Adrian

Just to keep everyone up to date with Kris and his C4 in Malaysia. We have been able to determine that Kris has been the victim of some unfair activities. The PDAS and ABS warning lights have been disabled or removed and the power to the PDAS/ABS system solenoid has been removed. What we have to do before actual troubleshooting begins is to try and find where all the systems have been disabled at and repair this first. Once the system has been returned to operation, then we have to find the cause of the actual problem. This is a tough job but we are getting there. I am sure that the control unit is okay. Reason for this is because we have discovered that the green PDAS activation light which was disconnected by Porsche in March 1989 (was far too annoying) has been connected up by a PO this is flashing on and off. This tells me the control unit is trying to activate the system. The warn horn has also been disconnected. This is a sad state of affairs. If you all rememeber, the mechanic Kris saw suggested that both the PDAS/ABS control unit and the Hydraulic Pump had failed. By coming to this forum Kris just saved himself at least $US2,000. Possibly more. The brakes are working the ABS is not. If the pump has failed ona C4, you will not have any brakes.
Ciao,
Adrian
911C4

apw964

Adrian,

Just wanted to say thanks for continuing to post on this. For me and others who do not possess the in depth technical knowledge you and others have, this forum is a great place to learn and validate what we are being told by our mechanics

Thanks again,
Andy

Bill Wagner

Adrian, your last statement says:

"If the pump has failed ona C4, you will not have any brakes."


Ehhhhhhhhhh?!?!?!?!?!

Bill Wagner

Adrian

Dear Bill,
I know you have the maintenance manual. Go to the section on brakes. You will see that it takes 25 pushes of the pedals to remove the pressure. This is the procedure to bleed the pressure off the system. If the hydraulic pump does not crank up (under normal intermittant operation conditions) you will have no brake pressure or an ever reducing brake pressure every time you push the pedal. I can tell you from some testing I did some time ago that it takes around 10 pushes of the pedal and you is in deep trouble if the pump does not crank up. "NO BRAKES" and warning lights on and warn horn blasting away. Three or four pushes will bring on the warn light if you are stopped.
No pump running and you have no brakes, after quite a short time,
Ciao,
Adrian
911C4

PS: The 80 bar accumulator is insufficient to power the entire system of a C4 which you must know under normal operation is fully open.

Adrian

Now after that little diversion here is the latest. Following the instructions I sent to Kris, he went to work. He has found both the PDAS and ABS warnlights disabled and the warn horn disabled. He already knows that the PDAS/ABS soleniod is disabled. No power so no pressure to the system, it is sealed off. Searching to find out how this has been disabled and what else has been disabled continues.
Once the system has been returned to its correct configuration we will actually then try to work out what has failed. Obviously the PO had a problem and the fault could not be located or was not even looked at, but the system was disabled and the C4 sold on.
When people do this type of thing it does make things very difficult and some owner patience is required. These things are not in the manual. As I have said beofre, this is now the third C4 I have been asked to assist with that has had some form of system disabling carried out,
Ciao,
Adrian
911C4

Adrian

There is something extra I should point out for all readers, especially owners of C4s. You often hear, as in the post from John about what a system under the law should do. The pressure accumulator on the C4 when "NEW" was charged to 80 bar of pressure. It cannot be recharged by the way. If you read the maint manual the brake accumulator has to be replaced at around 60 bar. How many people have carried out accumulator and brake pressure checks on their C4s. The answer I would bet is very close to Zero. A working hydraulic pump will cover up a low pressure accumulator very easily. Most people do not even know what kind of brake system is installed.
It is too late when your brake pedal goes rock hard (no pressure) and you plough into the rear of somebody else.
There are certification requirements when new and there is reality 13 years down the track.
I also recommend every C4 owner reads the owners manual and see what the instructions are when the brake warn light comes on. If the pump fails this light will come on very quickly and you can check this very easily by pumping the brake pedal whilst stopped.
This is pure experience talking here because this was the very first fault that ever happened to me on my C4. Nobody in 1999 could help me, via bulletin boards or the rennlist mail lists. I had to go find out myself. This I did and this is what I share today.
Ciao,
Adrian
911C4

slant911

Adrian,

Do I understand correctly that normal mode of operation then is that at an idle if you step on the brake pedal 3-4 times the warn light should blimp on then go back off? And those warn lights should be the "brake" light and the "4 wheel drive looking" light. Correct??

borion

Sorry to butt in here, but could someone out there test this "no brakes" theory Adrian has put forward? If you have a C4, find a nice safe place to get the car up to a low speed, like 10 or 20 mph, cut the power to the car, and see if the brakes fail to work. I don't even know if this can be done (safety first everyone!) but I find it very very hard to believe Porsche would be stupid enough to design a car that would lose its brakes once the power was lost to the car ....like in an alternator failure. I also believe this is an AES design violation if it's true.

Bob

slant911

Borion,

I have a C4, but, am not willing to purposely test your theory with mine

I know for sure that there is braking power upon power failure (at least enough to stop) because I had an issue of stalling a while back and stopped many times after the engine died. Now I would not want to be traveling at 100 MPH and test this theory, but, from a reasonable cruising speed you will certainly be able to stop.

I think if you look at Adrian's post again. He is not saying there are NO brakes if power is lost. He is saying that you only have so many pushes of the brakes before there is no more pressure. Obviously, if the power were to be lost then you would press the brakes just once to stop yourself and that's all that would be needed.

