MB965

Bob,

Either you have a problem understanding the language Adrian is using or you refuse to accept that you are WRONG!
The braking system will NOT continue to operate with the hydraulic pump not operational, after the system accumulator has bled down. The C4 and C2 Turbo braking systems are not like standard power assisted braking systems. In fact there is no direct input from the brake pedal to pressurize the system, the brake pedal simply actuates a valve which then ports hydraulic fluid under pressure (pressure developed in the pump)to the brake calipers.
Porsche specifically developed the "Central Warning System" to address the pump failure by providing several levels of warning i.e. brake failure light, flashing "!" light and the warning horn.

Both Adrian and myself are highly experienced mechanical engineers, and have specific, detailed experience with our personal Porsches (C4 and Turbo 3.6). From your statement that "we in the engineering field call failsafe" I guess you are an engineer. If you are, you should consider some additional training as you have done a poor job of trying to understand the specifics of the system design and operation on this unconvential braking system. You seem to be relying on your generic understanding of braking systems and are making a basic mistake of assumming you know how the system works. Get a system diagram and a service manual and study them, all of the information is there.

Michael Behrman

Bill Wagner

Well, I for one am going to try the "borion test" this weekend if I can. There's a large parking lot (flat) about 1/2 mile by 1/2 mile at Wolf Trap Farm Park in Vienna, VA (hopefully it will not be closed). "borion" may not own a C4, but I do, and if there's some way the brakes could fail I WANT to know about it. After looking at the owners manual it seems to imply that the brakes will still work. After looking at the service manuals the only thing I can conclude is that they're the worst service manuals I've ever owned in my life (and that's the ONLY conclusion I can reach on this issue from THEM ). Is there a picture or hydraulic schematic of the hydraulic circuit somewhere that I'm just missing?

Should be an interesting weekend

Bill Wagner

Adrian

Dear Mike,
Thanks for your input and kind words. I get really worried when people advocate driving around with warn lights on, or integral parts of the auto, like brakes hydraulic systems failed. Please 964 C4, Turbo, Carrera Cup, Speedsters, TLs, do not do this. The warn light is there for a reason. If the brake pressure drops, get off the road and find out why. Borion Bob also fails to understand that failsafe does not mean continued operation. The C4 and other 964s with Hydraulic brakes system have a certified fail safe system. If the hydraulic pump fails there is a pressure warn system and if you all read my explanation below an accumulator which provides sufficient brake pressure for you to stop safely in all situations. The 964 is also fitted with a mechanical emergency brake. The 964 is "NOT" fitted with a second brake system which allows you to continue driving around whilst you decide how best to fix the first one. This is not a certification of any auto under any regulatory authority in the world. If the pump on the C4 fails you can stop. Easily and safely. You cannot continue on your trip until you get it fixed.
Dear Bill,
I do not recommend you do this test and it may not be as conclusive as you think and please keep your hand on the emergeny (hand) brake because when that pedal goes hard you cannot stop. Been there sdone that bought that t-shirt and it was my onw fault as well.
However to check the system out I need to explain a little better.
The C4 brake system is fitted with a pre-charged (80 bar) accumulator. When the pump is running it charges up the accumulator to 180 bar. When you use the brakes, or the traction control the pressure is supplied by the accumulator and the pump cranks up at a predetermined pressure (no idea what) and replenishes the accumulator (hence the very loosely applied word of power assist). Now if the pump fails this function is lost. Therefore as you use brake pressure the pressure inside the accumulator is bled off and eventually you have nothing but the precharge left which cannot power the system. This is a standard backup system which is installed in many systems including our aircraft (which for info bleed 500 PSI per brake application off the accumulator).
Now where the Borion test is going to provide some unreliable results is, if the C4 is not powered then all the solenoid valves are going to be shut. This means the accumulator is only going to provide brake pressure. This means the 180 bar (if you let the system charge first) in this test is going to last far longer than in a real situation.
The only reliable test really is to having the C4 running, and powered, disable the pump by the direct removal of power only to the pump and then try it.
You know you are in trouble when the brake pedal goes hard.
Good luck and please be careful doing such things. Being the conservative person that I am and not wanting you to create problems for yourself can I suggest the following. Regardless of your results when you power up the C4 again please time how long the brake warning light and PDAS lights remain on for. If you get to 90 seconds and the lights are still "ON", please turn off the ignition. Wait 5 minutes and then power up again. The pump will start to overheat above 90 seconds of continuous operation. Only takes 120 seconds and you can terminally damage the pump and the relay contacts. The number of pushes on the pedal you have will be an indication of the condition of your system and the precharge level remaining in your accumulator. However I still warn you that this is not a safe test to do. The other point I forgot to mention is that at very low speeds you intend driving at, the brake pressure used is going to be much less because the inherent superiority of the 964 braking system will have you stopped very quickly. The you have to get going again. I reckon this test you wish to perform if you can do it without stopping you will have 15-25 brake applications in the accumulator. Maybe even more. You will not be able to do this test, unless you are going downhill (a big hill needed as well)but at say 50mph you will have a lot less brake applications. Please remember in your tests the object is to actually stop the C4. You will easily achieve this. The question I do not believe your test will answer is, how many times can you stop before you run out of brake pressure. The other flaw in this test is that how long after the brake pressure warn light comes on do you actually run out of brake pressure. One last comment on the Owners Manual. I am getting a copy of the US version to read for myself. I do agree entirely with you that what you are saying is in this manual is not accurate. I suspect because the owners manual (well mine anyway) cover both the C2 and C4 (C2 with vacuum assisted brakes is obviously quite different) they have sort of missed something or were never told.
Ciao,
Adrian
911C4

