PSM locked all wheels solid

[leggedj]

Hi all,

Newbie here I'm afraid... I've had a search for this one, but can't find anything obvious. If anyone's got any advice on this then I'd appreciate it!

Car: 2004 996 Carrera 4s, Manual, UK (RHD)

Driving in icy conditions today where the PSM was obviously doing some work to keep things tidy and the PSM/ABS failure lights came on then the car stalled. It was on a reasonable hill and wouldn't drive up, nor would it go down. Turned out all 4 wheels were locked fairly solidly. Disconnecting the battery made no difference, nor did disconnecting the ABS/PSM module electrics. Got it taken home on a low-loader and jacked it up to investigate - sure enough all 4 wheels were locked. Decided there must be hydraulic pressure being held by the ABS/PSM unit, so I cracked open a couple of unions where they left the ABS unit (one front, one rear) and let a small quantity of fluid out. This freed the wheels (which was good as it meant I could get it back inside my garage). I have not reconnected the ABS/PSM unit and the brakes feel OK. Applying the brakes does not result in the wheels locking solid again. I have not attempted to make the PSM work to see if the fault is reproduced and obviously I'm not going to drive the car on the road!

The error code reported was 4460, which I believe is a pressure fault. That seems plausible - there's pressure downstream which it is not expecting. I know that code is reported if the sensor is faulty or if the brake light switch isn't working, but the switch seems fine. After clearing the code, it didn't come back (but neither did the fault)

I'm guessing here, but this sounds like there's a valve in the ABS unit to relieve the downstream pressure which has failed. I'm also betting that this is not something that can be fixed and I need a new ABS unit.

Has anyone else come across this?

Thanks in advance,

Dave.

[wvicary]

I have never heard of a fault like this occuring before. I can only suggest flushing the entire brake system, clearing all fault codes and road testing the vehicle.

[Pacific996]

Yah I'd suggest a qualified porsche guy flushing the system and see if he comes up with something irregular. I'm not sure if you have had the car stored for a long time but water can enbread into the fluid and that causes expansion brake fluids don't do when heated. The icy condition I'd imagine caused a lot of braking which heated things up possibly which possibly caused any moisture to expand. Had an old hot rod that would start applying the brakes when doing long hals on the road due to water in the lines.

One of the main reasons I figure a porsche dealer doing the work is if somebody not familiar with ABS and porsche's arrangement of things trips the ABS into a warning light like many ABS cars out there will, then you'll be going to porsche or some other qualified porsche mechanic anyway. I feel if there is a bad part or system out there that they will be looking into checking everyones ABS for problems. Locked brakes isn't what a man wants flying the autobahn.

[leggedj]

This gets more interesting....

I thought rather than pay Porsche the fat end of £3,000 to look at this, I'd do some investigation. This car is a daily driver and the fluid was replaced within the last couple of years (by Porsche) so I don't think water or other contamination is a likely culprit. I put the car on axle stands and tried running it in 1st. Obviously it's not going to be too happy since the wheels are not all going at the same speed, but the PSM doesn't cut in until you're going more than about 4mph. Tried braking, and the ABS was very busy. No sign of wheels locking up and all felt well. Increase the speed a bit and the PSM kicks in. After about 10 seconds of the PSM "doing stuff", I was back to the situation of pressure in the lines locking all 4 wheels solid again. So this fault is reproducible and definitely PSM related. With the wheels locked, it now reports a PSM/ABS failure. Bleeding off a little fluid from the front left wheel makes the fault go away and the ABS performs normally again.

I then disconnected the PSM booster pump, which of course means that the PSM reports a fault and is disabled. Now it's possible to rev up to speeds higher than 10mph without the fault recurring. ABS seems OK still (but I've not used it on the road). Brakes do not lock solid even when "driving" above 4mph. Reconnect the pump and rev it a bit and I'm back to wheels locked solid.

