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Thread: Wiring Servotronic in 540 for M5 steering box retrofit

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    Wiring Servotronic in 540 for M5 steering box retrofit

    Background:

    So as many know the M5 steering box is a nice drop in for the 540i that changes the steering ratio dramatically, taking away the soccer-mom school-bus ratio of the stock 540 and making the car feel much more responsive and nimble.

    The bolt-in process is fairly straight forward but somewhat of a PITA beyotch. Splined pitman arms tend to be seized on old cars, the subframe needs to be dropped a bit for clearance, and if you have headers that doesn't help. I had it done on a lift and watched but didn't do every step so I won't pretend to give every step of the physical bolt-up. There is however somewhere on the web a write-up that covers this. "Straight forward if a pain in the ass" sums it up I think.

    However, the M5 box, unlike the US 540 OEM, uses Servotronic - ZF's brand for variable-assist power steering. The standard 540 has no electronic control over assist, and it is purely driven by fixed hydraulic pressure. On the M5 and E38's, the steering box has a Servotronic module, which is a solenoid/servo that activates to vary the amount of power based on roadspeed.

    If you do not properly set up Servotronic on the M5 box you are faced with 2 options - either NO assist, which is going to be pretty brutal in parking lots on a 4k lb car with wide performance tires, or, you wire up 12v to the servo, and in that case you have mega-power at all times and the steering is light and Cadillac-ish at speed.

    Some sources you can find on the interwebz talk about wiring up a variable resistor to "vary the voltage and dial in the power steering". This is a pile of poo, because the Servotronic is controlled by pulse-width modulation and current control, not voltage, so trying to voltage-regulate that servo is going to be not only potentially hard on the servo, but potentially hard on your electrical system, and is likely to not work well anyway, AND will certainly not be speed-dependent.

    Good news is that it is pretty EASY to do this THE RIGHT WAY. However it is not exactly the M5 way...

    Getting it done:

    The M5 engine uses the MSS52 engine management system, nearly the same as the MSS50 in the Euro E36M3, and MSS54 in the E46M3. That DME platform has facility to drive the Servotronic directly, so in an M5 the Servotronic is wired straight to the DME itself. The 540 however uses a totally and completely different ME7.2 system which knows nothing about Servotronic.

    However, all is not lost, because on non-US 540's and US E38's that do HAVE Servotronic, the Servotronic is actually happily driven by the GM Module behind the glovebox.

    Therefore to get properly functioning Servotronic for your M5 steering box, all you need to do is:
    1. Acquire a servotronic end-plug connector X120 to connect to the M5 servo. This seems to be the same as some of the ABS connectors etc. in the wheelwell, but you can also buy one new (which I did - you need to order a housing and 2 pins) or if you can find an E38 in the yunkyard, you can steal the wiring from that. In the wheelwell there is a connector box that OEM should hold this connector next to the ABS connector, and its a nice tidy place to snap this in, BUT, unfortunately the location on the E38/E39 is different, so the tail for the servo won't quite reach, so we had to get a little creative to mount the connector on the wheelwell ourselves.
    2. Run 2 wires from this connector, in the drivers side wheel well, through the fender grommet into the engine compartment, then through firewall, into the passenger compartment (I ran it through the boot for the hood latch cable as the wiring interface is a beast to open up on the firewall) and across behind the dash to the passengers footwell.
    3. Get 2 new pins for connector X254 for the General Module (ZKE). At the passengers footwell, terminate your 2 wires to these pins and put them in locations 9 & 11 of connector X254, which should be empty from the factory. Plug X254 back in. Almost done.
    4. Use NCSExpert or PASoft to code the General Module and turn on Servotronic option. You can also code the Servotronic option into the ZCS and then recode the entire car, but AFAIK all that is needed is to tell the GM/ZKE that it has Servotronic now.


    That's it. Done. Works fine if you got the physical bolt in done right.

    But wait... what about the servo:

    One more thing. The part number for the actual Servotronic servo module on the M5 box is different than the one for the 540/7x0 cars. That servo does seem to crap out from time to time, and you can find M5 guys complaining that it costs $300 or something to replace. My used M5 box was a good deal but came w/ a damaged servo, so it was in digging around for a replacement that I discovered this difference, and I easily and super cheeeeply pulled an E38 servo from the yunkyard and installed this on my M5 steering box - fits totally exactly the same.

