2003TT 89K, Stock wheels and tires. Started happening yesterday for the first time

Release brakes from a full stop, begin forward roll (or reverse roll for that matter), at under 1/2 MPH both left and right front brakes randomly pulse because the ABS is firing. Some starts worse than others, but you can actually hear the ABS unit firing off. So each side will fire off 2 to 10 pulses at strange intervals then stop once I'm over 2-3 MPH. Seems to be happening in half of my starts. Actually stalled the car the first time as I wasn't expecting it.

The only PSM light I saw (and only once) was when I was rounding a very familiar corner and the PSM kicked in when there was no meaningful slipping going on. PSM light flashed as it should, but went out after the event.

About 500 miles ago I DIY'ed a laundry list of upgrades (that's for a later post, all of them I will highly recommend) but none involved anything with the ABS/suspension/wheels/tune or braking system. I did replace the MAF with a Bosch, but am not seeing any other symptoms that OP's describe with a MAF/PSM problem.

Thinking wheel speed sensor or dirt, but why would both front's be firing all of a sudden, wouldn't it just be on one side?

Wiring Harness? Didn't screw around much in the trunk except for GT2 master/slave conversion.

Stealership right around the corner, but kills me to put it in their hands.

Idea's? Can't find any other threads which describe this condition.

I'll post what I figure out.

As a teaser on some recent DIY work...

From Kevin/UMW: LWFW, sachs disc, GT2 conversion (yes, I took a step bit to my trans), Bosch DV's and MAF, coils/plugs, 964 motor mounts, 997 trans mount, water pump (why not?).

From Markski/911 Tuning. Billet K16's, 5bar FPR, 1 bar WG, Flexible tubing intakes and the crown jewel a 3" crossflow exhaust with quad tips. Ordered a tune from 911tuning, but I can't stop driving the car long enough to ship the ECU out for programming.

And about 40 other little things from Suncoast while I had the motor out.

I could go on at length on how the car has been transformed...wonder how that chip is gonna feel?

Just need to figure out this ABS thing....anything would be appreciated.

 

If your stopped foot off the brake turn off psm do not touch the brake pedal and start off will it still do it?

 

Good thinking. Tried it, no change with PSM on or off, brake used or not (tried all 4 combinations). Same result.

Weird that sometimes one side almost locks, other times a bunch of pulses, other times it rapid fire switches from right to left. All at under 1MPH. I wonder if the rear's might be doing it as well but I certainly can't hear it BC of Markski's exhaust!

Thanks...any other idea's?

 

Unplug the psm pump that right under the brake master. If it still does it then its a straight up abs system problem. Not sure how deep you can get it to that with out the pwis.

 

Progress. With PSM unplugged, no ABS chatter. Suspects?

Thanks for jumping in BTW.

 

Don't have an answer for you (but I did send you a PM). Sub'd to see what the resolution is. I hope it's quick and easy.

 

Because the engine was removed and you did not have this fault before....I don't know if it can have the effect you are having but there is a vacuum pipe for the brake booster that you have to disconnect on top of the engine before lowering it. Do you know if it was connected properly? Located on top left side. It got a small gray plastic clip....

 

Thanks jpflip...I did re-install said vacuum line with a new o-ring IRRC but I'll give it an extra "push" just in case.

Power brakes working fine...I know because I have been beating the hell out of this car since I got it off the lift a month ago. Cannot believe how much I like all of the mods. ABS issue just showed up yesterday...and suddenly.

 

Ok. The psm pump is not the same as the abs pump two different systems. This means to me that your problem is in the psm system. That does not rule out speed sensors tho, because those are common to both systems but on the input signal side.

 

Thanks NYC. The PSM disconnect trick will get me to Saturday at least. I'll get it back on the lift, clean the sensors, hook up the durametric and see if I have any codes etc... Thanks again. I'll post findings for all.