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Installing a 997 GT3 RS Steering Wheel in my 91 928 GT (w/ horn, w/o airbag)

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Old 06-23-2011, 06:20 PM
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GMM928
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Default Installing a 997 GT3 RS Steering Wheel in my 91 928 GT (w/ horn, w/o airbag)

This is my first post - very sorry if I went way overboard with information. Also, I'm not yet expert at including the photos; I hope they come through and do so in the appropriate order.

I purchased my 928 in April (first one). The original owner died and his wife sold it to a friend of mine in March of this year (I actually found the car for him). He decided to sell after only 1 month (too many cars), so I’ve owned it since then. The car is black with classic grey interior and has about 51k miles.

Thanks to the incredible amount of information and support from the people on this forum, the 928 Owner’s Club (many are the same guys, I’ve found), and the dedicated 928 shops (who have provided me with great advice as well as parts), I’ve managed to get my car from a “semi-neglected garage queen” state to a reasonably well-sorted, dependable driver.

I’m about 6’5”. While I ‘just’ get by in terms of headroom, the stock steering wheel was too far from my seat to be comfortable or safe. I always wear my seatbelt, so the steering wheel airbag in my car (while very nice to have), is not as critical to me as having a well-located steering wheel.

Jim Mayzurk and Gary Knox have both posted excellent write-ups on steering wheel upgrades. Following their posted info and PM guidance (thank you both very much), I was initially headed down the route of a MOMO (7004) hub adapter and MOMO or similar wheel. However, Jim highlighted to me that a regular, flat MOMO wheel would land at about the same distance from the instrument cluster as compared to the stock wheel because most MOMO steering wheels are not dished, and the stock 928 wheel clearly is... It seems Gary was able to successfully adapt a 996 wheel and airbag directly to the 928; the similarity of the wiring seems to have helped a lot there, but his solution didn’t seem to solve my primary concern (getting the wheel closer to me).

I checked out all of the MOMO (and other after-market) dished steering wheel options I could find, didn’t like them, and elected to buy and mount a 997 Porsche wheel with an integrated airbag. I purchased a 2008 GT3 RS 3-spoke steering wheel covered in Volcano Grey alcantara (stock alcantara color for most GT3 and GT3 RS cars I think, close match/nice contrast to my classic grey interior). I suppose I could have tried to adapt my existing wheel, but it’s a bit dated anyway, and I wanted to retain it in case I want to return the car to stock for some reason. My goals in order of priority (all of them important):

1. Mount the steering wheel 3-4” closer to the seat.
2. Make sure horn works.
3. Retain steering wheel airbag functionality.

Some obvious (key) hurdles:
1. The RS wheel isn’t set up to mount to the MOMO adapter.
2. The horn needs to be re-wired.
3. The RS airbag is 2-stage, the 928 system only supports a single stage airbag.
4. The clockspring wiring harness (essential for the airbag system) must be retained to arm and control the steering wheel airbag. The MOMO adapter isn’t designed to easily allow for retention of this part.

Before I go further, please note that I’m not really endorsing this solution (use at your own risk), but it works great for me.

Hurdle 1: Mounting the RS wheel to the MOMO adapter

The 928 spline diameter is much smaller (approx 0.65”) than the 997 spline (approx 1.00”). This doesn’t really matter, though. They key is the diameter of the MOMO adapter (end that attaches to the MOMO steering wheel) vs. the BACK of the RS wheel:

Photo 1
Photo 2

The MOMO mounting ring easily clears the spline mounting hole on the RS wheel, but there are four ridges at 2:30 and 9:30 (see Photo 1) which must first be machined flat in order for the adapter to sit flush against the surface of the steering wheel (an air-driven grinder works perfectly – a dremel tool would probably also get the job done).

The finished product looks something like this (no need to be too elegant):

Photo 3

The next step is to locate and drill the 6-bolt pattern on the RS wheel. Obviously, centering and alignment is hyper-critical here. I definitely would have done a less than great job of this on my own, but fortunately a good friend of mine owns tool & die company. His solution for this next step was to measure and then fabricate an alignment dowel using the precise inside diameter of the MOMO hub’s spline hole and RS steering wheel’s spline hole:

Photo 4

This “tool” enabled us to exactly center the MOMO hub on the RS Wheel.

