ColinB

Recently I carried out some repairs to my instrument cluster including replacement of the printed circuit boards. I was most surprised to find that very little information was available on Rennlist or elsewhere so with the help of several other Rennlisters I’ve done some research. I’ve collected quite a bit of material and would like to record what they and I have learned for the archives. It is applicable to ‘89 and later digital instrument panels.

The digital instrument panel introduced in 1989 is of VDO manufacture. It incorporates the warning system adapted from previous years and the trip computer but it also passively records data, especially fault information on several vehicle systems, eg ABS, Coolant pressure, Oil level etc. The fault information can be read in summary directly from the display or more comprehensively by a Porsche workshop’s diagnostic equipment. Instructions for the use of this feature were published in the 1989 workshop document WKD 495 821 entitled ”Diagnosis of the instrument cluster”.

The heart of the digital instrument cluster is the printed circuit board (PCB) of which about 30 variations were produced. They were progressively developed with successive model years, each with a different part number and a software ‘level’ tailored to the various requirements of the countries of intended sale and the specification of the vehicle. For example, manual or auto, MPH or KPH, airbag or not, Saudi Arabia speed warning buzzer or not, etc. For this reason the boards are not completely interchangeable.

The PCB is in two sections joined by flexible links and is fitted into the black plastic housing which forms the rear of the instrument cluster. The innermost of the two sections is glued into place and is not easily removed from the housing though the housing and PCB assembly can be unhinged from the main section of the instrument cluster. This is also the method by which the instrument bulbs can be replaced once the pod and cluster have been removed from the car. They cannot be changed in situ.

Failures of the PCB are fortunately not common. They can be caused by damage to sensors or other hardware outside the instrument cluster and by transmitted shock from severe impacts even if the instrument cluster itself is not directly involved. Spontaneous failure has also been reported. Symptoms include inoperative or erratic gauges, inoperative, always-on or flickering warning lights, failure of the trip computer and/or warning system, and total or intermittent blackout of the panel. It should be pointed out that several of these symptoms can also be caused by loose or dirty edge connectors on the PCBs.

Due to the sandwich construction of the boards some circuits are embedded within the board material and are inaccessible. A competent electronic workshop could probably effect repairs to the external components but Porsche appears to have published no documentation or drawings of the circuits within its technical literature and at present replacement of the circuit board is the best option. New boards are in very short supply. At the time of writing (Oct ‘04) only four boards remain in stock in Germany at a cost of about $1000 each though individual dealers may of course have stock of their own. This remaining stock are of P/N 928 641 991 30, a late model configuration which should hopefully be backwards-compatible with most others.

Language and value changes
Depending on the software status (level) of the circuit boards installed certain information displayed on the instrument panel can be changed to suit the owner/driver. The software status is shown in the square centre box of the computer display when the special tool 9293 or jumper, described later, is fitted. It is also printed on a rectangular paper label stuck on the back of the instrument cluster and inside between the two circuit board sections.

The digital panel was first introduced in 1989 with “software level K18”. The dashboard language could be supplied in German, French, Italian or English and could NOT be changed but on US market cars the units of measurement could be changed from Metric to Imperial and back by selecting the appropriate display function then pulling the operating stalk twice within one second.

From MY 1990 and software level K25 the language could be changed by a Porsche dealer by the use of special tool 9293 fitted into the19-pin diagnostic socket under the floor panel between the passenger’s seat and the door. In fact the special tool is merely a bridge between pins 5 and 13. A wire jumper will substitute. For those wishing to do this the language change procedure is.

Turn on the ignition
Fit tool 9293 or the jumper (NB, it won’t work if the jumper is fitted before the ignition is turned on)
Hold back the control stalk for 3 seconds until “LANGUAGE” appears in the display.
Pull the stalk sequentially to scroll through the four available languages.
Turn off the ignition and/or remove the jumper to store the language displayed.

The language and the selected units of measurement revert to the default if the battery is disconnected or the instrument power supply is lost by other means.

From MY 1991 and software level K26 the selected language and units were no longer lost when the power was disconnected. If a new circuit board was fitted K26 also provided for the reading of the odometer display to be set to the vehicle’s correct total distance travelled but note that this only applies to a factory new circuit board or one with less than 250 miles or km on it. The procedure to do this is covered under “Specific Functions” as described (badly) in the document ‘Diagnosis of the Instrument Cluster’.

