up the techstream and did not find any ABS/VCS/TRAC trouble codes...

There did not seem to be any play in the steering linkage. The ABS sensors appear to be reading the same speeds while driving.

Since you ha e techstream, might be worth doing the ABS bleed.

It's a simple procedure, outlined in This thread: Caliper line and ABS flush using Techstream

Also possible you have the master cylinder piston leak where fluid bypasses the piston in the master cylinder. A rebuild kit is about $45

I will try the flush and bleed today. The brake fluid and AHC fluid both appear pretty old and dark and need flushing...Would I notice an exterior leak around the Master cylinder? I don't see one...
No exterior leak. Most common symptom of the internal leak is spongy pedal that just keeps going to the floor when held at a stop. Additionally my pedal wasn't returning all of the way back up when I let off of the brakes.
closer inspection....yeah, the outside of the master cylinder is a bit corroded in the center at the end cap...the car was "detailed" before I picked it up, so everything looked shiny and clean....looks like there has been fluid seeping out of the end of the cylinder....

The brakes feel pretty good though. Super firm and responsive. Pedal immediately returns after pushing.
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If you are throwing master cylinder codes, particularly in conjunction with VSC, it could be the ECU in the braking system. I don't know very much about the AHC, but could it be that the functions for the brakes and the AHC are also integrated into the electronic control unit for the braking system?

The braking system consists of several major components including; master cylinder, booster pump, accumulator, and electronic control unit (ECU). It is a tightly integrated system that includes 12 independently controlled solenoid operated valves managed by the ECU that effect assisted braking system (ABS), TRAC, and vehicle stability control (VSC) instantly and simultaneously at each wheel via sensor input to the ABS controller in the ECU from each wheel while the vehicle is at speed. This system operates at around 2000 psi so as to be instantly available at all times the engine is running.

It is complex to say the least. It is not your fathers brake system - IMHO It is a highly sophisticated unit that belongs on the space shuttle, not a four wheel vehicle – it also carries a price tag that NASA would be proud of - best case around $3K - $4K installed. I know. I just put one in my 2000 Hundy and my checkbook still has bite marks on it. DIY would have saved me some, but not enough to take it out of the warranty. Bought the part through LHM in Albuquerque - was $2,400 and deinstalled/reinstall was ~$850. I'm in CA Silicon Valley and we pay a huge zip code tax for just about everything just for living in this once beautiful state. Would have done it myself but as far as brakes are concerned, to me its like do it yourself brain surgery; it may be cheaper, but the risk is high.

Because the brake system is so tightly integrated, matching and installing part components from different manufacturing dates (I.e., different months in the same model year) can be problematic and may actually cause more damage over time if installed. Hence, I bought the entire unit new and replaced everything.

Again, I don't know how the AHC system fits into or is related to the braking system, but if they both run off the same ECU or get sensor data from the same sources, it could be a fairly major problem. It could also be that the master cylinder (MC) system and the AHC run off of separate ECU's in which case the codes you indicate above suggest there could be issues w both.

Apparently the braking problems can start with water getting in through he bleeders at each wheel (I'm guessing it's pretty humid/wet in Houston?), particularly if the neoprene caps have fallen off over time. I'm also told that inactivity can cause issues with the brakes overtime as well. Changing the GVW (the way we all love to do!) can also cause height and vehicle angle/attitude sensor data issues, possibly confusing the ECU('s). Maybe AHC is affected similarly?

Lastly, I was told by Toyota techs in Santa Fe as well as here on The Left Coast that the only safe way to bleed and calibrate the brakes on these trucks is to use the Toyota special service tool (SST) and that complete removal of all old fluid and replacement with spec fluid, while not mandatory is highly recommended. You can DIY, and/or use alternate test systems but again, that could end up creating subsequent failure conditions overtime.

Hang onto your hat... And your checkbook. Sometimes free stuff just costs too much. Ping me if you have questions. Best of luck!

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