leggedj

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.

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.

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.

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.