Adrian

Dear Bill,
I tried your test on my driveway when I got hoe from work. It is nice and steep. Please take care if you intend to go ahead with this test. If you try and run with the ignition off the steering lock engages and will not release. Not very safe and I nearly hit the fence. I did manage to pump the brakes and the C4 stopped very quickly. I pumped the brakes about six times whilst wrestling with the steering lock. I got myself into a position where I had to start up. I timed the brake warn light and it was around 15 seconds. It should have go off instantly under normal circumstances.
Ciao,
Adrian
911C4

Bill Wagner

I just started a new post on the brake-test that I did today. This wasn't supposed to be a brake related thread, so those that want to throw stones at me can do so in the new thread.

CHEERS,

Bill Wagner

Adrian

Latest Update from Kris in Malaysia with his C4.
Now with all the warn lights working we have made some further progress. Kris has been able to confirm that the hydraulic pump relay which controls power to the PDAS/ABS control unit direct from the battery (B+) and the differential lock dual solenoid pack is engaging for around half a second and tripping off again. The relay was then jumpered and the system went through its normal start up procedure, ABS off, PDAS Brake Warn off for the normal 30 seconds or so, then the PDAS/ABS warnlights come back on. This is telling us we have a hard component failure. What has to be done now is to find the failed component. The list is actually quite short. It is not a pressure related failure because the brake pressure warn light is not coming on. I have asked Kris to check if the pressure switch circuit has been bypassed or messed around with. Just to be sure.
I have a spare control unit which I can loan to Kris. I do not believe it is the control unit but it may be required to prove it. With these faults a Bosch Hammer will always show a control unit failure. My first choice for a failure is a shorted (damaged) acceleration sensor. These items are all easy to check without special test equipment and Kris will do this, when he gets time,
Ciao,
Adrian
911C4

Tom Sharpes

Oh what he heck, I'll jump into this fray.

Sonny Vo (who is on the list) has a turbo 964 (a 92 I think) and he had the brake systen exhibit the same thing Adrian is talking about.

He told me of how he would start the car and everything would be OK, but when he used the brakes a few times the light would come on and the car would not stop the car after a few more pumps of the pedal.

If I am not mistaken it was the pump that failed - to the tune of $600 ! I'll alert Sonny to this thread and have him comment directly.

SonnyV

Everyone,

I've read all of the threads from top to Tom's post. Here is my $0.02 cent with regard to the brake system and hydraulic brake servo on my '92 C2 Turbo.

First, if you search the archive, you will find more information on this issue as I went through it about 7 months ago.

Here is the original symptom on my car. I start the car as normal after two weeks of no driving. The brake warning light comes on, then a short while later a series of beeps come on and then the beeps stop. The warning light remains on. Remember I had the problem for about 2 weeks before I finally identified the source with the help of Ardian and others on the list.

At first, I still have the brake for about 1, or 2 applications of the brakes. I have a slop drive way and unable to stop the car when backing out of the garage. The source of my problem is the failure of the hydraulic brake servo in the front trunk on the driver side, near the battery.