Photo 2 is a bit blurry, but you will note there are “locating screws” in the hub bolt holes. Using locating screws in all bolt holes (all set at the same height), the next step was to center an align the hub (top of hub at top of wheel) using the alignment dowel:

Photo 5

With the alignment setup done, we marked the center points for each bolt hole by giving the adapter a tap with a shop hammer; the locating screws left marks noting each drill point. If you intend to do this conversion but don’t feel like (or cannot) make this alignment tool, please send me a note and I will loan you mine.

Well before you get to this point, you will notice that the various holes already machined onto the back of the RS steering wheel will prevent you from locating all six MOMO bolt holes into solid metal on the steering wheel – you will only get 4 new holes. The top bolt hole and the bolt hole at 8 o’clock both go through existing “windows” on the 997 wheel (you will need to machine a very small amount of overlapping metal to fully expose the 8 o’clock hole).

At this point, you will have four precise marks on the back of the wheel where you now need to drill holes. My advice here (learned from the experts at my friend’s shop, is to start with very small “pilot” holes using the smallest drill bit available. And, take all the time you need to locate the drill bit EXACTLY onto the location marks. Once the first hole is drilled, each successively larger drill bit will naturally follow the first hole. Then, simply increase the size of the four holes to the appropriate diameter.

For the two “existing” holes, I used small sections of hardened steel bar stock as “bridge washers” to mount the bolts. Given four direct through-bolts and 2 bridge washers, I think the wheel locks onto the MOMO adapter as well as any stock MOMO wheel you might purchase.

Source the appropriate length bolts, slide them through the newly drilled holes and thread them into the MOMO adapter (thread lock might be a good idea during final install).

Final assembly of the wheel and adapter will look something like this:

Photo 6

Notice I’ve fed the MOMO horn wire through the center, and I’ve attached a brown ground wire to one of the bolts (also for the horn).

Hurdle 2: Rewiring the 997 “airbag” horn for the above setup

I bought and installed the steering wheel contact from Porsche (part # 964.652.104.00) as described by Jim and may others. I checked the connection between the copper contact on this part and the back of the MOMO hub adapter and found I needed to slightly bend the copper contact in order to make positive contact with the hub. Make sure you check this connection before completing assembly of the hub. Here’s a picture of the 964 part installed (only installs one way):

Photo 7

The round airbag from the 997 steering wheel has an integrated wiring harness with 6 wires. The solid black and solid brown wires attach to a perimeter “tape” via a black quick connect. There are four pressure pads along the tape; pressing the airbag anywhere causes at least one of these pads to complete the connection between the 2 wires, thereby sounding the horn.

While we’re talking about wires on the airbag, one of my local Porsche service departments has explained to me that the green connection on this airbag controls airbag inflation for low speed collisions and the yellow connection controls inflation for high-speed collisions (someone please correct me if they know this to be wrong). As the 928 was designed with a single stage airbag, I presume most would agree the wires from the yellow connector would be most appropriate to use with the existing 928 airbag system. I have also been told (but have not been able to confirm) that the airbag activation signal that a 928 system would send is the same as that from a 997 system, meaning the 928 system should be able to activate the 997 airbag. I don’t have a 997 wiring diagram, and I don’t have the technical specs for the 997 system so I have no way to verify that this is actually true. Back to the horn…

Photo 8 – Horn wires have a black quick-connect on the back of the airbag

On the terminating end of the airbag wiring harness, I clipped the horn wires from the orange plastic connector and added quick connects to each. I later added a length of wire to the black wire (Photo 10) to allow for easy connection of these wires once the steering wheel was in place.

Photo 9
Photo 10

This completes the wiring connection of the 997 steering wheel airbag horn. Anyone who purchases this wheel will already know the following: the airbag is held in place by a retaining wire. To remove the airbag from the wheel you insert a flathead screwdriver in the hole below the airbag housing at the 6 o’clock position and press firmly upward. This will deflect the perimeter wire and cause the airbag to pop out of the steering wheel airbag housing. No torx bolts, not screws, nothing. To install the airbag into the steering wheel, simply align and press until the clips in back clip onto/behind the retaining wire.

