I have this problem on my car as well and I found out that it's the PSM unit that gets "spunky" for some reason. The problem happens when I'm going fast and braking hard on a bumpy surface or braking in a fast turn. I solved the problem by disconnecting the electrical line to the pre-charge pump this disables the PSM fully but keep the ABS system fully active.

 

I got a PM from 996TT_STEVO but I'm not allowed to answer him(smart function? NOT!). It is the cable that goes from the ABS unit to the pre-charge pump, the pre-charge pump is located under the Brake master cylinder. The ABS unit is FULLY Active.

 

You know this sounds exactly like my track toy problem. I have a 94 rx7 for the track and usually runs with nt01 or dunlop racing slicks...i have upgraded brakes etc...2600lb with 610hp...and while testing my new set of hawk HT14's I experienced serious lock up only when pushing hard for a long period of time. No one knew what it was until we called up the ST champ and he helped chime in. He said it was the temp of the brake fluid that was causing my ABS valve (in the abs unit) to fail. When temps hit sky high the valve would stick and not release pressure. We changed the entire ABS system (including abs unit) and added heat protecting wraps because the engine bay does get really hot from the large snail... not taking any chances at 180mph...now I can push hard and have not had a similar scenario since. He also recommended to go with 2pc wide and tall rotors...but thats not been necessary. Since then i hate thru my wiring harness that killed the abs pump...low is good but too low is slow.

Hope this helps in anyway.

 

I had a situation similar to this in a mod'd NSX with Brembo brakes, ended up that the ABS sensor was contaminated and the ABS pump needed flushing, I'd guess these directly related to each other.

[subscribed]

 

KWERKS and D33PBLU3

I don't think this is a problem with the ABS unit because when I disconnect the Pre-charge Pump for the PSM unit the brakes and ABS works just fine. I think this is a problem with the PSM.

 

I get the brake fluid level light on track, unless the fluid level is precisely at the tip of the arrow marked MAX. It seems that even a mm over or under eventually lights it up. It also occasionally triggers the red WARNING about brake system failure. Students riding in the car sometimes get a little edgy when they see it, and I just flip the lever off and keep going.

I once had a lockup under heavy braking at Turn 14 at Road America, but that was because I had hosed up the alignment and the steering angle sensor was off, so the PSM turned itself off.

The fact that you get yours under light braking does suggest a stuck piston somewhere.

Jon

 

Thanks Speed

 

Any updates on the problem?

 

That is a crazy story for sure. As I understand ABS/PSM light did not pop up. How are you going to assess the issue if you have not done so?

 

I have had this exact same problem in TWO of my Cars at the track. The first was in my 944 turbo... the problem was with larger stickier tires.. on VERY hard braking the brakes would just stop functioning.. What I found out is that with so much more grip and with the track orientated tires, the ABS module goes haywire.. It is calibrated for a specific set of parameters and specific tires etc.. When we change those parameters, the ABS module can't deal with it.

The same issues happened with my Corvette Z06, but it was even worse.. rather than the brakes ceasing to work for a bit.. the Corvette brakes totally locked my front left wheel.. i MEAN locked solid and flat spotted my tire and caused me to go off track at a high rate of speed.

On NEITHER occasion did my ABS warning light come on to give any indication that there was an issue.

On the 944 Turbo, I got a brake proportioning valve to change the front to rear split percentage and moved it closer to 60/40 (stock was 80/20 I think) and I didn't have the issue again...

 

This is a scary behaviour and I had it happen twice in a factory 1998 993 RSR racecar with a 500+ hp TT engine swap. The car also had 4 caliper brakes for endurance and an ABS unit that I have to research more to find out what it was.

Temps were fairly cold, say 18 celcius, appr 70 F, it was qualifying so only had a warmup lap and this was the fast lap. Speed was about 265kmh at the end of a very long straight of Scandinavian Raceway. I nearly rammed a competitor due to this who was braking hard for the upcoming turn, it said "snap" when I hit the brakes and the wheels locked with no release even as I modulated greatly / foot off the brakes. Luckily it let go about a car length behind the competitor and I steered of the track in the opposite direction of the track and surfed throught he Leca-pit.

There were loong loong black flat lines from the slicks being locked up.

My team went over the brake system, looked for any bad sensors etc. Could not find anything. Later the same weekend, with about 30 minutes to go in a 3h Endurance race, I brake at a different spot, lower speed, brakes say "snap" and locks up, rear mainly this time, car danced of the track and got stuck in the swampy grass. DNF instead of podium.

I never happened before in this car, and never in the following races. It is interesting to read this (and worrysome) as no one ever found out what happened, and it never came back in the car.

My guess is ABS unit, valve in the unit or something like that. Please proceed the investigation!

