RSAmerica.net: Tech Article: John Miles of www.porsche964.co.uk

Emergency ABS override switch with streamer

Figure 1. The completed installation showing the red streamer that activates ABS override rocker switch.

Fitting your RSA with an Emergency ABS override switch

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 may be contrary to the “intelligence” our vehicles now possess. One such instance is going to be our topic today.

When a late model car such as an RS America with anti-lock brakes is being used on a track, the ABS system is unaware of whether the car is going backwards or forwards, it only knows that the wheels are rotating. 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 minimal weight on them, immediately lock up when activated. The ABS senses this, and thinking the car is going forward it releases the pressure to the front calipers, virtually eliminating any effect the front brakes might have had. What you are left with is a smart car with significantly reduced braking ability when going backwards.

The remedy for this is to install a dash mounted emergency switch that can be easily activated in just such an situation to shut off the ABS system. The priorities for such a switch are to be highly visible, and above all, easy to use in a panic situation. I have come up with a system that satisfies both of these criteria while still remaining simple and relatively inexpensive.

Accomplishment Instructions:

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

Install a Porsche rocker switch part# 911.613.113.00 in the switch blank between the speedometer and clock.
Grind away a small amount of the top of the switch such that a cloth streamer will just slide into it. Pushing in the bottom half of the rocker switch reveals this gap in the top of the switch. Insert a cloth streamer (I prefer red) into this gap and then push in the top of the switch thereby trapping the streamer behind the top portion of the switch.
Run two 24 gage wires from the switch to the front fuse box in the trunk.
Disconnect the battery negative cable.
Lift out the fuse box and cut the .75mm brown wire at the #31 connection of relay R34 (see figure 2)
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.

Figure 2. Lift out the fuse box to gain access to the brown wire at the #31 connection of relay R34.

Activation procedure:

Should you be unfortunate enough to end up going backwards in your RS America at a rapid rate, as discussed above, the ABS will tend to negatively effect your braking ability. Your immediate response would be to grab the red streamer, pull it out of the dash and discard it. This pulls the top half of the rocker switch away from the dash which has the same net effect as pushing the bottom half of the switch in, this deactivates your ABS which in turn rapidly restores your braking ability when travelling in reverse. This could be the difference between finishing an off track excursion by stopping in a run off area .... or not losing enough speed and introducing the expensive end of your beloved RSA to the wall.

Testing the completed installation:

With the car turned on, pull the red streamer which will activate the rocker switch and, if the ABS override switch is functioning correctly, an alarm will sound and the ABS warning light will come on.

“Reloading” the switch simply consists of pushing the end of the streamer into the gap at the top of the switch and then pressing the upper half of the switch to trap the streamer once again - this is definitely something best done when back in the pits and not recommended to be attempted during the lap after you just spun and had to activate the switch.

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.

Steve Grosekemper is shop foreman at Dieters Independent Porsche Service in downtown San Diego. For other technical articles go to www.dietersmotorsports.com and click on Technical Articles.