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Cadillac STS Forum - 2005 through 2012 Discussion, MRC Diagnostic help in Past Cadillac Vehicle Discussion; Hoping someone who also has MRC can confirm my findings... Still trying to nail down the 120MPH limit issue that ...
  1. #1
    Ludacrisvp's Avatar
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    Question MRC Diagnostic help

    Hoping someone who also has MRC can confirm my findings...
    Still trying to nail down the 120MPH limit issue that I've been seeing.

    On the top of the shock there is a pair of wires that plug in to a white elbow cap that connects the top of the shock to the car.
    This is what would control the viscosity of the magnetic fluid.

    Can be tested with car 'running' or in 'on' but not running.

    1. Remove white cap from top of either strut (leaving connected to wiring).
    - check to see if a speed limited message was generated
    -- No message displayed.

    2. Remove white cap from top of both struts (leaving connected to wiring).
    - check to see if a speed limited message was generated
    -- Speed limited to 80MPH displayed.

    Turn off car, disconnect battery to clear DTC.

    3. Unplug white cap from Driver strut (leaving cap connected to strut).
    - check to see if a speed limited message was generated
    -- No message displayed.

    4. Unplug white cap from Passenger strut (leaving cap connected to strut).
    - check to see if a speed limited message was generated
    -- Speed limited to 80MPH displayed.

    Turn off car, disconnect battery to clear DTC.

    5. Swap white cap from Passenger strut to Driver side strut.
    - check to see if a speed limited message was generated
    -- No message displayed.

    6. Unplug white cap from Driver strut that now has the cap from the Passenger side on it (leaving cap connected to strut).
    - check to see if a speed limited message was generated
    -- No message displayed.

    7. Unplug white cap from Passenger strut that now has the cap from the Driver side on it (leaving cap connected to strut).
    - check to see if a speed limited message was generated
    -- Speed limited to 80MPH displayed.

    Turn off car, disconnect battery to clear DTC and your car should be normal at this point.



    I can verify that the car did silently limit speed to 80MPH while the front struts were disabled/unplugged.
    I can also verify that the car does still silently limit to 120MPH even just after clearing the DTCs by disconnecting the battery.

    By silently limit the speed I mean that there is no RPM drop or active display that says the car is limiting the speed at the moment it happens (like would be expected when hitting the 'governor' when the Deville DTS would hit this governor it would say 'top speed fuel cutoff') the car just stops acceleration like you just set the cruise.

    I'm wondering how/why the car passes its checks with 1 shock disabled in various means.
    My thoughts are that possibly one of those white caps are possibly malfunctioning or perhaps wiring issue connecting to them.

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    Subsailor613's Avatar
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    Cool Re: MRC Diagnostic help

    The way I understand it:
    The overall speed (Limiter) is based on what "Tires" the car was shipped with.
    and is controlled by the ECM.
    Whatever tires are listed on the sticker on the drivers door rim, where they list the pressures,
    Z or Y or W speed rated tires, that is what they set your ECM to.
    Even though the MRC limits the speed to 80 MPH, it does NOT limit the car if they are OK
    Happy and SAFE Motoring !

  4. #3
    Ludacrisvp's Avatar
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    Quote Originally Posted by Subsailor613 View Post
    The way I understand it:
    The overall speed (Limiter) is based on what "Tires" the car was shipped with.
    and is controlled by the ECM.
    Whatever tires are listed on the sticker on the drivers door rim, where they list the pressures,
    Z or Y or W speed rated tires, that is what they set your ECM to.
    Even though the MRC limits the speed to 80 MPH, it does NOT limit the car if they are OK
    Happy and SAFE Motoring !
    That is not what this post is about.
    My car came from the factory with W rated tires from the factory. My car was not a fleet or rental car so doesn't have a lower limiter than normally found with the 1SG package I have.

    http://en.wikipedia.org/wiki/Tire_code#Speed_rating

    This shows that the W rating would be 168+ MPH.

    The issue is that my DIC will display at random times "Speed Limited to 120MPH"
    It does not matter what speed I am going when it comes on (I can even be sitting in park).

    When this happens the MRC system is disabled which I do not like as the car's ride and handling goes out the window.

    In the owners manual it states if you see a message on the DIC that is "SPEED LIMITED TO XXX" to see your dealer as it indicates an issue with one or more of these systems: Braking, Steering, Suspension.

    So clearly there is some part of my car's safety systems is not 100%.
    I am trying to figure it out somehow the car doesn't leave a diagnostic code behind to figure it out.


    Every dealer I have been to I believe feels the same way you have written your post. You just believe that what I am wanting / needing is to exceed the limiter. This isn't the case. I am extremely frustrated that there is some kind of problem with my car that is causing features to become disabled and nobody can fix it.

    There must, absolutely must, be a table that shows what the different speed limited messages mean since they are various speed limits.

    I've seen 120 and 80 in my car, moistcabbage I believe had his limit him to 0 and I believe there is a 55 limit too.

    It all depends on what issues the car detects to determine what it limits the speed to.


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    EChas3's Avatar
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    Re: MRC Diagnostic help

    Are you sure it's not Stability Control? That could be an intermittent wheel speed sensor. It certainly seems odd it doesn't set a suspension system code of some sort.

    Where are you located? As long as this has been going on, you might be willing to visit another member's dealer with known expertise. I don't know where EWill3rd practices his talents but he has led many members to resolution of off-beat issues.

