evilruss

Purchased my 01 996tt a little over a year ago for the daily driver and so far absolutely love it. Put almost 30k miles on in the time I've had it already, and just installed the 3rd new set of rear summer tires. Previous two sets were continental conti2's and this time went with Sumitomo HTR ZIII's. The conti2's in the front were installed with the first set of conti2's in the rear, and since they are still having 50% or more tread left I decided to just swap the rears. And decided to try running the car rwd for a bit to try it out, and avoid any issues with too large of a OD difference front to rear before I replace the front tires with the matching HTR ZIII's.


After reading a number of posts about rwd conversions I just dropped the center driveshaft disconnecting the front diff from the transmission. Definitely notice psm kicking from the open diff in the rear under hard acceleration. The weird behavior that I noticed now though, is sometimes on hard rights like cloverleafs or even a few times trying to make a hard right lane change at speed I can feel the abs start pulsing. Seems fairly random when it happens, but I believe its just the front right wheel, and then the car plows straight ahead. At these times I wasn't applying any brakes until it happened, at which point I hit the brakes to slow down.


At first was thinking maybe it was an issue of the front diff and axles still being connected and now instead of the diff having drive from the transmission it's only being driven by the front wheels. I just reinstalled the center driveshaft a couple days ago after looking closer at the OD specs for both sets of tires, and seeing they are almost identicle. Was thinking that would stop the cornering issue, but its still happening. The Conti2's Fr/Rr are OD 26.1"/26.3" vs HTR ZIII being 26.1"/25.9"


Tried to start keeping the PSM off now for 2 days and that seems to keep the issue from occurring. Forgot to turn it off once and abs pulsing happened on the first cloverleaf I went around. I'm assuming the PSM is taking wheel speeds into account and now the slightly smaller OD of the rear wheels might be the culprit. Curious if anybody else has some thoughts on this. Was planning to order the matching front HTR ZIII's shortly to get the brand/model tires matching again, but if there is an issue with the rear tires being slightly smaller I'm thinking that might just make it worse.

Road King

When I first bought my car I had the same issue with what I thought was the ABS pulsing under hard cornering...freeway onramp and even tighter twisties. Really freaked me out when it occured, as I didn't think PSM was supposed to be that intrusive.

My front tires were 1/2 worn, if not more at the time. They were old Falken FK452s, and very noisy. My rear tires were brand new Falken FK453s with under 1000 miles. I bought new matching front tires (FK453s) and the issue went away immediately.

If I were buying all 4 new tires, I probably wouldn't have gone with the Falken FK453s, but I've got 'em and they seems to do fine for my driving style, which is mostly dry backroads and canyon runs. No tracking for now. BTW, Falken is the flagship brand of Sumitomo, which I didn't know until recently. I'm sure that guys running PS2s and Pirellis may have a bit more dry grip, but I really can't complain about the low priced Falkens at this point in time.

leftlane

Those Sumis are so cheap that you should replace all 4. Mismatched treads and wear from front to rear can make the car squirrelly for sure.

rmc1148

I refuse to run mismatched tires and agree that new fronts or even better full set will fix problem.

evilruss

Thanks for the info! Sounds like you had exactly the same issue I'm having right now with the brand new tires on the rear and half worn tires in the front. I checked the OD on the FK453's you have and they match the sizes of the HTR ZIII's that I misquoted earlier. They should be 25.1/24.9 not 26.1/25.9 Guess I'll get some new front ordered up then and see if that cures the issue.

Kevinmacd

Yup tire mismatch! The only fix get a pair of sumo for the front? You have to remember the tread on the front are worn down, so the diameter difference may be a little more progressive than you think! Been there done that!

Carlo_Carrera

Yes, mismatched tires.