The final variation of the software, level K29, was introduced in the first GTS of ’92. Curiously this limited the diagnostic scope of the instrument cluster to internal faults within the instruments themselves.

The shortage of new replacement circuit boards is a concern as is the absence of circuit drawings or other technical details which would allow users to have their boards repaired. For those contemplating a PCB replacement the above information should, for the immediate future, help to identify a suitable used component.

Colin, 89GT. With thanks for input from John Speake, Roger Tyson, Wally Plumley, Doug Hillary, Tails, Schocki and others.

docmirror

Wow, good info here. The reason you don't have any info from Porsche on the circuit ASICs is that they are undoubtedly trademarked. I haven't been into my pod yet, but I will be early next year. If the prom which holds the software(actully, it's firmware) is a commercial type, it can be copied on a prom burner. I've done it before but don't have a burner any more. Other chips on the pod are probably gates, and switches and are commonly available, should one become defective. An IC ref book with automotive application would allow us to look up the IC types, and crossref them to other mfg or current stock. If you still have the board out, and could send me all the writing on the top of the IC's, I can probably come up with some cross refs. As for the microproc, I don't know who makes it, but it may also be commecially available from an OEM supply house. That would leave us with sourcing the prom chip, and sometimes they are proprietary packaging, and have anti-cloning traps built in. It's hard to say if the prom in a Porsche would go to the trouble of that. Never know.

At $1000 each for a new board, I would spend a heck of a lot of time diagnosing and refurbing mine, before shelling out that kind of money. I think Roger has a spare pod, and I may go over and have a look at his to figure out what's replaceable, if needed. It'll be a few weeks though, I post again when I know more.

I had a bunch of those warning messages that came into the board from various sensors in the engine bay, and fixing my big wiring harness near the jump point helped a lot. I wish there was some way to reset my "airbag malfunction" without the Bosch tool(hammer). Zims wants $86 bucks to hook it up and throw the switch. AARRRGGGGHHHH.....

Vilhuer

Colin, thank you very much for the information. If original program can be taken out of PROM it would open up possibilies in modifying it. How about replacing non working RDK with radar detector signal or making new display functions like boost gage for example. Wonder how many unused inputs circuit board has that could be used after some wiring and code changes.

John Speake

Doc,
Doesn't a battery reset clear the airbag error code ?

ROG100

John,
Not on my car - everytime I have had the warning light stay on (steering wheel removal) the hammer has been needed.
Best,
Roger

ROG100

Doc,
I have a spare pod from an 89. At least it will confirm if your instrument cluster is playing up or its wireing gremlins.
Roger

John Speake

Thanks Roger !

docmirror

Roger thanks for the offer. I think it's my wiring. Once I fixed that bundle, things act pretty normal. I even got the 'tail lamp' warning, and sure enough, one of the lamps on the left rear is out. Also, that pod's code K18 prom, and I have code K25 for certain. My car is the 4th to last off the line in 1990. I would like to have a look at the board, to see if we can source some things.

As for the airbag warning. I had the batt disconnected for at least 4 days. I grounded the + terminal to discharge it fully, and still has the warning after startup. I've heard from several sources that if full discharge doesn't fix it, there's either something wrong with the sensors or wiring (likely), or it just needs to be reset after the codes are pulled.

My RDK seems to have a mind of it's own. I had warnigns for RF and LR tires. I added some air to the RF and that's gone but the LR comes and goes. The pressure is at 42PSI cold, and I don't want to go higher. I think it's the sensor near the wheel, as I was going over some rough road, and the warning lamp went off.

We could find out what other data logger functions are built in, and try reprogramming, but that's gonna require a code writer with experience in assembly lang. I don't have much, and if someone else has those skills we could go wild, and have the display say anything we want, and collect info from external sources, like a GPS....

ColinB

Doc
As I understand it an airbag fault light which won't go out is a sign of a recurring or intermittent fault. Maybe you should shell out the $86. The RDK system is actually Herr Dr Ing Porsche's private joke. They very seldom work properly and most owners disable them. The system is notorious and was discontinued on later cars.

Gentlemen
I think 'cloning' the PCB would be a most excellent idea but I'd have to leave that to the experts. I wouldn't know a microfarad if you beat me over the head with one.

Colin

Tom in Austin

Colin, this is fascinating information. I have two spare clusters, and will go home tonight and look for the software rev and part numbers. The boards are not completely identical; each one has an odd wire soldered to bridge some points together as though they were repaired or modified at some point. Perhaps this was a means the factory used to update old stock for a new firmware rev?

