Shiv Sikand

Guys,

Car: CL65, 60K, 2005
ABC: replaced all hydraulic components
1) Every hydraulic hose
2) Pump
3) Both valve blocks, all accumulators
4) Front struts
5) Rear struts were replaced 20K miles ago

Car drives very well, no sags, excellent handling, etc.

But, under hard acceleration from rest, get a red ABC fault. Driving the car hard on twisty's and fast highways, mountain roads, etc NO fault.
Can't reproduce problem in reverse.
Can also reproduce by holding the car with the brakes in D. Hit 1.9K revs and DING fault triggered. Easier to trigger fault on hard left turn at speed. Hard right turn, no fault. Hard left or hard right in reverse, no fault.
Very predictable fault. Never fires intermittently, only under specific conditions described above.

STAR reports all sensors have failed. Switching off and on resets the problem.

DAS states that if multiple sensor faults are reported, then it's a short in the harness or in the sensor.

Replaced all the sensors. 4 Level sensors, 3 acceleration sensors, lateral and long sensors, everything on the fault list except the pressure sensor. Swapped out controller. All tests and diagnostics pass.

All connectors look good.

When the fault is triggered, it looks like a CAN bus short to ground. Every sensor goes to zero, pressure sensor goes to max.

Any ideas ?

Geraldius

On a CAN bus a short will shut down all communications over the bus. It could be an open on a branch of the bus. You should be able to measure a resistance between the two bus wires, at any node on the bus, of about 60ohms. Significantly different readings indicate a problem. In that case look for a loose or corroded connection at a junction that is common to the components giving errors.

Shiv Sikand

An open on a branch is plausible, but why does the open go away on reset ?

Geraldius

A CAN controller will typically shut off communication to an unresponsive device on the bus after some number of failed retries. So even if the malfunction ceases the device might not be picked back up by the controller until commanded to do so by software, or by a reset of the hardware.

My guess is that flexing in the wire harness at a connection is momentarily causing an open, or high enough resistance to let the bus become electrically noisy.

Holding the brakes while in drive while revving the engine, or hard acceleration, will both cause the engine to torque in one direction. Reversing will cause it to torque in the opposite direction.

Bad motor mount on one side? Have a helper watch the engine for abnormal movement while doing your power braking test. (don't run anyone over)

Shiv Sikand

If I watch the actual values very closely on DAS, the moment there is a large forward physical impulse, I see the actual sensor voltages for the components on the list go to zero virtually instantly and the fault light comes on.

I have thought about your hypothesis previously, but I don't know how to isolate it.

A loose or a corroded connector is a valuable thought but there are no obvious connectors beyond the sensors themselves and the harness for these connectors is all 9ver the car. The lat and long sensors are in the footwells for example, the rear acc sensor is under the parcel shelf etc.

principledchiro

Go over each major ABC harness plug. Unplug each harness, inspect for corrosion, etc.

knowbenz

Are there any CAN codes stored in any other module on the engine CAN? If engine CAN shuts down you will have communication faults stored somewhere

knowbenz

Like the above guy said check for corrosion. Then what I would do is get a multimeter and back probe the power and ground to the control module. See if you are briefly loosing voltage when the fault occurs.

Shiv Sikand

No other modules are reporting a fault. Just ABC.

knowbenz

Try monitoring all power and ground circuits to the module

hvacdude

There is a inerta sensor on these cars. maybe it has gone awry.

Welwynnick

Do you have to be Drive to see the fault, or can you reproduce it in neutral?

I'm wondering if it's speed related or load related.

Nick

driveability

Having worked in the Tune-up & driveability department for G.M. for a long time, I would get cars like this where doing what you described would get the car to act up I would hold the scanner under the hood and grab the engine wiring harness and move it around hoping to see a reaction. I would go that
way trying to isolate areas that might react to engine movement. I wish you good luck !!!!

Shiv Sikand

Hi Folks,

Thank you to everybody for their responses so far.

I have now analyzed the circuit diagrams and the pins. I have attached the image for your review. The problem is not CAN related, because these are all analog 3 wire sensors.

My next step now is to physically remove the power pin for each sensor, sequentially, from the control unit connector (power and ground are common for all sensors, but each has a separate pin thank heavens - except for the left and right front body acceleration sensors, and the longitudinal and lateral acc sensors which are shared via z splices) and then stimulate the fault. This will allow me to isolate the wire segment that is causing the problem.

Cheers,
Shiv

Shiv Sikand

Have to be in Drive.

biker349

good luck.

Shiv Sikand

ABC problem is black/yellow power circuit wire of right front level sensor is being crushed between the air filter housing and the right front strut tower. There is a sharp surface on the tower that cut through the wire.

Welwynnick

Well done finding that!

Nick

Shiv Sikand

Thanks Nick, but I can't claim credit. Shout out to Sean M for his detective work.

knowbenz

Nice find, did you snap a pic of where it was pinched? I'm trying to picture it in my head but having trouble

Shiv Sikand

I will post next week, sorry been on the road.

Shiv Sikand

Shiv Sikand

The bulging section is where we added a lot of rubber to prevent it happening again. Also, the engine mounts were replaced, so the air box is a lot higher in this pic than it used to be, so perhaps that was the root cause of the rubbing.

Geraldius called it !

mohsin52

had this same issue with my s55 and it was bugging me out. Couldn’t find the source, until I stumbled upon this thread. I had the same exact codes you did. Something seemed very off because it was only giving sensor codes. Turns out my wiring harness on the FR passenger side strut mount was cut into by my custom intake system. Luckily the wires weren’t in bad shape, other plastic sheath was torn but wire intact. Put electrical tape on it and it worked like a charm! Pm me would love to send you something as appreciation! Lol

-
Moe

Shiv Sikand

You are welcome. Please feed a hungry person or family in appreciation. Hunger is all around us.