Hurdles 3 & 4: Adapting the 997 steering wheel airbag to the 928 (not yet…)

In removing the stock airbag wheel, you will encounter and remove the clockspring assembly from the steering column spline (see Photo 10 below). This mechanism allows the airbag wires (internal wires with red connector) and the horn hotwire (copper ring around perimeter, analogous to 964 part) to “spin” with the steering wheel, and it allows these wires to simultaneously connect to the fixed wires in the dash (perimeter wires with white/yellow and black connectors). The yellow connector is the airbag wiring connector, and the black plug is the horn hotwire.

Photo 11

As I noted above, the airbag wiring on the back of the 997 round airbag basically nets to 2 wires from the yellow airbag wiring harness, and the horn wiring (discussed above). Ideally, this clockspring mechanism would simply be retained and re-used with the MOMO adapter; this would easily allow use of both the airbag and the horn (without need for the 964 part), but (so far) there seem to be a few problems associated with retaining this wiring clockspring:

1. The MOMO hub has a turn signal extension on its base; it would need to be removed to fit on top of the clockspring.
2. The base of the MOMO adapter is quite thick, and retention of the clockspring might not leave sufficient spline threads for the locking nut which secures the hub to the spline.
3. There is no window in the base of the MOMO hub through which the clockspring airbag wires can pass in order to connect with the steering wheel airbag wires.
4. Its probably also the case that the clockspring’s copper horn ring would need to be modified in order to contact the copper ring on the base of the MOMO hub.

To do a “complete” wheel swap to a 997 airbag wheel and retain airbag function using a MOMO hub, I think the base of the MOMO hub needs to be machined and modified to address the issues above. I think it can be done; I’ve purchased an extra MOMO hub and will attempt to modify at some point in the near future.

Until then, I’ve got a 997 wheel with a fully functional horn attached to a MOMO hub adapter (with a non-functioning airbag). This puts my steering wheel in a closer, ideal position given my height and seat position. Good thing the distance works for me, because its obviously not adjustable; anyone shorter than me who drives my car will be doing so NASCAR-style (wheel in lap). One thing worth noting, the increased distance of the steering wheel from the instrument cluster causes one to need to reach for the indicator and control levers. Very awkward at first, but I’ve quickly adapted.

For drivers who want to upgrade their wheel but are not as tall (i.e. they do not want the wheel as close), it should be possible to source shorter hubs / spacers in order to adjust the distance…

I did this project over the course of a week, and didn’t give up on the airbag (for now) until the last day. I re-connected my battery several times and registered the airbag absence/malfunction fault in my car’s computer before installing the 3 ohm bypass resistor, but it was easy enough to clear the fault with the Bosch Hammer once I installed the correct resistor.

I will post an update with the results of my efforts to modify the MOMO hub (good or bad). For now, though, here’s a photo of my installed new wheel (sorry, the MOMO hub is not clearly visible):

Photo 12

As many others have noted before, I had to sand a small amount of the center plastic trim piece to allow clearance for the MOMO hub adapter (tall people will also note that I ‘deleted’ the parcel shelf).

Thanks again to everyone who posted the results of their efforts to do other steering wheel mods; I was only able to get this done with the help of your posts and shared expertise. I’m sure I may not have answered all of the questions people might have; I’ll try to answer any posted questions if I can.

It was a fun project, and (oh, yea) driving the car is pretty good too…
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Old 06-23-2011, 06:26 PM
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ender928
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holy crap nice job!!
Old 06-23-2011, 11:50 PM
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cold_beer839
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damn thats a big steering wheel.
Old 06-24-2011, 12:20 AM
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GMM928
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I think its basically the same size as the original wheel, but the camera angle sure didn't help alot now that you mention it. Don't notice the size at all while driving. Anyway, you're definitely right that's its bigger than many MOMO wheels (which I prefer - have one on my 911...), but I couldn't find a dished aftermarket wheel that I liked more than this one. Now that this one has been paid for and mounted, I probably will...
Old 06-24-2011, 12:58 AM
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Jim M.
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Greg, great job. I originally had my doubts, but you made it work and it looks good.
Old 06-24-2011, 01:13 AM
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GMM928
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Thanks again for the help, Jim.
Old 06-24-2011, 01:54 AM
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S4ordie
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Great effort! But beware if you get in an accident and are injured your insurance or that of the other driver may not cover your medical bills. Removal of safety equipment usually turns out bad for the driver of the car.