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    MoistCabbage's Avatar
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    I only skimmed through this thread. Is it throwing a code?

    Quote Originally Posted by Ludacrisvp
    moistcabbage I believe had his limit him to 0
    Ha, I just thought of that when I was reading your post. That was the beginning of a major ground problem (a large chassis ground was almost completely severed. Things went REALLY crazy once it tore). The car wasn't actually limited to 0, and AFAIK, that's not a real message.

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    Ewill is Bill at Lindsay Cadillac.

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    Re: MRC Diagnostic help

    Quote Originally Posted by MoistCabbage View Post
    I only skimmed through this thread. Is it throwing a code?
    Ha, I just thought of that when I was reading your post. That was the beginning of a major ground problem (a large chassis ground was almost completely severed. Things went REALLY crazy once it tore). The car wasn't actually limited to 0, and AFAIK, that's not a real message.
    That is the hard part, there is no code left behind, the snapshot tool wasn't able to save any useful data according to Ryan Cadillac's Service.
    I've considered buying a Tech II for a long time and this might just be a reason to get one so i waste less time at dealers hoping they can 'find a code', this way when i see this come up i can pull over and plug in and see if there is anything in the system that is out of the norm, and to just run through as many diagnostic tools are in there that I can find.

    Quote Originally Posted by 1BadCadSTS View Post
    Ewill is Bill at Lindsay Cadillac.
    That is about 1,200 miles away from me

    Quote Originally Posted by EChas3 View Post
    Are you sure it's not Stability Control? That could be an intermittent wheel speed sensor. It certainly seems odd it doesn't set a suspension system code of some sort.
    Where are you located? As long as this has been going on, you might be willing to visit another member's dealer with known expertise. I don't know where EWill3rd practices his talents but he has led many members to resolution of off-beat issues.
    I guess I hadn't really thought about stability control too much; going back in my mind it does seem like almost any time I do a quick U turn it does activate stability control, maybe it activates more often than it should.
    Would a wheel speed sensor not leave a code behind?
    I want to say that 1BadCadSTS might have suggested a wheel speed sensor a long time ago when I first started seeing these issues, I asked the dealer about it and they basically brushed it off as not possible that it would create this type of message.

    The lack of suspension code doesn't really surprise me too much, there is a TSB for the CTS V 2nd Gen w/ MRC where it can trigger service suspension messages without leaving a DTC behind. I would expect that the two systems are much closer in design than some of the GM techs might believe it is. GM certainly is not going to re-invent the wheel on the MRC system for every car they make and with the V2 being a later design it is certainly possible that this issue was present in the STS and just wasn't discovered / common enough that there is a TSB about it or perhaps just owners that aren't very persistent about it.

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    Hard to test this but when the message appaeaes have you attempted to push the car past the advised speed?

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    Quote Originally Posted by 1BadCadSTS View Post
    Hard to test this but when the message appaeaes have you attempted to push the car past the advised speed?
    The car never exceeds 120mph even if the message hasn't displayed during the my drive. (Well I did get the HUD to show 122mph briefly thanks to a small hill). It just gets to 120 and it just stops accelerating. The rpm stays steady so it is not like what happens when you hit the top speed fuel cutoff where your rpm will drop due to fuel shutoff. I have tested this when the message had come on during my drive and when it had not (also pulled battery to clear DTCs before testing) every time 120 was the stop point.

    The behavior of it being limited to 80mph when I purposely disabled the suspension by disconnecting the shocks is exactly how it behaves when hitting 120mph.


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    I'm leaning to a failed suspension module in the trunk. Could have a short or some memory fault. It would explain the lack of storing a code.

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    Re: MRC Diagnostic help

    However you do have 4 wheel drive !
    Maybe Cadillac downgraded the top speed,
    because of that.
    The Abrams tank could do 65+ MPH, but the tracks
    limited them to 45 MPH ! Just a thought.

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    Re: MRC Diagnostic help

    Not so on this one.

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    Yeah awd or rwd are electronically limited at 168 however they are drag limited in the 150's.

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    Re: MRC Diagnostic help

    Quote Originally Posted by Ludacrisvp View Post
    The issue is that my DIC will display at random times "Speed Limited to 120MPH"
    It does not matter what speed I am going when it comes on (I can even be sitting in park).
    Ludacrisvp,
    I used to have this same problem, there was never a code, any lights or warning chimes, just the "speed limited to 120mph" would come up on the DIC for about 5 seconds and then fade away. I could be sitting at a red light or driving on the highway, it was totally random when it would appear. It never bothered me much as it didn't seem to effect the car in any way and I never drive 120mph so I kind of shrugged it off. About a year ago I had the two front leaking struts replaced under warranty and I have never seen the message again. Just thought I would throw that at you. Good Luck in your hunt.

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    1BadCadSTS's Avatar
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    Both of my front struts had leaked out and other than a rough ride and a clunk noise no suspension error was logged. Similarly a rear strut valve failed on my dads 07. It was stuck at full firmness and was jarring every time on a rough road. 3 trips to the dealer noted a rough ride but no issues found. I unplugged the strut so it would log a code and sure enough then it was finally replaced and the dealer confirmed it had no movement....

    I'm sure you can gather what in indicating you do off the story of my dad. Techs now a days need a code to do anything and be reimbursed from GM it seems. Gone are the days when they would har to investigate things without a tech 2.

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