Tom. M

Colin,

I have an 89GT from Canada and the dash is metric with metric digital display but with english warnings as in call range etc... I was wondering if there was a way to switch to US on the digital display. I can't switch via the typical US method. Is there a special one for Canada, or is it a variant of the German one with the language set to english?

Later,
Tom
midlman@rennlist.net
89GT Cnd spec.

ColinB

Tom in Austin
You'll find the software level on the small white paper label with rounded corners on the back of the cluster incorporated in other digits. It says INDEXHK25(or whatever, that's your software level,) followed by a seven digit number. There are other labels on the cluster, one of which which shows the 928 part number for the cluster AS A WHOLE. I don't have a cluster with me now but I think that one also has a bar code and straddles the joint between the white and black housings. As for the wires soldered on, Schocki has a theory that you can change the language that way but we've never proved it. Please keep me in the loop.

Tom M
See my original post. The software on '89 cars is K18 so the language is fixed for the target market. Canada certainly did have its own boards in '89 and I was searching there on the net for a replacement board because curiously only Canada and Japan have the same boards as I have. It's English/Metric and I'm afraid you can't change it. Only US cars have the Imperial/Metric change feature.

Colin 89GT

docmirror

Inst board wiring:

Those wires you see running on top of the traces of the board represent a modification or revision that was applied after the run of boards was etched & silkscreened. Sometimes it's possible to fix a problem with microcode(PROM code), other times, a change to the circuitry is required.

It's one of those dirty little secrets in the computer buiness that happens all the time. Look near the paper tag with the K18 or K25, and somewhere around you will also see either a paper tag, or possilby an ink stamp with words like "REV -04" or "VER -58" or something like that in German for revision or version. While you're looking about in there for the codes on the paper tag, find some of the writing on the chips on the board and either post or PM them to me. I'll try
to cross ref them. Try to get all the writing including the MFG stamp, which may look like the state of Texas for a TI part, or a bold font M for a Motorola, Siemens uses a big S with a box around it.

So, what happens is, the boards are silkscreened and tested then put into production. At some point Franz realizes that he didn't add a signal to some gate that will do something when a certain set of circumstances occur. It's typically pretty obscure, and you wouldn't find it in the normal course of operation. So, a sustaining engineer develops a board rev spec, and someone with a soldering iron and a spool of 32Ga wire starts connecting lines. When a board comes back for repair, or in the case of a serious mistake, all the boards may be recalled and rewired(rare).

Also, as you mentioned, some revs of boards are meant for certain applications. It's entierly possible that the board in your car from Canada(eh) is exactly the same as the board from US, but the trace wires have been ADDED to diasble the SAE measure display. Again, a car destined for Canada had a special stop at the wire station to add these wires and defeat the functionality that was built in.

Airbag Malfunction display;

I think my board is correct in saying that I have a malfunction. Since the control unit is in the center console, and my wiring for the center console has been seriously tampered with by some gorrila attempting to fit a stereo, it's my opinion that the wiring fault exists in the center console where it serves the airbag control unit. I checked the bags carefully and they haven't deployed, so it must be in the wiring, or control unit.

RDK;

Yes, I've heard of all the problems with the RDK, and the simple defeat of the system with a wire from pins 5 to 13 of the RDK control unit. I may go that way too, but first I'll look into getting it fixed if it's not to big of a job. I suspect faulty wiring from the harness at the LR sensor, up the suspension and then to the harness mating plug near the diff. Heck it might be just as basic as cleaning the RDK **** on the rim, and the sensor on the hub. They have some kind of proximity detection going on, and cleaning it may do wonders for the sensing circuit of the detector.

I'm headed for the bay area this week. Any get togethers going on there? I'll be staying in San Jose, and have rental car will travel. I was thinking of going up to the wine country on Sat 6 Nov.

ColinB

Doc
Very interesting board construction info if a little out of my depth. This sort of stuff makes for good archive material.

You're right, the RDK wheel/bay units are proximity sensors. They have about 2mm clearance and cleaning mine didn't help. You will find the harness connector in the front right corner of the spare wheel well in unit with the brake pad sensor and ABS wiring.

Luck. Colin

Chris

Can one of you guys post pics of the PCB ?? Thinking it may be possible to modify it somehow so that you could have any function you wanted.

Chris