Yes, the brakes continue to work with the servo's failure. But you do not have the hydraulic power brakes anymore. In other words, you brakes become manual brakes with no assitant what so ever. On my C2 Turbo, I would have to stand on the brake pedal in order to slow the car down. Yes, I drive around like this until the hydraulic pressure comes back. At first, it takes about 10 minutes to have the pressure again, and the warning light goes out. Once the light goes out, the brake system is good for the duration of the drive. As time goes by (each time I drive it), it takes longer and longer to build up the pressure. During this time, if you apply the brake, you will lose the pressure and it takes much longer to build it up again. It is best to let the system build up the pressure before start driving. This is indicated when the warning light goes off.

If your servo fails completely, then your warning light will not go off. In my case, it goes off after a very very long time after the car has been started and idling. I had to sit in the car and waited for over 20 minutes before I could drive home because I was at work at last day.

I paid ~$860 for the servo, brand new, and about 4 hours of labor of r&r. Since then, I don't have any more problems with the brakes. My car was pretty much 10 years old when the servo failed. So, I contributed this to just old age. Adrian will be able to tell you more about the life of the servo and how much pressure it will hold and for how long.

Since I don't have a C4, I can't comment on other systems such as ABD... on that car. My understanding is that C2 Turbos and C4s both have hydraulic brakes system and you must have the Porsche hammer to bleed it correctly.

My $0.02 cent worth.

Best regards,

Sonny Vo

Bill Wagner

Sonny:

Thank you for clarifying that. That is EXACTLY how the brakes are supposed to behave (or misbehave, if you prefer ) in the event of a failure. Something is better than nothing!

Thanks for your input,

Bill Wagner

Young_Turk

Wow, this is great information.

Does someone have a hydraulic schematic of the braking system of a C4 they could post? I am interested in seeing it.

A picture is worth a least a 1000 words.

John

SonnyV

Here is a picture of the hydraulic brakeservo. It is #1 in the picture.



Hope this helps.

Best regards,

Sonny Vo

Young_Turk

Thanks.

Young_Turk

Using Sonny Vo's supplied diagram in the earlier post

OK, let's see if I have got this.

Since this system uses hydraulic fluid pressure boost instead of the typical vacuum boost of many US type cars, the boost pressure backup comes from the servo pump #1 and the pressure reservoir #2 which acts like an accumulator in case of pump or power failure. The servo also regulates boost pressure at a constant. W/O this you would not have power assisted brakes, which would be tough to have adequate braking power, but you would have some if you stood on them. There are 3 anti lock circuits, right front (VR) left fronr(VL) and all rear (HA).

As in normal anti lock circuits, if there is a failure of the unit or any wheel sensor, the system defaults to a non anti lock condition, meaning operational brakes.

For brake bleeding purposes it appears you could easily bleed the normal brake circuits, but would still have contaminated circuits in the booster as well as the clutch since they share a common reservoir.

Does anyone else see it that way?

Would it have been a better design if they had a separate fluid reservoir's for the booster and the clutch?

Bottom line as I see it, is that if you loose pressure in the pressure reservoir (2) you would have hard brakes. This would be the same as loosing the vacuum reservoir in a vacuum assisted braking car.


John

Adrian

I have to apologise to Sonny. I had forgotten about your problem and it certainly would have been a good example to use. I also need to make a clear clarification. Whilst I am an engineer, I try my best to look at things as the average person would, not in an engineering way, which believe me is very different in cases like these. On this board and all other forums we are not debating, well I am not, certification requirements and meeting national regulations. We are talking ordinary people, owning and driving and maintaining 964s ( I classify myself as normal and ordinary btw). I try to make my explanations easy to understand and practical. I am not into playing with words. Please read this post and the previous posts with my clarification in mind.
"NO BRAKES" in my explanation means the loss of "ALL EFFECTIVE" braking ability. When the pedal goes rock hard the average 964 driver, is going to think they have no brakes. I know I did. Some may stand on the brakes with a really heavy force. I did and it didn´t help. Some drivers are not able to provide the necessary force. I doubt that my wife Gail could possibly force the brake pedal down sufficiently to create any pressure.
This situation is what I want to be avoided. Hence my statement about when the brake warn light which I have made many times. This is a serious subject.
The following is found in the Porsche Maint Manual. For the average person and their 964 the implications are quite clear in my opinion.
Brake Test for the Carrera 4 as per Porsche Instructions
It is possible to test braking efficiency on a single-roller test stand with the ignition switched off. The roller speed must, be below, 13kph.
In order to obtain sufficient brake force boosting, the brake must be operated as often as required, directly before the test with the ignition switched on until the hydraulic pump starts running and fills up the pressure regulator. Approximately 3 to 5 braking operations are then possible on the test stand with the ignition switched off.