Now this sounds to me as if there's a faulty solonoid in the ABS/PSM unit, but I've not come across a block diagram of how this unit is put together. I know there's 10 solonoids in it - 4 to stop pressure being applied to the brakes, 4 to relieve pressure from the brakes and the other 2 must be to allow the pump to swap between "pumping out" and "pumping in". I don't understand how the PSM can "lock" pressure in the lines, yet the ABS (and normal brake function) seems to work just fine. Remember when the fault ocurrs the pressure is never released - the only way to get the wheels to turn again is to remove fluid by bleeding. Even disconnecting the battery for a week didn't release the pressure.

I suspect that this will turn out to be a "replace ABS/PSM unit", but I wanted to be sure before shelling out £2,000 for the part.

Any further insights?

Cheers,

Dave.

[wvicary]

Possible to get a used unit from a breakers yard? How did you disconnect the PSM booster pump as I thought only one pump was used for both the ABS and PSM systems. I would still flush the system as maybe debris is causing a problem with a solenoid valve.

[carrera3.2]

OK, this may not be a relevant observation, but I had a situation many years ago on my 1986.5 928S, with ABS, where the brakes would come on by themselves while I was driving at freeway speeds. They essentially would lock up entirely, as if you were standing on the brakes. Very scary! It turns out that the cause was a failing brake master cylinder vacuum booster. I replaced it and the problem went away. It was too long ago for me to remember all of the details and the 928S, of course, did not have PSM. Does your wheel lockup occur even when you turn off the PSM?

[leggedj]

More investigation... I laid hands on a 2nd hand ABS/PSM unit which was apparently OK and the problem is still present. Of course, it could be that the replacement ABS/PSM unit is also faulty, with the same fault, but this seems unlikely.

I'm now convinced that the fault lies somehow in the master cylinder. I came to this conclusion with the following test: Make the PSM operate by running the car with all 4 wheels raised. Wait until all 4 wheels are locked solid. Disconnect the unions from the master cylinder 1 at a time. When each union was loosened, about 5ml of fluid was forcefully ejected and the wheels were released (fronts with one connection, rears with the other).

This almost makes sense with the discovery that electrically disconnecting the booster pump (which disables the PSM but seems to leave the other braking functions active) stops the problem from happening. The booster pump can introduce additional fluid into the system (it has a line from the fluid reservoir going into it), whereas the ABS etc cannot as it just works on what's in the master cylinder.

So - given that fluid can get from the reservoir into the master cylinder (I can bleed the brakes with a pressure bleeder), why can't it get back? Is there any sort of valve that prevents this? If so, I'd guess there's one on each circuit and it seems odd that both have failed. I can't see any electrical connection into the master cylinder, so I'm assuming it's a mechanical problem. Tempted to replace the master cylinder just to see if this fixes it - it's not a particularly expensive part.

Cheers,

Dave.

[carrera3.2]

You might want to perform the following test: with the wheels "locked" and in the air, go to each brake caliper, momentarily open each bleed valve (don't leave it open) and see what happens in every individual case. Is brake fluid expelled under pressure? Does the brake rotor come free?

It is quite possible for enough back pressure to develop in the brake system to prevent the brake pads from releasing from the rotors. If you think about it, the hydraulic/mechanical advantage when you step on the brakes to direct the pads against the rotors is immense. On the other hand, there is little back pressure to release the pads after you remove your foot from the brake pedal.

Your car is likely not old enought to have had much interior deterioration of the flexible brake lines. However, as Bruce Anderson has repeatedly warned, just because the outside of these flexible lines looks good does not mean that the interiors have not become deteriorated from constant exposure to brake fluid. What commonly happens is the the flexible material swells on the inside, reducing the actual diameter and making it more and more difficult for the pressure to be released from the brake pads after you remove your foot from the pedal. Like cholesterol in human arteries. You may have small pieces of debris in your system.

I have personal experience with this. I bought a 3.2 Carrera that appeared to have flexible brake lines in excellent condition. I flushed the brake fluid. However, when I drove along narrow streets where I could get feed-back noise from the car, I could hear what sounded like dragging brake noises. There was no serious indication that anything was wrong. One day I took this car to a track (instead of my usual track car). As I got more used to it, I pushed harder. Finally, I was doing about 100 mph on a straight that ended in an abrupt right. When I got to my braking point, all Hell broke loose. The front brakes completely locked up and I ruined my new tires by flat spotting them. New flexible lines solved the problem. The old lines appeared to be original and were then about 17-18 years old.

