Randy V

Recently my GTS has begun to exhibit ABS and Differential warnings.

Usually after driving for 20 minutes or more the brakes will start to feel 'squishy', then the ABS warning light comes on. Sometimes - but not always - I also get a Differential Control Off warning message as well. After the warnings the brakes resume feeling normal, without the 'squishy' feedback feeling. The warning lights remain illuminated.

Warnings clear after shutdown and restart.

Any ideas where to look?

Thanks.

928 DesMoines

I’m no expert, but I had similar symptoms on a 90 S4 back in the day. Had to flush the PSD. It was throwing the diff error which in turn computer disabled the abs.
could also be abs relay

dr bob

Hi Randy --

Does your first-20-mins driving include stops that might otherwise get the ABS system's attention? ABS comes into play only when the engine is running .and. you are using the brakes hard enough to get a speed mismatch among the wheels. The spongy-pedal and the warning light together suggest an event where the ABS tried to engage but failed.

Since the ABS and the differential depend on the same wheel-speed sensors, you'd be tempted to look at those early in the diagnostic protocol. I'd also be looking at easy stuff, like the ABS power supply wire that connects just forward of the power steering reservoir, and the grounds too.

The ABS unit itself has a couple relays on it. The primary power relay is a 53b type that actually runs the ABS pump motor. The motor grounds at GP X. The other relay is a special one

The controller is a combo ABS and PSD brain, easy since both systems depend on the wheel speed sensors. The Controller grounds at GP IV.

The warning lights are somewhat common to both functions (ABS and PSD), while the spongy pedal would guide me towards a sticking ABS solenoid if the pump and the motor itself check out.

----

I'd consider borrowing a diagnostic tool if you don't have one. The latest version has ABS and PSD test functions. I'm pretty sure that Rob has one and for sure Greg does. I'd probably start out with a full brake fluid system flush to make sure there's good clean fluid throughout, especially if the fluid is more than a couple years in the car. I don't remember seeing a ABS flush plush procedure similar to what we have for the PSD units, although the need is very similar. This might be a great Sharktoberfest program item, FWIW.

SteveG

I'm following this closely as I am in midst of ABS related malf. Happy to start sep thread if confusion results, Randy.

It started with an autocross day: First time for me (and the car, I'm sure). There were 4, 5-min runs on a short course. First sign of trouble was the yellow check engine light at end of 4th run. No "!" lights, just the yellow ch eng which I had never seen before. Oil, coolant were OK. So I drove the 2 hours home, no problem, light stayed on. Next day start checking fuses and relays. The XV, which is ABS seemed likely so I removed it for exam. Rained threatened so I put car inside and then all hell broke loose. When I turned key to off, The anti-theft alarm went off and the PSD text "System Disengaged/Off" light showed and the doors locked. I'm not sure what ABS has to do with anti-theft. The relay chart in the car calls XV "anti-block"??

I was able to manually unlock the door and window so I could exit and unhook the batt to shut off the alarm. The relay is dead, so I am waiting for a new one to arrive. I'm hoping the relay just gave it up during the autox exercise, but that seems unlikely to trigger a check engine light and nothing else.

Neither the green PSD nor ABS lights came on during the cornering. The rear end did break loose once or twice during the runs, and I thought it was odd no lights came on, but I guess it is possible the wheels were in sync enough to not trigger anything??

Anybody thoughts?

Randy V

Thanks for the leads, guys. No problem to tag along Steve-O!

vanster

My ABS is acting up as well. I cleaned all the sensors and it did not help. On my way to Sharktoberfest on Thursday, maybe we can chase it there.

928 DesMoines

your description of that turn should have lit up your psd I would think.
Also, if that psd green light isn’t coming up briefly when you turn the key, your PSD pump is not self testing. Any 90 MY or above should have the PSD maintenance done at least once in ownership If not more in my opinion. There is a DIY write up here on Rennlist. A failed pump will throw that diff error. However, in my case it was fluid sludge causing my problem. I got lucky.

Randy V

GB serviced my PSD 3 years ago. The green PSD light doest light up over a year now. Have replaced the cluster bulbs so know that is not the problem.

siscogts

My friend 92 gts had a similar problem. Relay ok, psd serviced 6 month before the problem.. When turning the key all was fine but I was hearing the psd pump working.... Kts and theo's tool were not giving any info... I discovered the right rear abs sensor was sending strange numbers at the multimeter.changed and solved the problem.

Randy V

Yeah, my issue seems to be pointing at a funky sensor as well.

Strosek Ultra

https://rennlist.com/forums/928-foru...procedure.html

SteveG

I didn't mean to imply the green light didn't come on briefly at start-up. I believe it did and then did not come on during the runs, but my eyes were never on the pod as I was working very hard to make the car perform. One of the instructions for the rookies that day was "Look where you want the car to go, then make it go there (cones)." If nothing else I was trying to do that. My assessment of the car regarding it's usefulness for AutoX are meat for different thread.

The PSD has been flushed twice during my ownership and worked when needed during normal street driving.

I'll follow up when I get the new ABS relay in place.

M. Requin

On my 86.5, the grommets on the brackets for the ABS leads for the rear wheels that kept them properly routed had disintegrated, which resulted in the leads dropping down and rubbing on the half shafts, with predictable results. I haven't completely rectified the problem, I still get an ABS warning light, but I am sure this is part of it. I have fabbed some new short harnesses with weatherpac connectors, we'll see how that goes.