Blitz993

The instrument lights on my 1995 have stopped working. I've got a blurry copy of the car wiring/schematics and have learned a lot about the system and conclude that the "Control Unit, Instrument Light", sheet 1, coordinate K-1, needs an inspection - as well as the ground point for the dashboard.

A couple of questions:

If the dashboard ground were flakey I'd expect to see more problems than just the instrument lights not working - does that make sense? Everything else is working fine.

Does anyone know where the control unit is located in a 1995? I was assuming it was behind the dash because it's connected the dimmer which is on the dash. Maybe thats true, but I don't know how to get into there to find out. Mike J. says it might be elsewhere, like under the dash behind the glove compartment. As I hate mountaineering around under dashboards, I was hoping someone could point me in the right direction

Note: The 1995 model's fuse tray in the for'ard hatch area doesn't match newer models. There's another thread on this topic referencing a relay R24, but on my car that relay isn't present in the spot mentioned on that other thread. If the unit is in there, chances are it will be in a different location than newer 993s.

[Edit] - By the way, I've read as many other threads on this topic as I could find but it's quality time spent with the wiring diagram that has been most helpful to date.

techman1

Just ran into this. I believe you have a bad relay in the box. I swapped the one from the sunroof into it, works fine now. I believe the legend in the lid id the relay as ....crap, can't remember. Something like terminal X or...anybody post a pick of the fuse diagram help out?

Found a post, it says use a new relay. I used my sunroof like I said.
https://rennlist.com/forums/993-foru...t-failure.html

nine9six

In the fuse box lid, I'm sure there is a corresponding fuse (18), as well...Did you check it as a first course of action?

Here is a diagram for you...

d ward

I'd look at the dashboard lights dimmer switch/rheostat, often that's the cause of non working dashboard lights.
If it is that sometimes twisting it back and fouth will get the lights to come back on, or try bypassing it to rule in or out if it's that causing your issue

Blitz993

Hi Everyone,

Thanks for the help, there were some simple things to try to help eliminate possible cause:

As a note, when the lights failed they were intermittent about 3 times before they went dark completely and haven't worked since - after 14 months now.
The intermittent failure suggests a flakey connection but I've rattled the dimmer switch, the main light switch and get no response.

I felt around behind the ignition switch and felt a metal box about the size of a large cigarette packet, but it also seemed to contain the actual ignition switch. I'll need to get right under there and take a good look.

Nine9six, thanks for the image of the fuse box. I compared it to my actual box and it's a match - the legend in your diagram indicates which parts are omitted for the various model years. Too bad I don't have a 1996 because I think the part I'm looking for is R24 on that year. In my car there is no relay in that location

techman1

Rory,
Are the dash gauges working properly?

ToreB

The schematics mentions that R24 (Control Unit Instrument Light) is placed in Luggage Compartment Left side. I guess this is along a couple of other relay-like units,(Gong/Flash relays) placed on the inside of the fender. They are easily reached through the oil temperature/pressure instrument opening.
Google 964 618 103 00 to see what you are looking for. See here for for some pictures on the location.
Extract the relay and jump terminal 58 and 58d on the relay socket in order to test the unit. If the lights works, you probably have a defective control unit. If not, you probably have a problem with the light switch.
Cheers,
Tore

Blitz993

@techman1 - everything is working except the instrument lights.

The main light switch also seems to be working properly but my car has been modified for use in Canada which means the marker lights are always on even if the light switch is in the off position. The only verification I can tell that the light switch is working is that a green lamp illuminates on the instrument with the speedometer on it when the light switch is in the first position. In the second position - i.e. headlights on - that green light goes out. So for now it's reasonable to conclude the switch is OK and move on to the next thing.

Don't know about the dimmer rheostat beyond the fact that rattling it and turning it briskly doesn't make any difference.

A detailed review of the schematics indicated that if the license plate light worked, the problem was further downstream - the light does work and that's how I concluded the Instrument Light Control Unit needed to be checked. Don't have the schematics with me at the moment, but that's it in a nutshell.

@ToreB, thank you for the reply and link to the PDF - that is good information. Removing the instruments to get at the inside of the dashboard looks like a necessary step forward, but I'm still not sure of the location of that ILCU.

I'm confused over one thing you said about the Luggage Compartment. The PDF shows access to the inside of the dashboard, I didn't see any reference to a Luggage Compartment. Can you clarify what you meant?

Thanks for your posts, this is good information

Rory

ToreB

The relays pictured in my PDF doc can be reached both from the luggage compartment and through the instrument opening. However, the latter is the most easy route, it's done in minutes.
Cheers,
Tore

Blitz993

Thanks - I've never taken a close enough look from the luggage compartment to notice that. Still, instruments it is.

