Active suspension bummer

I have a 99 Cadillac SLS with ~50K miles. I think I have a fairly common, as well as fairly expensive problem on my hands. At start up, The Driver's Information Displays: "Speed Limited to 90". The speed is
definitely limited to 90, but more annoyingly, are the two other problems. First, the rear of the car always sits too low, as if the shocks are worn out. Second, while the car rides ok from a handling standpoint, the body shudders at freeway speed (~60 - 80 Mph). From what I can tell, the DIC message is related the the car's sophisticated active suspension. I'm just wondering if these problems are related and if so, what kind of money pit I'm looking at. Any advice is very appreciated.
Stephen
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I'll take a flyer at this, only because I have a '98 STS where I had a similar message. Since I live in Canada, the message I received was "Speed Limited to 145" (kph). In my particular case, it was a simple fix.
What I found, is that all of the suspension position sensor levers attached to each of the four control arms had become disconnected from their sockets, probably because the car had just previously been lifted on a chassis hoist (as compared to a suspension or wheel hoist) at a local department-store garage for an emergency tire repair. Apparently, when the suspension was allowed to achieve full extension by lifting the car from the chassis, the sensor levers weren't able to accomodate the movement range required, and the lever rods were pulled from their sockets, thus disconnecting the sensors. It follows that, once the car was back on the ground, the active suspension computer could no longer determine the position and attitude of the suspension, so it went into 'default' mode. This had two effects: 1) the air compressor for the rear shocks was disabled from achieving level ride height, since it couldn't determine when the proper ride height was achieved in order to stop pumping air into the rear shocks. Thus it defaults to a predetermined pressure which invariably isn't enough to level the car, even when there is no load. 2) active suspension is disabled since the computer cannot determine the position of each control arm in order to either add or release air pressure, or adjust the internal valving (for soft/firm ride) in any of the shocks/struts in response to road conditions. Again, a default pressure and valve setting is maintained, which may or may not be sufficient for higher speed maneuvering, so the computer restricts the vehicle from achieving higher speeds.
The solution? With the car on a suspension hoist (or jacking up the car by the suspension (control arm), remove each of the wheels, and check to ensure that the suspension position levers are connected at each control arm. It's a simple rod, about four inches long, which fits into a hole in a plastic socket connected to the control arm at the bottom, and a plastic lever connected to the chassis at the top, which is attached to a rotary sensor. At worst, the rod may even be missing. You should easily spot it with the wheel removed. It can also be inspected with the car on the ground (or on a wheel hoist), from underneath without removing the wheels, but not as easily. Turning the front wheels would also make inspection easier without removing the wheels. Make sure that the rotary sensors are free and moveable... if it's seized, the rod will just pull out again when the suspension moves.
If you find that one (or more) have been disconnected, it's likely the problem. However, if you find that they're all connected, then it could be an electrical connection in the sensor network, or a leak in the air compression system (i.e. unable to maintain air pressure in the shocks and struts), or failed or worn shocks/struts. This would likely require diagnosis and repair at your local dealer. Note that the adjustable shocks/struts on these models are rather expensive. If it's a failed shock or strut, I'd recommend genuine OEM replacement in pairs (both sides) at your dealer. Alternatively, the dealer could install regular shocks and struts at a lower cost, but this would require dealer deactivation of the suspension computer, and the loss of the active suspension and auto-leveling functions. Your choice.
FYI, my '98 STS has just over 205,000 km, and still riding on the original shocks and struts, and still riding fine with functional active suspension and auto-levelling. I only let myself, or a Cadillac dealer perform tire service (replacement, rotation, etc.), since they (and now I) are aware of these suspension position levers, and should inspect them to ensure they're still connected and functional after the car is lowered and once again resting on it's suspension. Someone unfamiliar with Seville suspension technology (like your local or department-store garage) probably don't even realise these things even exist.
Bullitt

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