Speedometer, OBC and Spoiler weird Problem

frf · 03-30-2008, 04:08 AM

I'm a newbie 993 owner since last February. Finally, I had the chance to fulfill the dream of a live. I've bought a MY95 993 C4 Coupe, with almost 72K km, a black leather interior impecable and the Midnight Metallic Blue painting is in a pretty good shape if we don't forget the car is almost 14 years old. I'm looking for help to a weird problem I'm having with the Speedometer, the OBC and the Spoiler. In what I've read so far, I've learned that if the speedometer dies, the spoiler will die with it, this meaning if there is a problem with the speedometer and it stops sending the speed signal to the spoiler control unit the spoiler will stop working too. However, in my case, the OBC is also not working. It stopped counting the kilometers, the consumption average is gone mad, no speed count, etc. I've also read that this problem might be related with a problem in the speed sensor, which is one of the ABS speed sensors located in a wheel. But, which one? The Porsche Workshop manual says it's the left rear wheel sensor, several forum I've searched say the left front wheel sensor and a Portuguese 993 expert I've talked with says the right front wheel. So, I've started having a look at each one of these sensors to check them.
I started the Portuguese way with the right front wheel sensor and, after having unscrewed the screw and sprayed WD40 around the sensor head I was not able to have the sensor removed because it look like glued. Anyhow, I let the WD40 working all night and, surprisingly, next morning everything was working again. So, against what I've been told in the Porsche dealer, the speedometer is not broken and is working so, the
problem is somewhere else, probably the speed sensor, I presume. A few days later, after everything having stopped working again, I tried the same treatment in right front wheel sensor but, this time, without success. So, why not to try the left front wheel sensor? And I did'it the same way and, also, without being able to extract the sensor. After a while, everything was working again until last week, when them all decided to go on strike again. So, I tried the left rear wheel sensor which, after I've unscrewed the screw, jump out quite easily. It was completely dirty with a kind of a black paste glued to the reader part of the sensor. So, I cleaned it carefully and screwed it back in place hoping that would have the problem solved. Unfortunately, it didn't solve it and I'm again looking for ideas about what is going on with this.
If it was the speedometer out of service, only the spoiler should be affected, because that's the only one receiving a speed signal from the speedometer. As the OBC is also affected, it must be something else and, as far as I have understood the way it works, the speed signal to the speedometer and to the OBC is being provided by the ABS Control Unit which, in turn, receives that information from a speed sensor in a wheel. The problem is that I have never seen a ABS/ABD warning light on and, as far as I've tested it, the ABS is working well. So, as a newbie, this problem has become a mistery to me and, my last hope, is that someone among this vast community which have faced and solved the same problem could provide me a solution.
Thanks in advance for any help, Francisco

930gt-40r · 930gt-40r's Garage

I would try and clean all of them first and see where that takes you. Are you throwing an abs light at all? is the abs acting up at all?

frf · 03-30-2008, 09:08 AM

No ABS or ABD warning lights coming up and the ABS system seems to be working OK. For me, this is the strangest thing because I thought that if, at least, one of the ABS/ABD sensors was failing I would receive the corresponding ABS/ABD warning light alarm and the ABS should not be working properly or should have been stopped.

930gt-40r · 930gt-40r's Garage

Try cleaning them all off- I think it may be the rear sensors that it reads from- which 1 I dont know.

ischmitz · ischmitz's Garage

If I am not totally mistaken the 993 has a control unit located in the fuse box that converts the ABS signal from one of the wheel sensors (>100 pulses per wheel revolution) to the speedo signal (8 counts per wheel revolution). The unit looks like a relais (small black box) but it has electronics inside. It is a frequency divider.

The fact that ABS is not complaining points to that control unit and wiring from it downstream to the speedo, OBC and spoiler. The speedo signal is used by all three units. As a first fix attempt just pull that unit and re-seat it. That might take care of the issue by cleaning the contacts due to the re-seating process.

Ingo

frf · 04-01-2008, 01:07 AM

Hi Ingo,

Thank for your suggestion but, in what I've read so far, is the ABS Control Unit that provides the speed signal. I just had a look at the Central Electric fuse and relais list and the only thing there related with ABS is fuse 16. Can you tell me the Part Number of the relais you're talking about? If so, I could have a look at the Central Electric looking for it. Cheers, Francisco

ischmitz · ischmitz's Garage

Francisco, you are right. The control unit I was referring to is used on 964. While most of the electrics are very similar between 964 and 993 the speed signal is handled different. The speed sensor signals from all wheel sensors are conditioned and processed in the ABS control unit. The conditioned rear left (RL) wheel speed signal is fed to the central informer. The central informer converts the high-frequency signal to the 8 pulses and feeds it to the speedometer. On the 964 there used to be a back box in position R52 on the central fuse panel. However, for the 993 this function has migrated into the central informer.

From speedometer the signal is conditioned and fed to all the other untis (transmission, spoiler, convertible top, heating unit, cruise control, alarm unit)

Chances are that the central informer has an intermittent issue. Maybe try to unplug and re-plug it. If the wheel sensor was bad I would think the ABS warning light would come on. But I am not an expert on ABS. I don't know if the unit only checks completely missing signals or also complains about implausible signals. As a test you could unplug one sensor and drive. If the ABS light comes on you have your answer.

