testing ABS module

j-bone

Well-Known Member
I'm testing a suspected bad abs module , the abs is kicking on at low speeds for no reason and it is killing engine power at low speeds as well .
Data logged the ABS module yesterday and found some interesting information.
so in park with engine running all 4 wheel sensors read 2mph
after driving around finally got the abs turning on on data log , front 2 wheels sensors drop to 2 mph rears still read 3 and its dumping left front wheel valve .
there was a stored but not active code for steering angle sensor as well .
is this enough info to consider the module bad ? are there any other tests to perform ?
i can send a copy of the data log if anyone would like to check it out as well.
 

MRRSM

Lifetime VIP Supporter
The ABS Wheel Sensors use Fixed, Fine Wire Induction Coils that generate an Alternating Current vs. the movement of the 'Fine Teeth' of the Reluctor Rings mounted to the rotating portion of the Wheel Hubs. If the Body of the SUV was vibrating enough to shuttle back and forth while the vehicle was in Park... (...think... Bad Motor Mounts here...) THAT might be just enough motion for it to move the Tip of the Fine Wire Induction Coil back and forth vs. a Single Tooth of the Reluctor Ring inside of the Wheel Hubs and trigger a reaction from the PCM by inducing just enough AC flow to create the False MPH reading. If the Wheel Hub Bearings are badly worn and allow for this "play"...that might play another part in allowing your parked vehicle to behave like a Gentleman Tourist... ambling slowly along on foot past the Savoy Theater in London, England. :>)

The problems normally encountered with the ABS come in two parts:

(1) The individual ABS Fine Wire Induction Coil Sensors can get "pushed away" from the Reluctor Rings inside the Wheel Hubs due to the fact that when Oxygen and Iron and Steel combine to form FE2O3 (Ferrous Oxide) the MASS of the two expands as blobs of Rusty Metal ...and that expansion causes the ABS Sensors to Malfunction as the Tip of the Fine Wire Induction Coil in the Wheels get moved further away from the Teeth of the Reluctor Rings. The Fix for the problem is well illustrated in this Brilliant Video:


(2) The Modern ABS System is not just an Electronic System... it is married well and truly to the reality of Hydraulic Brake Fluid moving through the Brake Lines and is just as dependent upon having No Air or Water inside of the Brake Lines and Brake Cylinders. Have a look at the 4th Video down inside of Post #9 at this Thread. You will need a “GYMKO” Tech 2 or a similarly capable Bi-Directional High End Scanner to perform the actions herein described involving a Dealership Quality Brake Bleed and ABS Purge:

 
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j-bone

Well-Known Member
I fully understand how the abs system operates and the information it needs to operate.
While I'm Not doubting you on the sensors pushing away from the pick up ring I find it very hard to believe all 4 sensors have the same problem and an hour of driving every time I stop all 4 are still at 2 mph even with engine off it still reads the same .
And yes I am using a tech 2 .
I will do the ohm and voltage test as I have to change pads and rotors this week .
Along with all the other problems I’m chasing it doesn’t seem like one sensor would cause so much havoc but I’ve been wrong before
 

MRRSM

Lifetime VIP Supporter
If the ABS demanded that "all four wheels" must sense the same slipppage simultaneously... say when having "Black Ice" on only one side of the roadway... they would never work ...and the purpose of having an ABS would be defeated. It should only require having One Wheel sending information that it has ceased to rotate to trigger the proper system-wide ABS response.

Please understand @j-bone ... that whenever I provide information in ANY posts... it is with the idea that I can never guess what a person knows... or does not know... about the problems they are asking to get help on. Ergo... I try to include as much information as possible because more than one person will visit your posts and many other similar posts, looking for additional, complete and useful information based upon your experiences...and the help rendered by the many contributing GMTN Members.

Your knowledge, acumen and skill set about Mechanics cannot be determined from what you wrote in Post #1. And so my replies are meant to be as comprehensive as possible and never written as a challenge towards your mechanical understanding. They are always free of any malice or pejorative intent in mind. You will determine in time whether or not any of these ideas are worth investigating by the success or failures of your repairs.

"When the Student is Ready... The Teacher Emerges..." Confucius
 
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j-bone

Well-Known Member
Yeah sorry I didn’t mean to sound arogent , I understand you can’t know the skill set on the other end of the forum .
I take everything with a gain of salt no matter the source.
 

budwich

Well-Known Member
As you plan, I would do some resistance measurements towards the sensors but also do some voltage measurements (or resistance measurements with power disconnected) towards the module inputs to see if anything is out of place. It is unlikely to be sensor issues. My guess would be either ABS module OR battery / ground conditions at the module as it is unlikely that all sensors have the same "error".

I guess one other question would be... when was the last time you checked your brakes (calipers)? frozen calipers could be a problem with brake pressures.
The earlier comment about "killing engine power at slow speeds" is somewhat puzzling both in terms of what does that mean (ie. how did you determine this) and how is this an ABS system function... ie. kill engine power.

Forget the "kill engine question"... I see abs (EBCM) has a function to request the PCM to reduce engine torque. :smile:

sorry one other question: what is your pcm indicating is your vehicle speed while in park?
 