Not trying to take sides. I think you are both saying the same thing and don't realize it.

krish

Borion, you must stop letting out all this bad karma. Open your mind and for Pete's sake read and assimilate everything before coming to conclusions. Adrian didn't say you would have no brakes immediately, he said you would gradually have no brakes as system pressure dropped if the the pump didn't work. And all modern servo brake systems in fact have even less in reserve (the word servo sounds good but all it is is engine vacuum that's used to assist the system- really no space to get into details here..). You could confirm this on your car by finding a stretch of road, killing the engine and then attempting to brake. Safety first of course and observe all precautions and you do this at your own risk etc etc.......

Kris
89 C4

krish

Hello everyone,

I was quoted USD$6,000.00 for the repairs. Based on this I went back to the seller and 'convinced' him that it would be in his best interest to split this cost with me and was given a USD$3,000.00 rebate. And the repairs I was quoted for wouldn't have fixed the problem as the mechanics diagnosis of failure was based on swapping over a PDAS/ABS control unit from a 993 onto my 964. Back when I was in never never land it seemed plausible, I have since learnt among other things that the systems are different and not interchangeable. Which is the crux of forums like this. You save money yes, but you also learn and that to me is priceless.

cheers,
Kris
89 C4

Bill Wagner

Adrian:

My owners manual states quite clearly that in the event of an ABS/PDAS failure the brakes will still work... they just won't work with ABS, and the AWD system won't be working properly either. The section you are referring to in the manual is about BLEEDING the brakes. If the brake lines aren't open like they would be in the bleeding process the events you're describing could not and would not occur. If on the other hand someone is driving around with their brake fluid leaking out like it would be in the bleeding process, I think they probably have MORE to worry about than their pumps!

Borion:

Your test is interesting, but probably unsafe UNLESS someone could do it at VERY low speeds (like 10mph) on a large parking lot with no obstructions. If I cut my power completely, the only way to totally do it without fooling with the electrical system itself would be to turn the power completely off. Since my car has a steering wheel lock, I would really have no steering. This would not be the case if the pump is not supplied electricity when the key is in accessory mode, but I don't know if the pump is receiving electricity at this point or not. If I ever find a place that big where I could perform this test of yours I will do it, but (no offense) I'm not going out to hunt for such a location. I already KNOW the brakes won't quit.

CHEERS

Bill Wagner

Adrian

Guys, that is not so smart test. Having found out with my own C4 what happens when you lose pressure it is something that I do not need to test.
Now things are yet again getting out of hand with people taking my comments literally, without reading the context of the thread. The context of my statements was that Kris mechanic told him the pump had failed. I said Kris had been driving around for some time so this is not possible. Without the hydraulic pump you have no brakes. Now we have progressed to as soon as the pump fails, turning power of the car in a car park.
If the pump fails the first thing that happens is the brake pressure warn light will come on (assuming it has not been disabled). When this light comes on you have approximately 10 full brake applications left. If you continue driving after these ten applications of brakes and the pump does not crank up to repressurise the system, "you is in trouble". Want to try it, I do not recommend it.
Certification of these systems requires the auto to be able to stop after a failure of the hydraulic pump. The pre-charged accumulator (set at 80 bar) performs this task. Therefore you will have brakes so you can stop. There is no requirement in the law that you can continue driving around with brake warning lights on and ignore them.
HOWEVER!!!!, how many C4 owners have actually pressure checked their system and how many actually know if the accumulator is still holding 80 bar. How many are actually serviceable. How many are sitting at 10 bar. A serviceable pump will cover this up.
So lets get back on track and stop this literal nonsense. I have grave concerns of people driving around with warning lights and they are warning lights for a very good reason.
After your hydraulic pump fails in a C4 you "Should" have sufficient pressure provided by the accumulator to stop. If you decide to continue driving and your pump has actually failed, (very rare failure) then you are going to have a hard pedal real quick. The accumulator cannot keep the system pressure charged. It was never designed to do so. As I said you can see this very quickly when stopped. Pump your pedal and see when the brake pressure light comes on. If you want to believe you can drive around with the brake warn light on all day, please let me know when you are driving so I can make sure you are not behind me.
Resuming normal channels. Pump failed, get a tow truck,
Ciao,
Adrian
911C4

PS: The power steering is "power assist", ever tried driving around with a broken pump belt. Exactly the same principle.

Adrian

Gettinmg back to our regular channel and the updating on Kris and his C4. All the warn lights have been re-activated. After start the brake warn light is going off far too quickly for my liking and the PDAS goes off as well. Only seconds later the PDAS and ABS warn light come back on. This is indicating a static hard system component failure. I am convinced that the system hydraulics (all 23 metres of it) is still isolated. This is why the brake warn goes off so quickly. We are now trying to identify which solenoid valves are closed and why. Whoever was behind this system disable had obviously traced the problem and decided it was too expensive to fix. Either a major leak somewhere or most likely a failed differential lock. The diff locks are not hard to replace nor are they by Porsche standards that expensive. However I get ahead of myself, we still need to locate how the rest of the system was disabled. Results of this effort will be posted as soon as we find the answer,
Ciao,
Adrian
911C4

PS: ABS relay has been jumpered and to no affect.

borion

Getting back to the brake issue.....

If the pump fails the brakes regress to being like old simple hydraulic brakes, and they stay that way. It's what we in engineering acll a failsafe mode. If Porsche didn't put this in they would have been sued at least 100 times in this country for at leats $50M a pop. Their reputation would be that they build unsafe cars.

The brakes worrk with the pump deactivated. They won't work very well, but they will work.

I don't have a C4 so I can't do the AWD test. Unless of course someone wants to give me their car :-).


Bob