ABS, Differential, Speedometer not working
 

I've had this intermittent problem for over a year. Now it's not intermittent. 93GTS 49Kmiles.

I have all three things happening at the same time. Warning on the digital dash for ABS off, Differential off, and my speedometer/odometer won't work. I also have the small yellow ABS light on the dash, and after about 5 seconds from starting the car, the small red light comes on that has the diagram of 4 wheels connected together.

Here's what I've done so far. I've cleaned all 4 wheel sensor ground points. I've cleaned the following ground points: battery strap, inside spare tire area, inside left front fender where the ABS hydraulic unit is, front of engine compartment. I've unplugged and plugged the relay and fuse in the central electric system, the two relays on top of the ABS hydraulic unit, and the three multi-pin connectors in the spare tire area.

After I did all this, I was looking up under the dash and I found that the plug that plugs into the ABS electronic unit was UNPLUGGED! So I plugged that back in, all the way so the metal tab clicked in and it was secure.

Still have the same problem.

I need help with what to do next. Is there a way to test relays? Was something damaged because of the ABS plug that was unplugged?

There must be some connection between the ABS, PSD and the speedometer. I'd understand if it was just the ABS/PSD, but why the speedo too?

Any help would be appreciated.

Have you tried to erase the ABS/PSD fault codes by disconnecting the battery? PSD (differential) is always disabled when there is an ABS fault code stored in the ABS/PSD controller.

Unlike earlier cars, on your year model, the speedo signal is taken from one of the ABS speed sensors at one of the front axles.

WSM warns about plugging or unplugging the ABS/PSD controller plug with the ignition on.

You didn't say anything about cleaning the brake dust from the ABS speed sensors. The magnetic dust collects on the sensors, eventually making them non functional.

I'll try disconnecting the battery.

I read through the WSM's and noticed that same warning.

Do you mean the gear-like things on the wheel hubs? The front ones were cleaned recently. If not, please be specific about what part to clean.

Thanks.

The front and rear sensors are different. On the front, I believe I cleaned mine when repacking the front wheel bearings. You might be able to remove the sensor and clean it without removing the wheel hub. They are magnetic so they attract brake dust which is mostly iron oxide.

Look at the Porsche "ABS service guide" on the Morehouse CDs, it shows how they mount and look.

the speed sensors that Borland is referring to are bolted into the front spindles and the rear hub carriers, you should try to remove every sensor and clean it , put some grease on the sensor before installing it so it wont corrode , also take a look at the ABS ring that the unit runs next to by looking thru the hole of the sensor try to clean off the speed rings with a rag wetted with some brake cleaner. Also check the connections in the junction blocks at every wheel, You may have to get a spanner to clear the fault codes

I have all three things happening at the same time. Warning on the digital dash for ABS off, Differential off, and my speedometer/odometer won't work. I also have the small yellow ABS light on the dash, and after about 5 seconds from starting the car, the small red light comes on that has the diagram of 4 wheels connected together.

There must be some connection between the ABS, PSD and the speedometer. I'd understand if it was just the ABS/PSD, but why the speedo too?

Greg
The diagram of 4 wheels connected together is the RDK tyre pressure warning system. The car normally has to be moving to initiate it. It appears that all these faults have a common point. Speed sensor. The right way to check it is with the diagnostic tool known as the Bosch hammer or John Speake's
'spanner'.

Colin. 89GT

Speedo uses one ABS sensor from mid '90 MY IIRR. This was very practical move from Porsche as ABS box needs exact same speed info as speedo. There was no need for diff speed sensor anymore.

At first one rear sensor was used and in '91 this was changed to left front wheel IIRR. So all it takes in GTS is that left front sensor is dead and all three systems are down.

I only just got the RDK warning light after plugging in the plug for the ABS electronic unit.

At least I think it was the ABS electronic unit. It wasn't exactly where it said in the WSM. In the WSM it says the ABS electronic unit is above the hood release lever on LHD cars. But the unit that I plugged in was under the dash, just above where my left knee would be while driving. Silver box @4" x 5" with "Bosch" and I remember the letters "RKS" on it. Did I plug something else in that had to do with the RDK tire pressure warning light? Maybe that's why it was unplugged (on purpose)?

That's because RDK is a separate controller. You've mistaken the controller. Tire pressure has nothing to do with the ABS/PSD function.

I know the tire pressure has nothing to do with the ABS/PSD. I don't know what unit I found unplugged under my dash.

I cleaned the front left ABS sensor, cleaned the ground wire connection on the front left again, disconnected the battery for a few minutes, and I'm still having the same problems.

Where do I go from here? Is there a way to test an ABS sensor? I don't want to start replacing parts without knowing they are bad.

All the controllers have part numbers identifying them. You could check these against the PET parts database. I'm sure you'll find that it's the RDK, and in the correct location, as depicted the WSM wiring diagrams for your GTS.

Unless you know someone with a 928 diagnostic tester, you're better off taking it to a 928 mechanic with one.

You could swap the front left and right sensors and see if you get the same result, but swapping parts is a poor way to troubleshoot the ABS/PSD system. You can see in the guide, that Porsche had an elaborate tester made up, at a time when the Bosch Hammer was non-existant.

Maybe someone else can suggest a plan. Good luck.

I have confirmed that it was the RDK unit that was unplugged and not the ABS electronic control.

What tire sizes are you running? If the tire sizes are too wildly different, I think there is a possibility of the PSD/ABS system getting confused if fronts and rears are way off.