    Aside purely from the cost of a new M5 one, there is also an electrical compatibility argument to be made to use the 540/7x0 unit, since those are factory setup to be driven by the GM/ZKE instead of the M5's DME. Honestly if my M5 servo was intact I probably would have never picked up on this fact, and would have wired it up anyway, and maybe that works fine I don't know, but this is possibly a more 'perfect' compatibility match. I am guessing the servo units are not really that different and the M5 guys are just getting screwed on the replacement cost but without testing inductance of the 2 and measuring the orifii etc. I can't say.

    I know in the Audi world guys mix-and-match swap around the ZF Servotronic driver relays between cars to get stiffer/softer response, and it also could be that the servo for the M5 gives a different feel, I don't know, but I'm completely happy w/ the E38 servo setup, and it certainly isn't a world of difference from driving my bud's M5 though we have yet to do back-to-back...

    TTIWWOP... so... some pitchurz:

    Crappy shot of the M5 box in place and where the Servo sits (sorry for the blurry...)


    The wheelwell connector from WDS:


    Here's my custom mount location:


    The GM/ZKE location and connector from WDS:


    Here's my lead with the 2 pins wired up ready to go in:


    Here's the X254 connector:




    Here's the ZKE/GM all wired up and back in place:
    Last edited by geargrinder; 02-23-2015 at 10:21 AM.
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    Nicely done! I will definitely consider doing this myself.
    -Chris

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    Wow. First time I saw this detailed out so well. Great job. I have been considering this change, but the servo hookup was keeping it out of reach. You closed that gap. Thanks. Now to find some parts and some time.

    Let us know when you compare feel to an actual M5. I wonder if the E38 servo is equivalvent? Thanks.

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    Awesome post!

    I have an M5 steering box in my 540, with the servotronic wired into the programmed GM. I'm glad you have done the same, and documented everything.

    Just a couple of quick notes for anyone else out there:

    If you plan to do the M5 box/with servotronic setup, make sure your GM is a "high" version. There is a transistor on the GM board that is required for the servotronic to work properly. Not all GMs have this transistor.

    If you study the servotronic solenoid, you'll notice it can be adjusted. I've never played with the adjustment, but I'd like just a tiny bit more assist. Guys over at the M5 board have been tweaking the the speed/assist curve via the DME. While the mechanical adjustment of the solenoid won't affect the curve, it would affect the overall assist.

    If you install the M5 box, it still has assist even without the servotronic solenoid connected. I drove my car for over a year this way. Its a bit stiff around parking lots, but fine at highway speed. The servotronic solenoid DOES NOT control all of the power steering assist, only partial.



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    Quote Originally Posted by philly98540 View Post
    Wow. First time I saw this detailed out so well. Great job. I have been considering this change, but the servo hookup was keeping it out of reach. You closed that gap. Thanks. Now to find some parts and some time.

    Let us know when you compare feel to an actual M5. I wonder if the E38 servo is equivalvent? Thanks.
    No prob. That's how I try to roll w the DIY and the info sharin'. ;^)

    Quote Originally Posted by CBi View Post
    Awesome post!

    I have an M5 steering box in my 540, with the servotronic wired into the programmed GM. I'm glad you have done the same, and documented everything.

    Just a couple of quick notes for anyone else out there:

    If you plan to do the M5 box/with servotronic setup, make sure your GM is a "high" version. There is a transistor on the GM board that is required for the servotronic to work properly. Not all GMs have this transistor.

    If you study the servotronic solenoid, you'll notice it can be adjusted. I've never played with the adjustment, but I'd like just a tiny bit more assist. Guys over at the M5 board have been tweaking the the speed/assist curve via the DME. While the mechanical adjustment of the solenoid won't affect the curve, it would affect the overall assist.

    If you install the M5 box, it still has assist even without the servotronic solenoid connected. I drove my car for over a year this way. Its a bit stiff around parking lots, but fine at highway speed. The servotronic solenoid DOES NOT control all of the power steering assist, only partial.


    Great points.

    I had scoped out the High GM issue SO long ago on my car it totally slipped my mind. You are so right about that being key although I think anybody w/ a "low" version can swap an ebay used unit as long as you then code it to your car of course

    I am unfamiliar with any mechanical adjustability of the solenoid. More infos please? You don't mean the external adjuster on the steering box do you? That's only to take up long-term wear slop in the recirc-balls-setup and shouldn't be used as a 'steering feel' adjustment. Sounds like you mean theres some actual adjustments to the solenoid itself?

    And yeah, tweaking in the M5 DME should be getting easier because of the work of the M3Forums guys on deconstructing the code for those units and putting it in the public domain, but unfortunately we don't have such a luxury...
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    Owning both an e38 and an e39 M5, I can say both systems feel about the same. The M5 is slightly heavier then the e38s system in normal mode. When in sport mode on the M5 it is significantly heavier.
    However, I will also say that the e38's steering snaps back to center much better and faster than the e39 M5. The e39 M5 on the other hand likes to follow the slope of the road, it doesn't like to snap back to center. Both cars have excellent feedback compared to any other cars I have driven.
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    First post updated to cover wiring polarity error issues.
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    Nice write up. Would thst work on non TU engines, pre 09/1998 builds?