Sorry to throw a wet blanket on your project, it looks great and is functional too, but thought I'd let you know.

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Old 06-24-2011, 04:57 AM
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I have seen others add a momo wheel with addition of a quick release adapter and that gained another 2 inches...

very nice write up!!
Old 06-29-2011, 02:23 AM
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GMM928
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Dan, Thanks for your fair comment on the insurance front. I actually called my insurance company post installation and highlighted the fact that my 928 no longer has a driver's side airbag. The insurance company acknowledged the deletion, adjusted my premium (actually, no change), and all is good. Not sure how I can deal with the "other guy's insurance", so I guess that is still a lingering point...
Old 06-29-2011, 11:26 AM
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SteveG
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HTML Code:
I did this project over the course of a week, and didn’t give up on the airbag (for now) until the last day. I re-connected my battery several times and registered the airbag absence/malfunction fault in my car’s computer before installing the 3 ohm bypass resistor, but it was easy enough to clear the fault with the Bosch Hammer once I installed the correct resistor.
Thanks for this detailed write-up. Could you expand on the resistor installation? Or a link. I assume going that route disables the airbag? I have the fault light after removal/reinstall of the wheel/bag. What kind of deposit would you want to rent your Hammer?
Old 06-29-2011, 01:16 PM
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GMM928
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Steve,

For the resistor, I followed advice from Jim Mayzurk's thread and ordered a 3 ohm, 1% tolerance resistor from Allied Electronics (www.alliedelec.com). I ordered part # 296-5559, and installed it by bending the resistor leads to fit into the airbag harness. See attached photos. I then used black electrical tape to secure it in place.

If you're in NYC, then we're only 45-60 minutes apart. Instead of sending you my Hammer (which I'm fine to do, worst case), I could just meet you somewhere convenient with the tool once you've ordered and installed the resistor. It should only take a few minutes to clear the fault from your computer at that point. My Hammer has software update 7 (the last was version 8), but the software cartridge is dated early 1993 so it should work on your '93 no problem (I bet the airbag fault can probably be cleared by most any software version anyway...).

One last thing, if your mechanic has a modern diagnostic system and the 928 cable, he'll probably be able to clear the fault for you as well.

Let me know how it goes, and if you want to get together.
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Old 06-29-2011, 02:17 PM
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SteveG
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Greg: The car is 90 min east of NYC. I'm the mechanic. Seriously, I have dealt with a local on problems I couldn't solve in my narrow, no-lift garage. Not to mention my expertise. He doesn't have a Hammer. I don't even know where the nearest dealer is, but I will check that before I go near NYC with the car. The car has been there a couple times, but it's just not worth putting it at risk unless there a family emergency. Thanks for the info.
Old 06-29-2011, 06:55 PM
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Got it. I'll probably be out on Eastern LI a few times this summer and we can sync up then if you're not in a rush. But I can also ship you the tool if needed sooner and you can't find a local solution. Just send me a PM whenever.

Btw, I just noticed that the orange airbag wiring plug is stamped as a BMW part. Maybe BMW were the only guys Porsche could source airbag stuff from back in 91. Go figure.
Old 06-30-2011, 12:52 PM
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Greg: I will meet at your convenience. I will PM. Here is link to Theo Jeniskens site and tools.

https://rennlist.com/forums/928-foru...to-access.html

Steve

Last edited by SteveG; 06-30-2011 at 01:22 PM.
Old 11-09-2011, 10:57 AM
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autodoctor911
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there is another solution for mounting the 997 wheel in an older porsche, but it does not help with the reach. there is a spline adapter between the steering wheel and the shaft on most volkswagens made between about 1991 and now that has the same outer splines as a 997 wheel and the same inner splines as an old porsche.


Quick Reply: Installing a 997 GT3 RS Steering Wheel in my 91 928 GT (w/ horn, w/o airbag)



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