1993 Ls: Air Suspension Dtcs (Error Codes) 11, 12, 13, 14 All Together

[LSPaul]

1993 LS with air suspension:

Driving along this morning, and noticed the suspension "HI" light flashing regularly. Got home after morning appointments, got it into the garage and pulled DTCs from the computers. The codes I got were 11, 12, 13, 14, for all four suspension height sensors, plus the usual "73" DTC for the car not running. Now, I could see that just one sensor could be wonky, throwing a code, but all four at once, and out of the blue?

First I thought that perhaps a sensor's wonky, given the weather today (damp/rainy, temp in the low 60s/high teens C), so maybe letting the car down from max suspension height could make sense--so I let the air out of the air struts. Uhhh, no difference, and ultimately not smart. Still have the codes, and now it looks very unintentionally VIP in my garage. :( I've tried clearing the codes from the suspension ECU by pulling the appropriate fuse, as well as disconnecting power at the battery. No difference. I've removed the air suspension ECU, disconnected and reconnected the harness and reinstalled. Nada.

I have bits and pieces of the air suspension section from TIS that a buddy of mine shared a few years back when I bought the car, but am missing the section pertaining to these DTCs (I believe it's pages 149-155 from the shop manual).

Ironically, I just sold my previous #1 car (an '01 Saab with 324k miles) yesterday, but luckily have another backup car.

Any knowledge, inputs and/or the TIS pages I'm looking for would be warmly welcomed.

Thanks,

Paul

[steve2006]

Is the AS pump running?

[LSPaul]

Hi, Steve--

Yes, when pins one and seven were jumpered at the switch block connector on the ECU body (next to the relay for the height sensors), the compressor did operate.

After finding TIS docs online, I've traced my way through the circuit diagrams, troubleshooting flow charts, etc. If it were just one or maybe even two height sensors throwing codes, it'd be straightforward to address. It appears that each sensor has its own harness for signal and power, all leading to the ECU main harness, so it doesn't appear to be a simple power issue. With all four height sensors going "out" at the same time, and no difference being made when I've cleared the ECU codes, even for a second, I'm leaning toward the suspension ECU having failed.

I located a used one at a recycler in my region yesterday for a very reasonable price, and had it shipped-close enough for normal one-day service with the shipping carrier they use. Hopefully it'll be on my door step this afternoon or Monday at latest; I'll update when it's arrived and installed.

Paul

[LSPaul]

Steve:

Update du jour: Finally laid hands on a used ECU (one originally thought to have been ordered had already been sold; roadtripped a few hours each way to buy another one yesterday--that car was junked due to a bad engine, not suspension). I plugged it all in and voila--the same result--the same codes--DTCs 11, 12, 13 and 14 in a row. Grrr.

I've printed off and have studied the schematic for the system--the six pages now spans the width of my kitchen table and I'm now tracing circuits with a DMM. I have followed power from the ECU-B fuse to the trunk harness, then to the #2 relay located on the ECU and verified there's juice. Following that B+ circuit, I've verified that the voltage (9-ish volts measured) is at three out of the four height sensors (pin 1 on all connectors to the height sensors). Still need to pull the RF wheel and check that side.

Am I missing anything here, or am I on the right track?

Paul

[steve2006]

You seem to be heading in the right direction, with all 4 playing up it would suggest you need to look for something that is common to all 4 which would be the ECU which you have now eliminated or a common 12 volt supply or common earth/ground missing.

No one has touched the AS switch in the trunk have they by accident?

[LSPaul]

Steve:

Nope, the A/S switch is in the correct position--that would throw a DTC 71 if it were turned off. The fourth level sensor also had voltage (9v key on/car not running and 14v running). I was able to successfully follow +12v from battery, through relay #2 and ECU, then to each of the four sensors. I'll study the diagram again to try to follow grounds, but on at least two sensors I checked, I was able to measure voltage between pin 1 and the ground pins on two or three of the sensor connectors. The ground lines on all of the sensors all go back to the ECU, near as I can tell.

When the system went into the current state the other day, weather was a bit damp, but not rainy, and it started about a mile and a half from home--the car was fine when I started and pulled out of the drive. The road I was on at that time is pretty ordinary (no major bumps, etc). Checking

I suppose it's not unheard of to have multiple level sensors die at once (someone's old post on CL indicated that it has happened)... It just seems really unlikely to me, that's all. If I can find a set inexpensively enough (I'll be calling the same j/y I visited over the weekend), I'll go that route. Otherwise, it's time to do the conversion to a conventional suspension setup. At least it's not a horribly difficult swap--a couple hours' work, at most.

Paul

[curiousB]

I'm not very familiar with air suspension but I agree it seems unlikely to be all four sensors failing at the same time. I would go buy new sensor set just yet. Maybe one sensor has shorted and is pulling down the reference voltage used to bias the potentiometer in the sensor. Do other codes go away if you unplug one sensor and reset system then fire it up? cable fault? A wire has broken in the harness that is common to all sensors (+VDC bias or ground)?

Is there an air leak. Is air pressure used to raise car or lower it? Maybe an air hose or fitting has failed stopping pressure from getting to shocks. Is the fail safe position for the car full height or minimal height? If you intentionally crack open a fitting to bleed out air does car fall to ground or raise to max height?

[LSPaul]

Good questions, all.

12V (b+) is a common circuit, as is the ground side. Ground leads check out from the rear two sensors to the ECU and body and 12V is common from the relay to all four sensors--and checked out. While I was checking each corner, I sprayed DeOxIt on the contacts (great electronic contact cleaner/conditioner, btw) in the off chance there was moisture, dirt or light corrosion present. No difference there.

The sensors in the A/S cars aren't a simple pot setup, god forbid--they're a sealed optical system (LED/phototransistor/toothed wheel acting as switches, sending inputs to the A/S ECU), unlike those on Gen3-cars that use height sensors to aim the HID lighting--I believe the later cars use pots as sensors. The sensor outputs are independent at each corner of the car.

The car uses an onboard air compressor, a bazillion solenoids and some valves to raise and lower the car. The height sensors tell the 'puter if the car's "level", which then activates the rest of it. Many of the car's electronic systems give inputs to the air suspension ECU. Pretty slick, really.

There definitely was not an air leak here--the car defaults to full-up (with no jounce or damping control) or full-down. The original (flawed) thought was that maybe the sensors are stuck "up", so I let the air out manually. Not the smartest move on my part--my suspension defaulted to "up" and now they're definitely "down".

I'm waiting on a price of a used sensor set from the yard in Gary that I dealt with over the weekend, and will probably call the yard in Rockford that supposedly had an A/S car. I also found a used lower-mile Strutmaster conversion kit on another Lexus forum, which I bought and am waiting to have shipped from Cal. Best case, if the price is right, I swap sensors. Worst case, I convert the car to the conventional setup--either way it's off the road this week. Thank goodness for the back-up car.

Paul

[LSPaul]

Epilogue: I had the occasion to roll through Chicagoland/northern Indiana over the weekend, so stopped at the yard in Gary and pulled all four suspension height sensors from the LS in that particular yard--not a fun removal project with wheels on and in the dirt/near mud, but they all came out. I got home tonight and figured I'd swap one of the sensors and see if it made any difference in the codes I was getting after a system reset. Ummm... no.

Implementing plan "B" is tomorrow evening--swapping in the set of used Strutmasters that showed up on my doorstep yesterday. I'll probably keep the air struts around for a while and eventually offload them. Hopefully the ride will still be decent with the conventional setup... I was so spoiled with the air suspension!

Paul