    Are you sure the m5 does not use Bosch dme 7.2?
    I may be wrong, but I am under the impression it does.
    Cheers

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    Quote Originally Posted by BMWPlanetDude View Post
    Nice write up. Would thst work on non TU engines, pre 09/1998 builds?
    Good question. The main thing it needs is the "HIGH" GM as pointed out above. You can probably swap one in.

    I think the only question would be if the older cars send wheelspeed to the GM. I would be optimistic but you'd have to do some research and see what the differences are.... around that time, BMW started switching over how they send info between modules and cutting out more and more hard-wire connections between modules and sending the info instead over the CAN-bus. (ex: famously to retro-fitters, the M5 cluster gets oil temp from CAN-bus instead of a hardwire to a sensor). What would be a positive sign is the E38 had Servotronic on the M62 for a long time so you'd think you could drop the right GM in...

    Quote Originally Posted by BMWPlanetDude View Post
    Are you sure the m5 does not use Bosch dme 7.2?
    I may be wrong, but I am under the impression it does.
    Cheers
    Yep yer wrong. Its MSS52. Google it. Same basic system as the E36/E46 M3's.
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    This is nice! My car already has servotronic, I guess that I'm only short of finding that M5 steering box and drop it there. IIRC M5's had 14.5:1 ratio while the stock is over 20:1 right?

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    I'm pretty sure the M5 has 14.7:1. Stock is 17.9:1
    Last edited by blackknight530i; 04-28-2014 at 04:32 PM.

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    Does the 2003 MSport also have the 14.7:1?
    Last edited by Da Beemers; 05-02-2014 at 03:05 PM.

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    I don't think so. I'm pretty sure that only the M5 got it.

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    Many thanks to the OP for lots of inspiration and great info! I'm reading this thread because I got the crazy idea to add the "sport" button to my dash as on the M5, and wondered how much trouble it would be. Now after reading and learning a bit, I find myself more confused than ever. If the M5 is controlled by the DME, and regular E39 is by the GM, then what controls the '03 MSport box? My original thinking was that the MSport has all of the same steering as the M5 (only because I know it to be very different from a regular 540, not because I have driven an M5), so I thought I could just add the button and turn on the function in the DME. Now I'm not so sure... Sorry if this is a bit off topic, I have so far been unable to find any info on adding the sport button elsewhere. I love the steering on my MSport the way it is, but after all, I do have the spot on my dash... having the button would be more for a parking lot or city driving. Forgive me if I sound clueless. I am!

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    Quote Originally Posted by Da Beemers View Post
    Many thanks to the OP for lots of inspiration and great info! I'm reading this thread because I got the crazy idea to add the "sport" button to my dash as on the M5, and wondered how much trouble it would be. Now after reading and learning a bit, I find myself more confused than ever. If the M5 is controlled by the DME, and regular E39 is by the GM, then what controls the '03 MSport box? My original thinking was that the MSport has all of the same steering as the M5 (only because I know it to be very different from a regular 540, not because I have driven an M5), so I thought I could just add the button and turn on the function in the DME. Now I'm not so sure... Sorry if this is a bit off topic, I have so far been unable to find any info on adding the sport button elsewhere. I love the steering on my MSport the way it is, but after all, I do have the spot on my dash... having the button would be more for a parking lot or city driving. Forgive me if I sound clueless. I am!
    I think you are confused. The M5 has a different steering box than the 540i (which in the US doesn't have servotronic and is not controlled by the DME or GM). The sport button on the M5 is connected directly to the DME and since the servotronic on the M5 is controlled by the DME, it changed based on the sport button status. The sport button also changes the throttle response on a M5. The M5's DME is completely different from the 540i DME. The 540i DME doesn't have a clue what the sport button is and doesn't use the pins for it.

    -Paul
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    Quote Originally Posted by Da Beemers View Post
    Many thanks to the OP for lots of inspiration and great info! I'm reading this thread because I got the crazy idea to add the "sport" button to my dash as on the M5, and wondered how much trouble it would be. Now after reading and learning a bit, I find myself more confused than ever. If the M5 is controlled by the DME, and regular E39 is by the GM, then what controls the '03 MSport box? My original thinking was that the MSport has all of the same steering as the M5 (only because I know it to be very different from a regular 540, not because I have driven an M5), so I thought I could just add the button and turn on the function in the DME. Now I'm not so sure... Sorry if this is a bit off topic, I have so far been unable to find any info on adding the sport button elsewhere. I love the steering on my MSport the way it is, but after all, I do have the spot on my dash... having the button would be more for a parking lot or city driving. Forgive me if I sound clueless. I am!

