|
959 ABS Confusing situation...
How's that for a teaser. So, here's the story - it's developing. I've already asked a host of experts on this topic and no one has experienced anything like this so far.
1987 Porsche 959. I'm replacing the tires on the car, so I jacked up the car yesterday in order to remove the wheels. It's been many years since I did that, and I forgot that you need to have the car on the ground to first loosen the center nut using the special tool. I thought that since the tool holds the wheel steady, you could remove it in the air - it does hole the wheel steady, but it still spins when you turn the breaker bar. Easy solution to this of course (done this many times) - just have your assistant hop in the car and press the brakes. I had him do this, and stomp on the brake pedal. Weird thing - front brakes didn't work! Rear brakes were locked (although typing this at home right now, I'm not 100% sure we had the parking brake off). When I mean the front brakes didn't work, I mean I could spin the wheels (both left and right) with a single finger. Nothin. The brakes work fine when driving, although when Derek Bell drove the car the other day he thought they were "too sensitive". I've driven my friend's 959 recently and his brakes were nearly identical, so I thought nothing of it really. So, we turned on the car, and powered up the system - brakes work fine. Turn off the car, press the pedal a few times, and then they go back to "not working". Now, this system is basically the very first ABS system (by WABCO) and is fairly primitive. No one I've asked so far has been able to give me an answer as to whether the brakes are supposed to only work when the pump / system is powered up. No one has actually been in a position to really check this either. What situation would occur where one would check this, except for the accidental finding that I made? Very odd. We're going to start with the basics and go through the entire system to ensure that everything is working okay, but I figured I would post our progress here, and also see if anyone has experienced anything like this in the past? So far, everyone I've talked to said "hmm, that's odd." :) -Wayne |
Give Bruce Canepa a call and ask him and his crew. He probably works on more than anyone else in the world at this point.
|
Quote:
-Wayne |
Try Jerry Pellegrino at European Performance Engineering in Massachusetts. I can almost guarantee Jerry won't say that.
They work on a lot of 959's. European Performance Engineering Mark |
Any fault codes Wayne?
|
Quote:
It was the precursor to the 964/993 electro-hyd boosted systems. But different in that the pressure pump is a separate part from the ABS hyd control unit 2 possibilities 1? the 3 pressure switch's only work w/ key on 2) the pressure accumulator doesn't store enough pressure w/o the pump on, either through design or a leak, the 964/993 system have a similar failure if the system is left for a long time and the system bleeds down. |
I was actually the OEM ABS product manager for Bosch back in the late '80s, and worked on the development for the 964 ABS (in Germany). I do not know the WABCO system, but the key question to understand is whether it has an integrated booster in the master cylinder. If it does, you will not get much pressure, even to the front brakes, without power on.
The 964 system (and most systems after that in most cars) have a conventional vacuum boosted system, and the ABS unit (automatic hydraulic pressure release) fits in the hydraulic line system after boosting. The ABS unit (via electronic controller) relieves line pressure many times per second, according to wheel speed sensed. In these systems, the regular hydraulic brake system works "independently" of ABS (until it is needed), and you would be able to provide enough pressure in a non-running car to lock the front wheels. If the WABCO system is like this, something else is at play... |
Hmm. I took a very close look at the factory manuals this evening. Seems like the system is setup to be this way. Very unique (as is typical with the 959). In a nutshell, the brake fluid recirculates in the front corners in order to minimize brake fluid boiling. So, the fluid is not contained in a static system like nearly every other car out there, but is a pressurized, recirculating setup. Failure of the system only activates the rear brakes:
http://forums.pelicanparts.com/uploa...1692766250.png http://forums.pelicanparts.com/uploa...1692766250.png http://forums.pelicanparts.com/uploa...1692766250.png http://forums.pelicanparts.com/uploa...1692766250.png http://forums.pelicanparts.com/uploa...1692766250.png http://forums.pelicanparts.com/uploa...1692766250.png http://forums.pelicanparts.com/uploa...1692766250.png Interesting stuff. Doesn't look like this is a failure mode after all (having only the rear brakes work with the engine off). -Wayne |
Quote:
|
Probably should quantify why.
|
Wayne, PM'd you.
|
Wayne, just run down to Autozone and get a new pump and ABS unit!
