pchristley

1999 Audi A8 D2 Pre facelift 110K. My question, "Is there a speed input sensor somewhere that provides input to the Computer or whatever?". I say this because at idle, I have zero assist on the power steering. If I barely touch the pedal, and pump the rpms slightly, it will kick in. My car also has developed a funky downshift between 60-70 mph off throttle if I let my instantaneous fuel mileage display go to ------- and not have a reading. If I press ever so slightly on the gas pedal to where I have a reading on that display, it will decelerate and not give me the harsh downshift. I'm thinking these two things may be related but I don't know where to look for a culprit. I've tried shortening the throttle cable slightly, and that took some play out of the pedal, it didn't fix the issue. Is there a known fix to this and I'm just missing it?

Thank you,

spitanddirt

On my 2000, I disabled my low speed P/S assist. I was having the same issue you were describing. The culprit is the P/S rack solenoid or valve. If it isn't working correctly, your steering will feel like has a mind of it's own at low speeds. My fix was to disconnect the vavle so the computer wouldn't have control over it. I still have power steering, it's just always on the heavy side.

The cause isn't that the P/S rack valve is bad or malfunctioning, it's that the wires that go to it are corroded and broken. The area where the valve and wires are is a area that collects road debris and it also absorbs a lot of engine heat from the exhaust. The valve and it's wiring isn't well protected. You can replace the valve separately, but it's still pretty expensive. You more than likely don't need a new steering rack either. I'll have to do some searching for the part number, but for me, it wasn't worth the price. Not only that, but trying to remove the valve in the car is damn near impossible without partially dropping the steering rack. If you want to disconnect it so that you can have consistant steering feel, the connection is behind the driver side wheel. I had to remove the rear piece of the wheel well to find it.

As for your transmission shift issue, first order of business is to change out the fluid and filter. If you still have the original fluid, it's definitely due. Fuel filter changed? Spark plugs? They all could be contributing to the shift issue.

pchristley

I've serviced the transmission and even tried switching out the pressure valve for the trans go one. I don't think its an internal tranny issue but I don't know for sure as I haven't disassembled it. It's never gone into limp mode and I didn't have any chunks in the trans pan. It just seems ironic to me that I have two separate issues with a similar fix, giving the car slight throttle, and things seem to straighten out. I just thought I'd ask here. I've put a lot of mile on it as is and it's ok, it would just be nice if I could get this sorted.

Could you elaborate on why the spark plugs could cause a tranny issue?

thank you,

spitanddirt

According to the RockAuto catalog, your 32V 4.2V (you didn't specify so I'm guessing), takes a Bosch spark plug 7411. I don't know if the original plugs are installed or if someone has changed them from original, but in my experience, using the factory 4 ground electrode plugs work the best. According to the factory manual, the spark plug replacement interval is every 40K miles, or at least it is for my 40V motor. It's a low mileage interval because the plugs are not tipped with Iridium or Platinum.

Anyway, when the spark plugs start to wear out, you can sometimes have odd misfires at times. The misfires might contribute to a jerky sensation at any vehicle speed. It might just be that yours likes to act up on the highway upon deceleration.

As an added item of note: Your car is 20 years old and more than likely your vacuum lines are probably garbage and should be inspected for leaks. Air leaks will definitely cause erratic shifts. I don't know why, but the Japanese cars I have worked on always seem to have better quality vacuum lines. My A8 by comparison has been very tough on vacuum line. Just something to look for. Good luck.