Cheers,
Ingo

frf · 04-01-2008, 10:56 AM

Ingo, thank you again for your precious help. In the cars with ABD (M224) the speed signal is being provided by the left front wheel sensor to the ABS Control Unit through pin 35 and, then, is sent to Central Informer using pin 40 - if I've read correctly the diagram in ABS Sheet 4A of the Workshop Manual (the diagram images in the PDF copy I have are very bad).
I could try to go further in the diagrams but, due to the quality of the diagram images I have, it is like playing a guess game, which makes rather difficult to find an answer for a newbie like me.
Since yesterday everything is working and tomorrow morning I will follow your suggestion about the Central Informer and the unplugged sensor.
Cheers, Francisco

ischmitz · ischmitz's Garage

Fancisco, my (also bad) copy of sheet 4A 993 ('95) shows that pin 40 of the ABS control unit provides an ESO signal to a contact point labeled F57 on M224 cars. Despite the poor quality I am pretty confident that it say RL (rear left). The pin assignment on the central informer is not legible in my copy. It is the third pin from the left in the diagram. Maybe I/10 0.5 WT/RE but not sure. The roman (I) denotes the white plug. Just unplug and replug that one and see if it gets better.

Ingo

frf · 04-04-2008, 02:29 AM

Hi Ingo,
Yesterday I did a travel of about 200km and everything is still working fine. I was very careful loading the luggage in the front compartment avoiding to touch the ABS Control Unit cable in which, as I suspect, probably may reside the problem - a broken wire. I will perform further tests as soon as I return home (next Monday).
Chers, Francisco

frf · 06-07-2008, 03:07 AM

Finally, almost a thousand kilometers after I spotted the problem for the first time, the problem seems to have been solved. Last Thursday I took the car for the annual revision in an OPC in a small town in the center of Portugal. I've asked them to have a look the speedometer in order to trace the reason for the absence of speed signal.
Friday a received a call from them telling me that the problem was in a bad connection in the ABS speed sensor of the left rear wheel. The speedometer is working again and, if it fails again, I might have to replace the sensor (a new one it will cost about 80 euros). What shall we think about the diagnosis made by an OPC in the capital of Portugal - speedometer is damaged and needs to be replaced. And I had to pay 800 euros so that they can order it from factory!!! So, I'm not amazed at all with the guy from Porsche talking in the last Newsletter about the 356 as being the first Porsche equipped with a flat-six boxer engine.

frf · 06-27-2008, 07:06 AM

In order to correct my previous information I must say that the day I was supposed to get the car I received a call from the OPC to inform me that the speedometer had stopped again. After having performed further testing they detected that the problem was a defective Central Informer - the speed signal being received from the ABS Control Unit was not being provided to the speedometer. So, after having waited more than two weeks for a new Central Informer (€545,68 - 858,79 USDol) because of the strikes going on all over Europe I received the car today and everything is working again like new. I hope this will be the end of this saga.

Don Plumley · Don Plumley's Garage

Thanks for the update. Glad your car is working fine. Now just drive it and enjoy the summer - you'll forget all about the time and expense after the second apex.

Cheers,

Don

Sonic dB · Sonic dB's Garage

Just found this thread...thanks Francisco...

I have been having this very weird, intermitant problem with my car. Often it does this all day, then suddenly it switches and everything works:

The symptoms your car had Francisco....., are very similar to mine: no lights on instrument panel go on when key is turned.... except the immobilizer light.... then when car starts...no guages work at all, no speedo, no tach, nothing except the clock works...no indicator lights work either...

When this occurs, the spoiler does not work... neither does my cab top or AC.

Again, the weird thing about it is..this happens from the start up of the car, and sometimes after driving it around a while, it "pops" back and all of a sudden all of the guages and everything then works fine.

I just replaced the alternator and checked for ground issues and fuses...everything checks out fine... so Im now thinking: Central Informer is going bad...I am assuming, from reading above,? that the reason why my cars spoiler or cab top does not work during these problem occaisons is due to the speed not being sensored.

This problem started out very intermittently a few months ago, but is getting worse and worse and is happening the majority of the time right now.
Anyone have any thoughts or experiences with the Central Informer? Im thinking of getting a used one and popping it in to see if it fixes the issue...thx.

frf · 05-04-2009, 02:10 AM

First of all I must excuse for not having updated this thread earlier with the final solution for the problem I had. The Central Informer was fine and didn't have any problem because, after having been replaced by a new one, the speedometer only worked about an hundred kilometers and then it stopped working again. I have a friend who manages a Mercedes-Benz workshop and he offered to have a look at the car to find out what was the problem. And I'm glad that I accepted the offer because, it was a Mercedes-Benz electrician that discovered the problem affecting my speedometer. The ABS-ABD control unit was passing the right signal to the Central Informer which was transferring it without problems to the speedometer.
The signal received in the speedometer was OK until the speedometer was connected to the ground. Then, the signal dropped from about 12 volts to 0.5 volts which was not enough to make the speedometer work and could not activate the rear spoiler nor the on board computer. I regret that any of the Porsche centers I've visited were not able to tell what was the problem showing me oscilloscope signal graphs as I was given by MB.
So, and to resume, the problem I had is different from the one you have. As I'm a newbie like you and not an expert, I will not dare to make a suggestion on what may be behind the problem you are facing except, if you can't find a Porsche center which is able to provide you a solution... pay a visit to a Mercedes-Benz center.
Cheers, Francisco

Sonic dB · Sonic dB's Garage

Thanks Francisco for the update on your situation/car... mine still has the problem and im sorting it out...or trying to.