January 2001

ABS EMERGENCY OVERRIDE

                                                                      By Steve Grosekemper

The level of sophistication that our cars have achieved over the last several years is nothing short of miraculous. We now have cars that will shift exactly when they should, put power down to the wheel that needs it and stops the wheel that has the best traction!

These are all truly wonderful features of our new "smarter vehicles." However there are times when the driver needs the car to do something that mat be contrary to the "intelligence" our vehicles now possess. This is going to be our topic today.

When a late model car with anti-lock brakes is being used for racing, the ABS system is unaware of whether the car is going backwards or forwards, it only knows that the wheels are spinning. The reason that this is an issue is as follows. The front brakes normally do most of the braking and are therefore much larger in size than the rear brakes. This is because under braking the weight of the car will shift towards the direction of travel, placing a much higher demand on the front brakes. Unfortunately, if the car gets completely out of control and ends up going backwards, this high braking demand is now placed on the forward facing rear brakes, while unloading weight off the rearward facing front brakes. Since the rear brakes are too small to do much stopping at all on their own, they are of very little use in this situation. The larger front brakes which now have no weight on them, immediately lock up. The ABS sees this, and thinking the car is going forward it releases the pressure to the front calipers, virtually eliminating any effect they might have had. What you are left with is a smart car which will not stop in a safe distance.

The remedy for this is to install a switch that can be easily activated to shut the ABS system off in such an emergency. The highest priorities for such a switch is to be highly visible, as well as easy to use in a panic situation. I have come up with a system that does both of these while still remaining simple and cost effective.

The following installation was done on a 1993 RS America but translates to most other Porsche models with ABS.

1- Install a Porsche rocker switch part# 911.613.113.00 in the switch blank between the speedometer and clock.

2- Grind a slot into the top of the switch so a cloth streamer will just slide into it. Insert the cloth streamer (I prefer red) into the top of the switch and push the top of the rocker switch in. The switch will now be in the on position. ABS activated.

(See figure 1)

2- Run two 24 gage wires from the switch to the front fuse box in the trunk.

3-Disconnect the battery negative cable

.

4- Lift out the fuse box and cut the .75mm brown wire at the #31 connection of relay R34

(See figure 2)

5- Solder these two cut wires to the wires which run to the newly installed switch in the dash. Install heat shrink tubing to the wires and reinstall the fuse box.

Testing:

With the car on, pull the red streamer and notice an alarm sound as the ABS warning light comes on.

Warning: Modification of any brake or safety system should be left to qualified personnel. This modification is for off road (racing) use only and the user assumes any and all responsibility in the event of system failure.

Good Luck

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