Ha, I love reading about the issue on 959s, but no one needs to tell you they are a unique vehicle. I hope you get it figured out. |
Wow, thanks for posting the system details, Wayne. This is actually a bit of a hybrid system: not a fully integrated unit (where the hydraulic master cylinder and ABS valving are all in one unit), but a separated system with full hydraulic system/booster as one unit (including hydraulic accumulator), and the ABS valve controller in another. However, note that the system is always pressurized when the vehicle is running; this means the master cylinder acts more like a valve than a conventional system, where pressure comes from your foot acting on the brake pedal, through the piston to the MC (with added vacuum boost in between). The only other OEMs I know that did this were Rolls Royce, and Citroen (this is actually how modern aircraft system are set up). Very trick, but very expensive (but you already know this). Some of the other features in the 959 system are also really cool (like the high-lateral force de-pressurization of the inside wheels on a turn).
|
Yes indeed. Everything about this car is complicated beyond one's imagination!
-Wayne |
I'ld love to have your problems :)
|
Quote:
-Wayne |
Okay, so I got a call from Jerry at EPE this morning. They have a car in the shop right now, and when the car is off and they step on the brakes, the front brakes stop the wheels. This counters the information that I have, and I found it confusing. Still, another data point in the puzzle. I would like to take a look at another car if I have the chance.
I did go through the hydraulic diagram a bit further, and I'm further confused. Here's the diagram: http://forums.pelicanparts.com/uploa...1697226199.JPG I added some fancy labels. As Lyle correctly pointed out, this is a hybrid system - it apparently has a master cylinder / gate valve (that's what the factory manuals call it) - which is apparently like two devices in one. The reservoir has a total of four outlets. Interestingly enough, they are at different levels, so that if you run out of fluid, the purple circuit will run dry first, and then the light blue circuit after that. The purple and light blue are isolated so that a break / massive leak in the green/yellow side or a leak in the purple / light blue side won't affect the emergency operation of the other pair. Here's what I see. The green supply from the reservoir supplies the master cylinder portion. This is then output (22) to the rear calipers via the red supply. There is a check valve near the SV1 solenoid, but it looks like this passes the entire ABS Valve block to supply pressure to the rear calipers. Note: there is no "master cylinder" supply going to the front calipers. This is what the manual mentions (rear brakes operate only, when the system fails), and is also the behavior we're seeing on our car. The red supply also goes to a "bypass valve". This valve looks like it gets pressure from the valve section of the master cylinder. I'm guessing that when the system is properly pressurized, this bypass valve bypasses the "emergency" system that powers the rear brakes. Excess pressure in the red section would be bled off when the rest of the system is pressurized, and then "closed" when the pressure is not in the system. This would mean that pressure from the master cylinder part of the system would go to the rear calipers. Again, this is what we're seeing. The ABS gate valve in the master cylinder takes pressure from the pump and the accumulator (orange), and then distributes / modulates it from the green line? That high pressure is then fed to the ABS valve block which supplies the pressure to the calipers or interrupts the flow in the case of an emergency braking situation? The purple flow supplying the valve block shows the recirculating of the brake fluid (although I haven't been able to find where this actually is located on the car). It would appear that pressure in the front caliper section of the car (the left side of the diagram) is then fed back to the reservoir through the purple line. This makes sense that this would be a return line, as it's the highest mounted in the reservoir. Bottom line, I'm still confused. According to this diagram, when the pump stops and the accumulator loses pressure, then I only see a mechanism for the rear brakes to work. I do not see how the front brakes can continue to work if there is no pressure, the system is off, and the system is functioning correctly? |
|
Bottom line, I still think the behavior we're seeing matches both the documents in the manual, and also the fluid flow diagrams. I'm confused as to how / why the car at EBE would be showing the front brakes locking with this current setup - there must be either residual pressure built up somewhere, or a check valve is frozen?
The red section of the diagram is the “backup” or “static brake circuit” according to the manual, and there does not appear to be any mechanism to drive the front brakes? Indeed, if the engine (ABS pump) were shut off, then Porsche wouldn’t want the brake system to immediately fail? That’s correct – and it won’t (according to this diagram). The pressure accumulator is there to hold pressure in the system if the engine (actually more specifically, the ABS pump) is turned off. My system on my car did “bleed down” after a few pushes on the pedal. It does allow for several front brake stops when the pump is turned off and then stops. According to the fluid flow diagram, this behavior seems to correspond? I may be completely confused too… -Wayne Dunno... -Wayne |
| All times are GMT -8. The time now is 06:50 AM. |
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2025, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2025 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website