Other day I goosed it pulling out onto a busy street and the back end started slewing around ... I noticed "Diff Control Off" appear on the Dig dash display. Never saw this before or since (I think the PSD light might have lit briefly also)

So, what does "Diff Control Off" mean exactly, and what are all the messages that are possible on the the dig dash? Are these documented anywhere (haven't seen this in the WSM, and only the most general messages are documented in the Owner's Manual)

 

I think you would have that in your OWNER'S MANUAL. I've not seen it in the WSM. I love the color of your car.

 

Diff Control Off means PSD gave up trying to lock up, usually because both wheels are spinning.

 

The only time I see that error is when I'm on a dyno. I would think that if you see it during normal driving something is wrong with the PSD? Yes, I have spun both wheels for a long time on occasion

Dan
'91 928GT S/C 475hp/460lb.ft

 

Makes sense both wheels spinning ... nothing PSD can do to help, I do remember that happening ;-)

 

I wouldn't be so sure that this is normal. Just because both wheels are spinning doesn't mean the PSD should just give up and turn itself off.

Dan
'91 928GT S/C 475hp/460lb.ft

 

PSD only works to stop wheel spin under 30mph as per the PSD video on youtube. Above 30mph it works to keep oversteer under control when cornering. Could be the light is coming on when you are out side the parameters it controls.

 

Dump the clutch on any 5 spd Porsche with PSD and watch the dash light up....guessin that if an auto could spin the tires, same thing will happen. It's normal...go back to bed.

 

I get that message when I have the car smogged. Front wheels rest while the rears spin on the dyno.

 

No, it isn't normal for "PSD off" to show up under any more-or-less normal driving conditions. It'll show up on a dyno when the front wheels are not turning. It will not show up if you're on a track, trying to do donuts, or any other such thing including laying down an "eleven" on the road, unless the PSD ECU thinks there's something wrong with the system. Now, the green light on the dash should be coming on a lot when you do those things. But, the "off" message on the LCD should not be.

It is possible that the PSD stored an error code that can be read by The Hammer. If your PSD hasn't been flushed within recent memory then that's the first place to start.

EDIT: The PSD also uses an accelerometer (under the seat IIRC). Since the PSD complained when you where "slewing" it might be that the accelerometer was producing output that the PSD ECU decided was too wacky for reality. Also, the ABS wheel speed sensors are inputs to the PSD. However, I'd not suspect those immediately because if they were doing something funky you'd likely also get an ABS-off warning a few seconds after the PSD-off warning.

 

Interesting subject.

MY 89 has a full fault code description in the Service Information Technik Manual, however, the MY 1990 with PSD was first year that it was fitted. I could not locate the fault description that you have described in the 1990 Service Information Technik Manual

However, the PSD lock control is provided on an additional platinum board in the ABS control unit. The control unit uses information from ABS wheel speed sensors to calculate:

wheel speed
wheel acceleration
wheel deceleration
mean axel speed (difference in speed front to rear) and
mean speed of car sides (difference in speed right to left for identification of curve)

A transverse acceleration sensor is located underneath the drivers seat in LHD cars and under the passengers seat in RHD cars, IIRC

Other possible input information:
operation of brakes (stop light switch)
ABS regulation
Transverse vehicle acceleration.

The control unit works in three different driving ranges:
1. Traction while moving off (comparison of wheel speeds)
2. Acceleration from driving in curves (The driven wheel on the inside of a tends to slip and propulsion is reduced when driving in curves with high transverse acceleration).
3. Ferraria effect (accelerator pedal release in a curve (rear wheel driven cars tend to oversteer when the accelerator is released suddenly while driving in a curve).

Lock Information Lamp

A lock information lamp in the instrument cluster lights up green each time the locks are operated and the rear wheel slip.

There is more information contained in the Service Information Technik manuals, I have only give an overview of what information I have to hand.

Seeing that I have not researched information further than MY1990, I would suggest that you purchase a copy of Jim Moorehouse's Technical CD to research the MYs up to your model GTS. You can purchase the CDs direct or via Roger.

The digital dash has 3 different set of software installed. The fault code you describe may be installed and described in the later model Service Information Technik manual for the 1991S4/GTs and the GTSs.

My assumption is that your car had too many parameters were being fed into the PSD CPU at one time and it has registered an overload condition and shut down. IIRC I have experienced as situation where the PSD has switched off, but I cannot remember where the indication was, ie a light indication or an indication via the fault windows. This happended when I was driving into a uphill drive was with a 180 degree hair pin turn in the wet where the back end (rear wheels) tractionend let go completely and there was probably different slip speed of the rear wheels.

I have only experienced the PSD system switching off once in 9 years. Unfortunately I don't have a Bosch Hammer or a JDS Spanner, so I service the system every 2 years with a complete brake fluid change and bleed as per the write up in Jim's technical CD, which I wrote.


If you purchase the Tech CD it will be the best purchase you will make to maintain your GTS.

Let us know what you find.

Tails 1990 928S4 Auto

 

This morning on the way to work the road was wet so I did a ~10 second burnout in second gear, both wheels spinning and the rear wiggling around. The green PSD light was on all the time, but never did I get any PSD error on the dash.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Lots of interesting information. Now I'm motivated to go spinning around wildly to see when and if I can recreate this (and to flush & bleed the PSD system of course). Maybe the parking lot at Rudy's Saturday would be a good location?

As I said originally, I've invoked PSD from time to time under various driving conditions (briefly seeing the dash light come on), but never before saw the LED message about "Diff control off".

 

Tom,
When the "diff off" light comes on that is terminal to the car.
As a big favor I will bring $2500 in cash on Saturday and take her off your hands.

We can load up the hammer if you bring the jack. Providing its dry and warm we should be good to go.
I will check with Sean as well.
Roger