    Agree w/ Paul - M-sport doesn't have any special steering gear as far as I know. Just different suspension.

    Unfortunately you are on the right track and the reason you can't find any info is because you can NOT add the sport button for sure, without changing the engine. If you put in an S62 you're all set but failing that, not an option.

    The M5 Sport button as you correctly note goes to the M5 DME, which is an MSS52 type DME. By pushing the button the DME then can change both the Servotronic curve it uses, and the throttle response curves it uses.

    AFAIK there is absolutely no such input into the ME7.2 type DME's that we have. Even if there was they don't control the Servotronic therefore could only switch the throttle response curves.

    On a 540 you would need to have some kind of a system that told the DME to change the throttle and the GM to change the Servotronic assist. Neither is possible without massive software reverse engineering.

    Read my other recent thread on custom coding the Servotronic for different assist feel curves though. There I talk about how we can adjust the feel to personal preferences through coding. Although that's "one time" changes, not on-the-fly, it at least means you can have the style of assist you like. I have a custom curve that provides plenty of assist at parking speeds but drops off rapidly as I speed up.

    - - - Updated - - -

    Quote Originally Posted by blackknight530i View Post
    I think you are confused.
    LOL sorry Paul obv we were both simul-composing replies.

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    Thanks for cluing me in! I did drive a regular '03 540 and then this '03 MSport back to back and they were very different when it comes to steering. Not only was there a big ratio difference, but the regular 540 had lots of assist at low speed and my MSport has almost none. Which is why I thought the MSport was like the M5.
    Now if I can just figure out what to do with that empty switch location...

    Thanks again guys!

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    Quote Originally Posted by geargrinder View Post
    Agree w/ Paul - M-sport doesn't have any special steering gear as far as I know. Just different suspension.

    Unfortunately you are on the right track and the reason you can't find any info is because you can NOT add the sport button for sure, without changing the engine. If you put in an S62 you're all set but failing that, not an option.

    The M5 Sport button as you correctly note goes to the M5 DME, which is an MSS52 type DME. By pushing the button the DME then can change both the Servotronic curve it uses, and the throttle response curves it uses.

    AFAIK there is absolutely no such input into the ME7.2 type DME's that we have. Even if there was they don't control the Servotronic therefore could only switch the throttle response curves.

    On a 540 you would need to have some kind of a system that told the DME to change the throttle and the GM to change the Servotronic assist. Neither is possible without massive software reverse engineering.

    Read my other recent thread on custom coding the Servotronic for different assist feel curves though. There I talk about how we can adjust the feel to personal preferences through coding. Although that's "one time" changes, not on-the-fly, it at least means you can have the style of assist you like. I have a custom curve that provides plenty of assist at parking speeds but drops off rapidly as I speed up.

    - - - Updated - - -



    LOL sorry Paul obv we were both simul-composing replies.

    JC
    No problem JC! I'm glad you went into more detail on it. I'm still at work trying to get a bunch of things done and I just kind of touched on it. I think the two of think a lot alike Helps having a couple of each model between the two of us, LOL.

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    Quote Originally Posted by geargrinder View Post
    Read my other recent thread on custom coding the Servotronic for different assist feel curves though. There I talk about how we can adjust the feel to personal preferences through coding. Although that's "one time" changes, not on-the-fly, it at least means you can have the style of assist you like. I have a custom curve that provides plenty of assist at parking speeds but drops off rapidly as I speed up.
    - - - Updated - - -
    JC
    I did read that and all I can say is WOW! I saw your curve and very much agree that it seems like the best compromise, with perfect linearity. You have my respect!! I have to go back and read it again now that I see it applies to me. Thank you very much.

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    Quote Originally Posted by Da Beemers View Post
    Thanks for cluing me in! I did drive a regular '03 540 and then this '03 MSport back to back and they were very different when it comes to steering. Not only was there a big ratio difference, but the regular 540 had lots of assist at low speed and my MSport has almost none. Which is why I thought the MSport was like the M5.
    Now if I can just figure out what to do with that empty switch location...

    Thanks again guys!
    I can almost guarantee that it's because of the different sport steering wheel. I just recently installed a sport wheel on my e38 and steering effort is much stiffer in both sport and non-sport modes (I have an S62 swapped in mine and therefore a working sport button). I also have an 03 530i Sport that has very stiff steering all the time and it doesn't have servotronic at all.

