AHC stuck in high

[greenflash]

Hi-

Looks like a great forum you have here- I was very happy to find it last night! I have a stock '99 LX470 with 105K miles.

I tested it a bit this weekend on a snowy dirt road (in Big Bear Lake, CA), where I had to raise the AHC to "high" and lock the differentials. (The section was only about 100 ft. long but it did great, by the way!)

So, my problem is that the AHC light started flashing "OFF" during/immediately after the ascent. Although the display showed that it's in "normal" mode, when I got out of the car, I found that the suspension is still raised. I drove 130 miles back home with it like this.

I tried lowering and raising the AHC, and turning the engine on and off a few times, but the light is still on and the suspension is still raised.

Thought I'd check here before heading to the dealer (Lexus Carlsbad). Perhaps there is a way to reset the system? Any advice would be appreciated.

Cheers!

greenflash

 

[uHu]

Selfhelp

Welcome to the forum.
I've found it very useful as well.

For your AHC problem, you should check your height sensors, and connectors to same. One for each front wheel and one on the rear axle.
Without any error-codes, I would say it is likely to be either a mechanical or electrical problem with one of the sensors.

If you can't see any problem, then the easiest is to read the error codes with a tester. Otherwise, there is a procedure to check the codes "manually", as well as checking the sensors with a multimeter.

 

[hoser]

I agree, I would first check your AHC height control sensors. Rear first, then the fronts. They sometimes get corroded inside. For each change in position in the sensor, there is a change in resistance. I don't know the exact numbers though. Like Uhu said, the Toyota hand-held scanner would be able to test the sensors easily--to bad I don't have one of those things.

 

[Jim_Chow]

This is what the '03 LX FSM says on how to adjust the AHC sensors to stock w/o Lexus tester: start engine, put AHC switch to "L", then to "N", inspect height.

Some distance definitions:
A=distance from ground to center of front spindle
B=distance from ground to center of ower suspension arm front bolt
C=distance from ground to center of rear axle shaft
D=distance from ground to center of lower control arm front bolt

Front: A-B=82.7mm (3.256 in)
Rear: C-D=71.2mm (2.803 in)

(a) Inspect vehicle height via the above for front/rear
(b) Inspect and adjust the height control sensor to th neutral position.
(1) disconnect the connector of the front & rear height control sensors
(2) connect 3 dry cell batteries of 1.5v to SHB terminal and its negative to SHG terminal and apply approx 4.5v between the two terminals.
(3) Measure the voltage between terminals SHB and SHG. The target voltage (voltage between SHFL, SHFR, SHRR, and SHB) is the value divided by 2.

HINT: (example) Terminal voltage between SHB and SHG: 4.5v, target voltage: 2.25v.

(4) Measure the voltage between SHFL (SHFR, SHRR) and SHG terminals when voltage is applied.

Standard values:
front: target voltage +/- 0.08v
rear: target voltage +/- 0.07v

front connector :
SHFL
(SHFR)
X -----------X SHB

-------X SHG (dashes are just blanks. Should be two terminals on top, one on the bottom, like an upside-down triangle)

rear connector: (three terminals side-by-side)
SHG SHRR SHB
X ------X ------X

(5) Loosen nut and adjust the positions of the height control sensor link and front upper suspension arm or rear lower control arm by moving them up and down to install them.
(6) Tighten nut (torque 49 in-lbs)
(7) front sensor: When adjustment cannot be done by performing step (5), loosen the two nuts of height control sensor link and turn the link.
HINT: To raise the vehicle, turn link clockwise.
To lower vehicle, turn CCW.

(8) Tighten the two nuts (torque 48 in-lbs)
(9) Coat the threads of the link with sealer (P/N 08833-00070, three bond 1324)

 

[greenflash]

Wow, you guys rock! Not sure I have the skills for this, but I'm going to see how far I can get on my own.

Many thanks!

 

[Jim_Chow]

One remark. It wasn't clear in the FSM that the engine be on or off. I presume you turn it on to get it to normal height, when you check the distances (A-B and stuff). But for safety sakes when working under the vehicle, you probably want the engine off when making adjustments to the sensors. If you're using a Toyota/lexus hand tester, you leave the engine on so the tester can work. Life is a lot easier if you have a tester, but in the end, the tester only tells you that something is broken with a lot less searching, as the FSM has flow charts for what to check and in what order to narrow down what is possibly going wrong. I was looking at the throttle position sensor part last night and basically if the TPS is bad, it tells you to replace the entire throttle body unit that contains the sensor!

 

[Jim_Chow]

Just checked the price on the Toyota/Lexus diagnostic tool...$2996, plus one has to download the software from the dealer's network into the memory card!

 

[tabraha]

Jim, I'd be willing to put in $6 towards a tool for the board to use.

 

[Jim_Chow]

Jim, I'd be willing to put in $6 towards a tool for the board to use.

I was looking at the FSM vol 1 last night and there is apparently a way to download DTC codes for the AHC only w/o the Lexus tool. There's a DLC1 connector on the passenger side engine compartment quarter near the PS resevoir whereby if you short out two pins, then turn ignition "on," the AHC off light will first shine for 2 sec, then start blinking. A 1/4 sec on, 1/4 sec off repeating pattern signals all is well. Unfortunately, the FSM only gives a few examples of the waveform for the codes, yet there is a page of possible code numbers for the AHC. These codes indicate where the problem is. There is also an active test. You short the same two DLC1 pins, turn ignition on as before, then push the AHC down rocker 5 or more times in < 5 sec. This puts the AHC into an "active" test mode where if the damping is set to comfort, the AHC on/off switch and up/down will raise/lower the front/rear ends independently from one another. There is yet another test one can perform to check the damping of the shocks (short the same two pins, turn ignition on) and each press of the brake pedal puts the AHC into "mode 1". There are 16 modes total, and each subsequent press of the brake puts it into the next mode. A higher number signals the shocks should be getter firmer (mode 1 the softest, mode 16 the firmest), so you bouce the vehicle to verify this. It also says if you drive (on a dyno?) at 3mph+, the shocks will stay in that mode number. This wasn't that clear to me, as the FSM seemed to imply that it would stay in the mode number regardless. One can also erase AHC & ABS codes in a similar way (can't exactly recall, but it's something like short the two pins, turn ign on, press brake 8 times in < 3 sec). Anyways, I only mention these so people know these methods exist and don't require any fancy test instruments. Maybe the 7 vol. '06 LC FSM has more detail?