T0M993

Last week I installed a Schnell front strut brace late Thursday evening and used a torque wrench to ensure not more than 20 ft-lbs were applied to the bolts.

Friday morning I hit the road on a 150-mile trip to Austin. About 60 miles in my Anti-Lock light came on. I thought maybe I knocked a sensor when I installed the brace the night before. About 20 miles later I pulled over for a break and when I restarted the car the ABS light was off and has not come on again since. About 20 miles from my destination, a strong rain storm came through and soaked the roads. This was the first time I drove my new-to-me '95 C2 cab in rain and at that time I started experiencing something very strange. Between the speeds of 40-55mph the steering wheel would begin to vibrate, sometimes pretty violently. Depressing the brake pedal or accelerating above 60 would make the vibration stop. Once the roads dried up there was no issue at all.

On the way back on Sunday I experienced the same problem when the rain started. Once it stopped the car was back to normal. I read somewhere in the car manual that there is a stability/traction control system that can brake the wheels independently to match rotation at speeds up to 70mph. Is it possible that the unit controlling this is faulty?

Sorry if I provided too much information above but I am trying to get the best advice possible on what this issue could be. Any advice would be appreciated.

morsini

Those cars don't have traction control. I'm wondering if you shouldn't be looking at a wheel bearing.

T0M993

Then what is the system referenced in the owners manual and why does the issue only occur on wet roads at a certain speed band? Seems like a wheel bearing problem would be prevalent on all road conditions and would not stop over a certain speed.

vincer77

Sounds like to me that you have a bit of hydroplaning happening and that a slight imbalance in the wheels is causing a vibration. How are the front tires? What brand/model are you running? With that widebody you are probably running wider tires than stock which will exasperate the hydroplaning.

vincer77

I looked at my manual. The system you refer to is ABD (Automatic Brake Differential)- it may be an optional system. I am not familiar with it, but it apparently only works up to 44mph.

T0M993

Thanks Vince. I'm running BFG 245/35R19 fronts and 295/35/R19 on the rear. I always thought than a tire imbalance vibration gets worse at higher speeds. Is that not the case?

nine9six

Thats what I was thinking...ABD is Porsche's predecessor to the development of traction ctrl. Fairly expensive option back in the day.

TMc993

+1...I would start by going to a good tire shop and having them start from scratch and balance your wheels.

A badly out-of-balance wheel will vibrate through the steering wheel at almost any speed. A slightly out-of-balance wheel will often be felt only within certain speed "windows" and sometimes only under certain conditions; coming in at a certain speed then diminishing at a higher speed, then reappearing as you slow down through the same "window."

Bad control arm bushings also tend to cause vibration through the steering wheel between 45 and 60 MPH, but once it starts, that vibration tends to be consistent under a variety of driving conditions.

Hope this helps.

Terry

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Quadcammer

Do you have a lot of dish on your wheels?

I've heard of water getting into the barrel and throwing the balance off.

Steve Weiner-Rennsport Systems

This one sure sounds like a tire/wheel balance issue. 19's can be really challenging to get smooth and certain wheels collect water which disturbs balance.

I fought this years ago with some Kenesis K58 wheels.

Further, certain brands of tires present challenges to get smooth, especially when large amounts of weight are needed to do the job. This is usually caused by QC issues with the tire maker.

T0M993

Many thanks to all of you for the helpful feedback. I will head back to the tire shop that installed these last week and have them re-balance them.

PNogC2S

With those sizes on the BFG's (KDW's?)you are getting pretty close to the maximum 5% differential in tire diameter front v. rear. I took a quick look on the TR site and the rears show a 27" diameter while the fronts are 25.8". You may be so close to the limit that any little bit of slippage (likely in the wet), could trigger the dash light and cause your ABD to get confused.
Double check those numbers just to be sure... but that may be the culprit.

Quadcammer

i believe abd is only good up to a certain speed. i want to say 45 mph but it could be more like 60