    -Paul
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  21. #21
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    That steering wheel really makes a difference then! It looks like Geargrinders curve is what I'm looking for.
    Thanks

    Quote Originally Posted by geargrinder View Post
    Read my other recent thread on custom coding the Servotronic for different assist feel curves though. There I talk about how we can adjust the feel to personal preferences through coding. Although that's "one time" changes, not on-the-fly, it at least means you can have the style of assist you like. I have a custom curve that provides plenty of assist at parking speeds but drops off rapidly as I speed up.
    - - - Updated - - -
    JC
    I did read that and all I can say is WOW! I saw your curve and very much agree that it seems like the best compromise, with perfect linearity. You have my respect!! I have to go back and read it again now that I see it applies to me. Thank you very much.

  22. #22
    geargrinder's Avatar
    geargrinder is offline Having No Trouble Here BMW CCA Member
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    Thanks! Tho' mine isn't really linear, it drops off harder than linear. I would say for somebody who is a n00b at coding, using either e39 01 or e39 05 pre-defined curves is going to be a good quick and easy improvement already from the default (the red-orange e39 03 is the default if you just code the car for the Servotronic equipment option) and less geeky and error-prone than coding up the custom curve.

    Once the Servotronic is in you can make that change to one of those in 2 minutes w/ NCSExpert and NCSDummy. If that's still not enough and you want even firmer, custom curve it..
    2003 M3CicM6 TiAg
    2002 540iT Sport Vortech S/C 6MT LSD TiAg
    2008 Audi A3 2.0T DSG (the daily beater)
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    Former:

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  23. #23
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    Great post, JC. I think I've got my head wrapped around this. I'm probably going to tackle it soon despite not yet knowing anything about NCSExpert or NCSDummy. I will learn that separately soon after I find a "DIY" for the actual physical swap. Doesn't look too hard but does appear to be a PITA. Since I need to replace the PS reservoir and lines and some balljoints up front anyway, this would be a good time.

    I assume I can bypass the connector in the wheel well and simply solder the 2 connection wires to a long lead which I then run into the cabin and into the GM. The question I have is, where do I buy the ZKE style pins? I looked around on the web but didn't find individual pins. Tried Digikey as well - I wonder if they are called something else?
    2003 Ferrari Red M3 3.5L wagon // 2011 Montego Blue tri-turbo 335d wagon
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  24. #24
    geargrinder's Avatar
    geargrinder is offline Having No Trouble Here BMW CCA Member
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    Philly just did it himself and agrees w/ me (he'd driven my car already hence he know but...) that it is a huge great thing. He dropped the whole subframe in order to do the front swaybar at same time. Hit him up on the mechanicals. I had help from my Indy doing mine and he just partially dropped the SF, not entirely (easier in a full shop w/ lift & tranny jacks etc.)

    Didn't I post the pin part numbers? Working now, no time to dig up the details...
    2003 M3CicM6 TiAg
    2002 540iT Sport Vortech S/C 6MT LSD TiAg
    2008 Audi A3 2.0T DSG (the daily beater)
    2014 BMW X1 xDrive28i (wifemobile)

    Former:

    1985 MB Euro graymarket 300SL
    1995.5 Audi S6 Avant (utility/winter billetturbobattlewagen)


  25. #25
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    Redshift- see my PM.

    GG- yes, you did post the P/N for the pins somewhere in the front of this thread. I found them online somewhere. (Edit: just re-read the thead, did not see the P/N for the connector pins. I must have got them from somewhere as I have the pins in a box at home. I'll update with the P/N shortly)

    All- I absolutely love this M5 steering box. All house of 8 guys should put this on their to do list. Not too expensive either!

    - - - Updated - - -

    Here is the connector pin part number for connecting to the GM. I got them at FCP.

    Attached Images Attached Images
    Last edited by philly98540; 11-11-2014 at 11:34 AM.
    98 540i 6, 525 whp, 120 mph 1/4, V3 Si S/C'er @16 psi, W/A I/C, Water/Meth, Supersprint Headers, HJS Cats, 3" Custom Exhaust, UUC Twin Disc, Wavetrac LSD, GC Coil Overs, Monoball TA, AEM FP, Aeromotive FPR, AEM Failsafe AFR/Boost, Style 65's w/275's, M5 Steering Box, Eibach Sways, M3 Shifter, Evans Coolant, 85 Deg Stat, PWM Fan, 10" Subs, B.A. speakers, Grom Aux/BT, Still Rolling as my DD!

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