"02996ttx50

the car will be far less sensitive to tread depth differences/rolling diameters, and even non existent ( well ..) once even the cardan shaft is removed.

if you are/ are? running open diff ( one wheel drive ) now and have pulsing abs issues? i would less suspect the tire tread depth differences, as the ecu isn't as concerned about nor would be as skittish, simply for having eliminated the awd however temporarily or crudely. gotta get new tires and go rwd with an lsd of some type to truly maximize the potential of the car. snow excepting lol

but..could be other stuff. but if awd? sure. all mismatched front to rear tire sets/depth will make orange dash lights and squirellyness happen.

Macster

While it feels like the ABS is being triggered unless you are engaging in hard braking it is not ABS.

What you are probably feeling is some front tire -- inside front tire -- scrubbing during the turn.

I get this on my Boxster (and Turbo) at very low speeds with lots of wheel lock when pulling into or out of a parking space. But sometimes I notice this at higher speeds in sharp turns especially when the front tires are close to being due to be replaced. While they do not get that worn they get hard from the heat cycles and their grip drops off.

At lower speeds the scrubbing occurs at a low frequency and is unlikely to be mistaken for ABS.

But at higher speeds the scrubbing occurs at a higher frequency that can be mistaken for ABS.

evilruss

I'm fairly confident it was the abs being actuated. Very pronounced sound of the pulsating pump and when I went to immediately apply the brakes the pedal had extreme resistance and was fighting against me trying to apply pressure to slow the car.

Every instance on a clover leaf I had already slowed to an moderate speed for taking the corner and was nowhere near the limits of the tires which is where it gets very unnerving as this pulsing causes the car to start plowing immediately. No flashing light in the dash either. The one instance it happened on the freeway the road was sweeping right and I went to make a quick lane change further right going about 60mph, and again the sound of the abs pump and the car immediately starts plowing straight ahead.

evilruss

Just went out and measured the tread on the front and rear tires. Was also a little curious if maybe the fronts had worn slightly more on one side or the other but they are fairly even at around 6/32" tread tapering to 5/32" on the insides. The front conti2's were brand new last July with the first set of rear conti's going about 8,000 miles. The second set of rear contis' this spring went about 6,000 miles before I just put on the new Sumitomo's.




The new HTR ZIII's rear tires are down to 9/32" from 10/32" after about 1600 miles, so I'd say so far they are lasting much better than the Conti's.

Macster

Well, that's not really ABS behavior. If you recall ABS is active under hard braking when the speed sensor detects wheel lock up and the brake pressure then is momentarily released then reapplied. This allows the locked up wheel/tire to rotate and regain traction so steering is effective.

What is sounds like is what I touched upon in an early post is the front tires are scrubbing and generates that high frequency vibration that can feel like the ABS is doing its thing -- though the few times I have actually triggered ABS on my car (only the Boxster so far not the Turbo) is the ABS cycles at a much slower frequency.

The plowing behavior you mention of the front end tends to reinforce my belief the front tires are scrubbing/slipping and ABS is not the culprit.

If the tires are worn and hard from heat cycles -- they can get real hard feeling almost like the front plastic wheel on a kid's Big Wheel trike -- and their grip goes down.

evilruss

Ok, well I could be completely mistaken. I need to read up more on the 996. I just assumed the PSM system uses the abs for modulating the individual wheels. Felt very much like when I was fooling around in the snow over the winter, psm seemed to kick in and had what sounded like the abs pump activating as it was trying to stop me from playing. To get the car to slide or drift at all I had to turn off the PSM.

Kevinmacd

I still feel this is a tire mismatch! The PSM monitors wheel speeds, yaw, under steer and oversrteer. It's integrated into the ABS system to control the speed of the wheel that is causing the PSM to engage?

evilruss

Ok, that's kind of what I was wondering. Seems to me like the current worn fronts and new rears might be causing wheel speeds to read in a way that on a sharp low speed turn, or quick attempt at a lane change at freeway speeds that are off enough that the PSM thinks the car is getting too far out of shape, and its trying to correct the car back to a more forward direction by pulsing or slowing the front right wheel.


As Sean said he had the same exact issue with basically identical conditions, half worn fronts and near new rears. I already have the fronts on order so see if that corrects the issue.