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j-bone

Well-Known Member
first off i never said what vehicle this is:bonk:, 2006 buck ranier AWD V8

Ok so some updates and more information on the whole scope of problems i'm chasing and why I think its something more then a sensor.
first an update on the sensors , curiosity go the best of me and decided to probe the front two sensors and do the brakes.
the sensors had a little over 1k ohm and only produced 200mv ( this is a live front axle so it has high resistance I set the fluke to min max and spun it the best i could ) the hubs did have the stainless shims which are completely useless at preventing rust . cleaned it up and got 4-500 mv doing the same spin test we will see if it helps anything.
front calipers were not seased and slide pins are free.
So the "kill engine power" situation has happened to me once and my mom a bunch of times but it was a very low speeds it kills the traction control ( light is illuminated for the rest of the drive ) and only had idle for a few seconds regardless of pedal position but returns to normal dive-able condition.
some other problems that i think are related somehow
the air suspension doesn't always pump up (pump is fairly new )
you can drive it on the bump stops and it will return to normal when it feels like it .
now there is no interior lights with door or key ( dash switch turns them on fine )
rear tailgate doesn't get power sometimes ( wont lock or work the rear wiper)
no dash information at times ( all gauges function even when the information does not work )
when the information center is working there is a " check stabiltrak" message often ( checking with my non tech 2 had provided no information )
you can see my original thread here
https://gmtnation.com/forums/threads/06-rainier-no-dash-information-center.19148/#post-577347
maybe they should be merged now ...?
I will check pcm mph in park and report back !
 

MRRSM

Lifetime VIP Supporter
The LGM (Lift Gate Module) in the Rear Hatch can get shorted in the Power Harness Connector to the LGM due to Electrical 'Arcing and Sparking' caused by excessive Body Vibrations and damage the internal Logic Board. Also, if you pull the Rubber Grommet-Tube containing the wiring leading to and from the LGM to the Wiper Motor... it is possible that the 12 VDC Wires AND the 5 Volt Reference Signal Wiring can get bent back and forth often enough during the normal opening and closing of the Lift gate to cause the Wiring inside to 'work harden' and separate.

The resulting loose wires can Short the LGM either to Ground or to 12 VDC Power and then play Hell with the Class 2 Network. You can use your Tech 2 to investigate the Lift Gate Module and see if you have any bi-directional communications. If not... Well... that would be a good place to start sorting out any Class 2 Network shorts that can often affect everything else you are having trouble with as a result, Module wise.

This video illustrates the problem. If you find any such problems... Pull any relevant Fuses to the Lift Gate FIRST before attempting to perform any soldering and Heat Shrink Repairs as you do NOT want to accidentally put power to that LGM before cross-checking all wires and-or having the repaired connections going "Hot' before the work is completed:

 
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Mooseman

Moderator
When you get that low speed ABS activation issue, it is never the module but one of the front wheel sensors that's giving erroneous sensor data. The trick is finding which one. Some have had good luck in removing the sensors and cleaning the rust under them on the hub. Sometimes it's the hub itself that is failing and allowing the tone ring inside to move around.

And disregard that speed reading you're getting while stopped. Most get it, including myself.
 

budwich

Well-Known Member
It is likely that the "absolute speed" reading of all sensors is probably of no value to the abs... just the differential. Since they are all the same, the ABS should be doing nothing as its intent is not to bring the vehicle to a halt, its to "equalize" the wheel system. As Mooseman indicated, it more than likely a sensor error and since you see a "dump" issued for sensor left front, I would suggest that one / hub may be a candidate for further inspection.
 
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j-bone

Well-Known Member
Yes I agree that if all wheels read the same but are off to actual vehicle speed the abs system is happy . If other have this non zero condition and abs is working correctly then I’m ok with that .
Upon further reflection on yesterday, the front left hub had more resistance then the right , both were not loose, also no grease or other foren objects on the ring or sensor .
Time will tell if the hub is on its way out .
Plan to bidirectional test the body / rear hatch this week .
On a slight tangent , I’m asked if the vehicle has auto or manual hvac when entering information in the tech 2 how would one know which is present ?
There is an “ auto “ button on the hvac panel is it that simple ?
 

Mooseman

Moderator
If it's the HVAC with a display and the auto button, then yes, it's auto. The manual is the one with two sliders for the temp.
 

16vcabman

Well-Known Member
I fully understand how the abs system operates and the information it needs to operate.
While I'm Not doubting you on the sensors pushing away from the pick up ring I find it very hard to believe all 4 sensors have the same problem and an hour of driving every time I stop all 4 are still at 2 mph even with engine off it still reads the same .
And yes I am using a tech 2 .
I will do the ohm and voltage test as I have to change pads and rotors this week .
Along with all the other problems I’m chasing it doesn’t seem like one sensor would cause so much havoc but I’ve been wrong before
Actually It can and there is a TBS on abs coming on at slow speeds. Pull the sensors block the hole, take of rust, spray with cold galvanizing paint lightly. You be amazed. Front hubs are the most sensitive part of the sensors doing exactly what your describing.
 

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