Regards

Rory

Blitz993

I finally managed to pull the instruments out of the panel [Edit] BTW - ToreB, your PDF was bang-on. After I got the instrument out I compared the plug-in relays to the ones on the PDF you provided and it was a perfect match. Thanks!

The ILCU is definitely getting 12V when I switch the lights on (both positions). The dimmer switch also seems to work, although it doesn't seem to match the wiring diagram. I'll let that pass for the time being.

I found a new ILCU for $140 on Pelican Parts. Here's where some faith is required because it would be very annoying to pay for it only to find the problem was somewhere else.

I also tested 1 of the dashboard lights with a continuity tester and it measured 12 ohms - so at least 1 of the lights should be working.

The ground point behind the dash also looks solid so I don't suspect a bad ground.

Question: [Edit] to test that everything seems to be working - [\edit] does anyone see a problem with running a jumper from signal 58 (instrument lights) to 58d (dimmed instrument lights)? I'd be passing 12V directly to the instrument lights - so the real question is how many volts can these bulbs take?

ToreB

There's no risk in feeding the 12V instrument lights with direct power.
Cheers,
Tore

Blitz993

Thanks for the support, Tore. In searching Rennlist for this problem I also found

https://rennlist.com/forums/993-foru...l#post12538682

which is a thread for the exact same problem but on a 1997. The relay is more easily accessed in the newer 993s because it's in the Central Control box.

I jumpered 58 to 58d and threw the switch - presto - the instrument lights came on! Only took me 15 months to get to this point, lol

I'll be ordering the relay from Sunset - if it doesn't actually fix the problem I'll update this post, but I am pretty sure it will resolve the issue.

I appreciate all the responses to my thread, thank you all.

Rory

Blitz993

I got the ILCU from Porsche and tried it. The problem seemed to be fixed but then came back again. The lights worked with the engine idling but as soon as the car was in gear and driving (revs around 3000 RPM) the lights go out again.

I've got a lot of practice reading the schematics, though. Note: there is no fuse for the power driving the instrument lights - it comes straight off the battery via terminal 30. Lots of current available there.

The symbol on the schematics for a wire junction is a square box with a number in it. The box usually has 1 wire into it and more than 2 leaving it. Does anyone know how these are actually implemented in the car? I need to start examining the wires at key locations. The junctions will need to be part of this.

Blitz993

With some help from a friend who knows 993's inside and out we tracked down the problem. In the last 20 months since I updated this thread I've been driving with the Instrument Light Control Unit (ILCU) removed from the socket so there has been no current flowing through this circuit in all that time.

A previous owner had installed a CDR-210 and told me that the illumination didn't work because it hadn't been wired yet. I wire with a bare contact had been hanging down from behind the radio - after a couple of years driving the car, I got tired of seeing it and tucked it behind the carpet forward of the center console.

I'd long suspected the cause of this problem was a short somewhere but had been assuming it was in the wiring harness somewhere behind the dash - hence the long delay in updating this thread. I concluded accessing the harness was beyond my abilities.

My friend Mike and reviewed the situation and decided to remove all the lamps on the dash circuit and check continuity. Sure enough the resistance didn't change very much at all. It was then that I remembered the wire - we pulled it from behind the carpet and noticed a small weld-like mark on it. We then tested the circuit by adding in more lamps, starting with a manually-jumpered one right off terminal 58 on the ILCU socket. It worked. Adding lamps back in groups and retesting with the light switch, they worked - even with all lamps reconnected.

We took a closer look at the dangling wire and discovered it had been spliced into the ashtray lamp - so when lights were energized this wire was too. It must have been touching metal and shorting to ground. Since there is no fuse in this circuit (terminal 58 and 58d) any time the lights should have been on they were shorted to ground.

My assumption had been that the dangling wire was from the radio and needed to find a connection to 58d - thus it would have been cold at all times and safe to be dangling. It is a lot more work to find a suitable wire (ashtray light in this case) and splice into it - the previous owner had done the heavy lifting and only needed still to wire it into the radio. Why they stopped here - and left the wire hot and not taped for protection - is a mystery.

The moral the story is - don't assume any wires dangling from behind the dash are harmless!

After re-installing the ILCU and running the car for a week everthing is fine and what a luxury to have the dash lights back again!

NOTE: If it hadn't been for this dangling wire, the information on this thread and others like it is accurate and good and learning it should be sufficient to address this problem.