 

Oops guys! I have missed updating this topic. I noticed this once again after years as I just went to my User CP and it shows all those negative reps from this topic. Somebody commented that I must learn to drive. Yep yep.

So, the whole situation once again: We had two 996 Turbos with these front brakes locking up (and the pedal stays in). Both upgraded to GT2 front brakes, other car was fine at once but I still had the problem. Local dealer remembered a case with either 964 Turbo or 993 Turbo and they ended up changing the ABS hydraulic unit.

I first tried all kinds of tires, more cooling, all sorts of parts inspected/replaced, factory diagnosis did not find anything strange either. I then went to the dealer to install new front speed sensors and no help. This was quite a bit of work as I had to drive to relatively safe race track to always go and test the updates and setups. The locking up did come only on hard driving. Then, the next step was installing a new rate of turn sensor. (Factory had newer part numbers available for all these sensors I changed). As these all parts are related to brake system now the car started to work! Of course, if PSM/ABS error lights visible, one has to be careful...

Notice that during these years or so I have drive all the time A LOT on tracks and the days are no 'education'...Anyway, I have not hit a wall even though many front tires have been destroyed due to flat spots.

So no problems anymore, but never say never. Because, it seems to be clear that the Turbo ABS (in contrast to GT2 and GT3 and especially to Motorsport versions!!!) is not suitable with really hard tracking: You can sense it! I am quite sure there would be problems (in my case locking, in your case something else perhaps) if you drive with slick tires and use the most aggressive racing pads and overall aggressive setup. It is no surprise people use new braking systems in all race car 996 Turbos etc.!

I have used r-compounds and sticky PFC 01's without problems, however the bite is somewhat too much for the ABS that I have went back to PFC 06's (oh, the durability may play a role as well) as the Turbo ABS is slowish with its reactions. So, the stock Turbo ABS seems to be a limiting factor quite soon, at least in my perspective and with this very car. Others may agree or disagree. You Cup/GT2/GT3 guys have it good

Hope this helps! I am quite relieved; no need to sell the car and how would you sell a car with a mystical problem! What I must say, I have had my fair share of driving back home from tracks with a single flat spotted front tire...Not nice...

Thanks for the various attempts to help!

 

I love you for this post...... Seriously.


You know, the Bromance kind of love.

 

Really, negative reps for this topic? This sneaky garbage is pretty annoying. People should have to man up and leave their name to give negative rep. This system lets people talk behind your back directly to your face.


That being said I was going to say that if you are engaging ABS on a regular basis in the braking zones that you are braking too hard and need to be smoother on to the pedal.

That was until I read that it was happening with very little brake pressure while just slowing down behind other drivers. If you aren't pushing the brakes very hard and ABS is coming on (while the tires are hot) there is definitely a problem.

I'd look into the brake booster, they can have funky issues sometimes and there could be an issue with the vacuum.

Personally I'd try a safe track and take out the ABS PSM fuse and see if the problem occurs again. If not, then you have narrowed it down to the PSM and ABS systems at fault. If so, then you know it's one or the other. Then try Speed's recommendation and disable the PSM completely. And so on.

 

Getting back on this thread, I had this lockup happen two days ago. It came gradually though, the fluid got overheated, the PSM/pump went frenzy and partially started to brake and within the next 1-2 seconds the brakes locked. Relatively slow place of track though and with a big banked turn so stayed on surface.

The problem for me seemed to be that with my car that has a little more "roatation" (oversteer) than the core PSM-function accepts (core PSM-function meaning even when the PSM is switched OFF), PSM was jabbing the brakes constantly and overheated the fluid in the brake system and somehow the whole brake unit went stiff pedal, pedal stayed in, and brakes would just lock. It took some cooling time and two restarts of the car before the unit would release the pedal again.

I removed the fuses from the entire PSM and ABS system and ran without and it worked fine, just had no ABS function anymore. It works but overall brake capacity sucks since basic bias is very much towards the front. My PBox shows I could brake 0.6G at best before locking up. At Cali Speedway in November I could brake at nearly 1.3G with electronic brake bias active (= ABS unit connected). At Cali speedway I did not have this problem, longer straights, less transitions to set off PSM.

Now, I want to try the pre-charge disconnect job, to find out what it does exactly. Did anyone else try this? Does the core PSM function still engage, or try to engage?
Or does the disconnect give PSM error code/shutdown right away, or on the first time PSM is trying to work? I guess I am trying to find out that if ABS is working with this pre-charge pump disconnect trick, will that only shut off PSM, or also shut off what I call "core PSM"?

2nd I want to know more about is the yawrate-sensor - is it just an improved sensor allowing wider "slides", which will still engage PSM if my slip angle is too large? (I suspect it will). Any part number for this?

It'd be nice for everyone to have this "myserious" brake issues fully sorted.