[wvicary]

I had an Audi once that would apply the front brakes after 20km's or so of driving and was the master cylinder at fault.

[leggedj]

You might want to perform the following test: with the wheels "locked" and in the air, go to each brake caliper, momentarily open each bleed valve (don't leave it open) and see what happens in every individual case. Is brake fluid expelled under pressure? Does the brake rotor come free?

Yes, you get a small release of fluid (about 0.5" in the pipe I'm using). This then releases the caliper and the wheel can be turned by hand. Now, you don't need to do this to ALL the wheels, just 2 of them. I can't remember how the wheels pair together - I'd have to do it again to check that.

Interesting about the brake hose deterioration - these will be the original hoses. As you say, they are only 5 years old so likely to be OK, but I've not checked them. I think the fact that releasing the pressure from the master cylinder releases the calipers probably indicates that the hoses are not causing the problem.

Cheers,

Dave.

[leggedj]

Possible to get a used unit from a breakers yard? How did you disconnect the PSM booster pump as I thought only one pump was used for both the ABS and PSM systems. I would still flush the system as maybe debris is causing a problem with a solenoid valve.

Hi,

Just to answer this - there's a pump on the ABS/PSM unit and there is a second "booster" pump between the master cylinder and the PSM/ABS unit. This pump has a 2-wire connection back to the ABS/PSM control unit and that is what I disconnected. This pump also has a low-pressure hose connected to the reservoir which I assume is required when more fluid is needed when the brake pedal has been depressed enough to cut off the reservoir from the master cylinder.

Cheers,

Dave.

[mattatk]

Dave,

Did you find out what caused the problem?

I've just experienced a similar problem with my 2001 Boxster S (986) with PSM.

- Reasonably heavy braking into corner & spirited acceleration out of the corner led to brakes locking on & PSM OFF & ABS lights on the dash illuminating.

- Pulled car over, turned off & restarted. Brakes still locked.

- Disconnected battery for 5 minutes & heard a few possibly ) gurgling fluid noises forward of the dash.

- Reconnected battery as the brakes had released.

- Restarted car & ABS PSM OFF lights exinguished.

- Car drove normally home.

Any suggestions please?

Cheers,

Matt

[Loren]

Dave,

Did you find out what caused the problem?

I've just experienced a similar problem with my 2001 Boxster S (986) with PSM.

- Reasonably heavy braking into corner & spirited acceleration out of the corner led to brakes locking on & PSM OFF & ABS lights on the dash illuminating.

- Pulled car over, turned off & restarted. Brakes still locked.

- Disconnected battery for 5 minutes & heard a few possibly ) gurgling fluid noises forward of the dash.

- Reconnected battery as the brakes had released.

- Restarted car & ABS PSM OFF lights exinguished.

- Car drove normally home.

Any suggestions please?

Cheers,

Matt

Check for water or moisture damage to the brake booster.

[mattatk]

Booster looks great, no obvious leaks/damage.

Master cylinder has a few drops of fluid hanging from underneath. Looks like a slight leak from reservoir attachment points to master cylinder.

Rather than replacing the master cylinder or reservoir could some form of silicon sealant be applied to the leak?

[Loren]

I would not try and seal a hydraulic system (especially brakes) with any sealant. For your safety any leaking parts should be replaced.

[mattatk]

I'll order a new reservoir & master cylinder for starters then. Thanks again Loren for your help.

[mattatk]

Just out of interest, I wasn't aware of the brake booster check in the Bentley Manual...

With engine off, pump brake pedal approx 10 times.

Start engine with brake pedal still firmly depressed.

If brake pedal sinks a bit lower on starting, then booster should be OK (I guess it's due to the vacuum assistance coming in)

[dennisvdw]

Hi dave!

What ended up to be the problem with your car? I have the same problem with mine, 996 4S manual6 2002 120.000 KM.....

grtz Dennis

[dennisvdw]

Dave???

[Loren]

Dave???

You replied to a two year old post.