Emanuele Ferry23

I read Adrian's issue about PDAS system, and I had a (purely theoretical) idea. The PDAS switches off automatically when brake pedal is depressed. This means there is a microswitch (the same which activates brake lights?) which triggers "PDAS off" signal. In this case PDAS may be switched off by simply introducing a dummy signal (button) on brake signal channel to PDAS.
The same effect should be obtained by giving wrong tension to PDAS (dangerous) or by giving a dummy error signal on accelerometers channel (with "christams tree effect").

...just a thought, I'm not rushing out to my 964 to mess wiring up! <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" />

joey bagadonuts

Nice thought. I just reread the <a href="http://www.porsche964.co.uk/rev_rep/images/Reviews/Wringing%20out%20the%20C4%20Part%201%20-%20P4~2.jpg" target="_blank">section</a> of the "Wringing Out the C4" article and it states that the application of brakes allows all four wheels to roll independently. This technique was used on the race track to execute faster turns, but unlocking your rear differential full time may be dangerous. During high-speed turns, your car will be very loose, so while I like your idea I think it may be too risky.

914und993

I installed a switch to supply a dummy brake signal to the PDAS on my C4. This worked to a point - there was less understeer on low speed, high-G cornering during autocross runs.

BUT, something else very strange happened on those runs that kept me from ever using it again: the brake pedal briefly went rock hard right after a power induced oversteer hairpin turn, and no braking action was available for that period <img border="0" title="" alt="[Eek!]" src="eek.gif" /> Needless to say, this rattled me a bit, and I never tried it on the track.

I do not know exactly what was happening, but if you choose to try this, and it isn't hard to implement, test it very thoroughly!

Chip

Adrian

Okay Guys,
Lets clear up a couple of points here. Firstly nothing Porsche is easy.
This is hard to explain but the ABS section of the control unit on the C4 does most of the actual computations not the PDAS section. There are three inputs related to braking activities supplied tot he control unit. Bypassing just one does not totally inhibit the system because the three inputs have to agree with each other.
The three inputs are,
Brake pedal pressed.
Deceleration registered by longitudinal accelerometer (C4 only)
Wheel speed detected reducing.
Note: The C2 only uses 2 inputs.
Hitting the brake pedal assumes you apply pressure to the brakes.

To the bypass question. I am not surprised you may have struck some strange braking issues. Remember that this is an PDAS/ABS control unit. The ABS section does most of the calculations and the brake pedal input is also used for other purposes within the ABS section. I will tell you that I suspect your ABS hydraulic unit put you into a mode like relieve pressure.
I do not recommend such actions. Life ain't that simple and messing with electronics without full knowledgeof the internal operation (which I do not even have) is not the way.
Why does everyone think electronic. How about thinking mechanical or maybe hydraulic.
Ciao,
Adrian
911C4

Adrian

Dear Chip,
I have sat down and thought about what you have done. I really think this could be seriously dangerous. In theory I can see you not have brakes or even any control over your C4 under certain circumstances.
By installing such a switch, yes you have turned off the PDAS. However you have armed the ABS. You only need two things to happen. A deceleration and a measured wheel speed deceleration of between 20 and 40%. If these two conditions are met then the ABS will firstly go into brake pressure hold mode which means you no longer control brake pressure to the callipers the ABS does and it may then relieve pressure if the impending wheel block (skid) is still there.
Nope, I really think nobody should ever consider such a modification.
Ciao,
Adrian
911C4

914und993

Adrian -

As my previous post indicates, I found it to be a seriously dangerous modification too. Thats why I only ever tried it out once under the controlled conditions of an autocross.

Thanks for the explanation of what probably was going on, though!

Chip

Adrian

Dear Chip,
I know but I thought an explanation was in order because others read this forum and well it is better to at least say we told people why not to do such things,
Ciao,
Adrian
911C4

PS: My idea would be to put a shutoff valve in the line between the accumulator and the dual solenoid valve. I have a feeling the PDAS will not like this either but it would not be dangerous. The reason the PDAS might get upset is when it applies the pressure to the slave cylinders it will not see what it thinks should happen from data received from the wheel speed sensors.