The C4 has a little more overall brake line plumbing than the Turbos (The pure brake system plumbing is the same and this is what is pressurised initally by the hydraulic pump). The C4 has 2 diff locks which use 180 bar pressure to operate. With ignition "ON" the ABS system is armed as is the traction control system in the C4. The circuits are physically isolated and closed. Therefore on start up only the primary brake system is being pressurized. The accumulator is being charged up from 50 to 80 bar to 160 to 180 bar.
The ABS system is a pressure removing system. A bypass system. The ABS circuit relieves each calliper of its applied pressure despite the efforts of the driver. It is a modulating system this is why the brake pedal moves under the foot. The C4 traction control system purely uses max pressure.
The question was asked "How long does the accumulator hold pressure". Well this is like asking how long is a piece of string. I say that tongue in cheek but it is true. The accumulator is pre-charged to 80 bar and can drop to 50 bar before it has to be replaced. How much pre-charge remains, system integrity. If the four ABS solenoid valves (all mounted in the hydraulic unit,the same for vacuum and hydraulic systems) are nicely sealed off there will be no pressure loss/bypass. If the dual diff locks solenoid valve assembly of the C4 is closed and not leaking into the traction control lines then there will be no pressure loss. Integrity of the brake system itself. No seals leaking etc. It is going to vary from 964 to 964 really. Do not forget the clutch circuit as well.
How many of us have had the accumulator pressure actually checked. How many of us have ever had a brake pressure leak check carried out. If the system leaks down to 100 bar in 3 hours (brake pressure warn light comes on at 102 bar) according to Porsche it is unserviceable. I have done a couple of checks myself since this all started and my brake warn light stays off after 3 hours. It takes around 8-10 hours before the light comes on. I am assuming everything is okay. However I should get it checked. Yesterday I had my oil leak fixed and a service carried out. I had my entire brake system including ABS circuit and Traction control circuit, emptied, flushed, refilled and totally bled. With the system full depressurized I was unable to depress the brake pedal. We did not try driving it. Fixed my oil leak as well but that is another story.
So the moral of the story is. When the brake warning light comes "ON" you have 102 bar of brake pressure left in the system. The traction control system and the ABS circuits will not be isolated until 102 bar. Now how long from 102 bar to no brakes or no effective brakes. Hard to say and I am not going to speculate except to refer back to my original statement. Do not drive with brake warn
light "ON" unless you can confirm the hydraulic pump is still operational. If in doubt, get help.
I am conservative yes, but better to be conservative than without brakes.
If you have confirmed your pump is failed. Please do not drive around. The accumulator pre-charge cannot power the brakes. One last point on this rather long post. The Hydraulic brake system drawing above uses the word Brake Booster. This is actually a pressure regulator valve system. Pressure porting is directly proportional to the amount of braking force required by the driver. Foot pressure and grittingof teeth. There is no real or effective usable pressure generated in this valve assembly called a booster, by the drivers foot on the brake pedal. There is a lot of plumbing full of brake fluid to try and pressurize. All I ask is for people to read and try to understand how the system works and it real or practical limitations. Saves banging the guy in front of you, up the bum, or knocking down your newly erected all electric, all singing and dancing garage door.
No hydraulic pump you have a finite amount of brake pressure left. This is not a defect, this is not a badly designed system it is quite normal. In fact you will have brakes for longer than with vacuum boosted systems. The hand brake is installed as a mechancial backup. However this system does not operational, normal redundancy built in. No second system installed which works and carries on for you normally like nothing ever happened. You cannot carry critical system component failures in a 964. When they break, they need fixing. For brakes this means immediately. Well in my opinion anyway,
Ciao,
Adrian
911C4

Young_Turk

Great to have you back Adrian, and thanks for the information. I know a lot more now.

John