Unresolved electrical issue with gauge cluster

I have been poking at this on and off for a few weeks, and since I'm pretty much unable to drive the car at night, I need to get busy on a solution. Thanks for your input and ideas. A couple of months ago, I had what would turn out to be the first event with the 996 where I noticed a pulsing (slowish, maybe 5 Hertz) in the center LCD display backlight, and the backlights for the two LCD displays on the left and right blinked off and never turned back on. I also noticed a small amount of smoke snaking up from what appeared to be the front and back of the gauge cluster. After a few seconds, I turned off the headlights (this was at dusk), and the pulsing and smoke both stopped. I assumed the smoke, pulsing, and missing backlighting meant a short in the gauge cluster, so I pulled that off and disassembled it to look for the section of burned wire or scorched PCB that would explain the smoke and missing lights - and I found nothing. Everything looked perfect; I spot-checked the backlight bulbs with my multimeter and they tested the same as the known-good bulb in the center. I could find NO source for the smoke or explanation for the unlit backlight bulbs. At about this time, troubleshooting was interrupted by the Vegas trip, and on that trip the car got speared with a section of pipe, so I was sidetracked with that mess and then got into several weeks of travel. I could not find any indication of scorched wire or burned insulation under the dash, but I'm worried about where it's coming from and why no fuse is blowing when it seems pretty clear there's a short. At this point, the car is still exhibiting the same issues. With the lights off, the only symptom is the missing backlight for the left and right LCD segments (above the warning lights, below the dial gauges). When I turn the lights on (my car is Litronic-equipped), the bulbs ignite as they normally do - but only on occasion, the cluster lights that actually still work will start to pulse, and I suspect I'd see more smoke if I left the lights on long enough. I have not yet replaced the headlight switch nor inspected the contacts to see if it's to blame, but that is my next step unless someone has a better idea. Thanks!

Some might say "mice" In the absence of visual problems nor open bulbs headlight switch seems suspect. I'll peek at the schematic when I get a chance.

Not really the ideal solution when you are talking about a potential fire, but pull the cluster apart but leave it plugged in and then turn the lights on. When/if it starts smoking, see if it is more identifiable as to the source. Might also get a thermal camera to watch things with to see if you can find an unexpected heat source. Edit: I'd try the thermal camera idea after getting it out for a good run (with the lights off) so that everything is already up to operating temp. That way you shouldn't have to chase too many ghosts.

After pulling the cluster and seeing how the dash is vented from below, I'm guessing the smoke was from below and one of the wires that goes to one of the cluster connectors melted through. I need to get upside down with the AC duct removed and spend some time looking around. If I can't eyeball anything, an IR filter for my camera is a good idea, but I really hate leaving whatever it is shorted long enough to heat up sufficiently to be an obvious heat source... Since the headlights are involved, I will be checking the headlight switch and testing the Litronics for a ground since the headlight harness insulation tends to crack and flake off...

Yeah waiting for smoke/heat is definitely not the ideal route, but if nothing else is finding it...

I would guess that it is something short circuiting that is causing too low of a current to trip a fuse. I would just take everything apart so you can see the wiring below the dash, then turn the headlights on an do a visual inspection.....I bet it will be obvious. Make sure you have the frunk open and the negative terminal bolt loose to ensure you can disconnect the battery if required. I bet your nose will find it before you even energize the circuit once you take everything apart. Good luck!

Since it seems to always involve the lights, I'll start with a new headlight switch. I don't necessarily expect that to fix it, but it's a fairly cheap part and relatively easy to replace. If that doesn't fix the issue, at least I have another known-good headlight switch for the parts collection...

Since I have a working bulb behind the center LCD panel, I can also try swapping it to one of the non-working spots and see what happens....

You have a fault in the wiring, about 90% sure. I had a very similar issue with my Audi A8 years ago when I did some major electrical modifications to the dash. I doubt changing the bulb or switch will solve your issue. The good news is that it will most likely be very obvious and you won't have to know anything about electrical troubleshooting. Wires are probably melted together at this point.

I don't expect swapping the bulb to work, but knowing the issue is not in the cluster will make me much happier and take one thing off the list. The cluster itself is pretty much solid state - I was surprised at how 1978 it looks in there. If the "working" bulb fails to light in the "not working" holder, I'll tone out the pin for that bulb and follow the wiring harness down inside the dash to find the melty bit. If necessary, I'll start sacrificing lower-amperage fuses to see if one circuit seems more inclined to blow when the lights are on...

Might be worth taking a quick look to see how the cluster is grounded, and if the ground is clean. May not be the cause of your gauge problem, but a dirty ground can cause all sorts of electrical funkiness.

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It's definitely mice. Classic example too. Nicely done Wild Billy!!!

If it's mice, they failed to eat anything important except for something headlight-related. I can live with that.

You Tube

Well, after a month of d!cking around with this thing, I have gleaned a few things: 1) The cluster is functioning 100% normally apart from the backlighting circuit. 2) There is a short in the backlighting circuitry. As a subcomponent of a turd, I shouldn't be surprised how crappy it is. 92% sure the culprit is the flexible circuit "board" or one of the two stone-age electrical connectors that provides power from one half of the cluster sandwich (the part with the gauges and the windows for the LCD elements) to the other (the half with the three LCD segments displays and all of the backlighting). 3) There are no visible burns or tears, and none of the incandescent bulbs test bad. There is no indication of any unusual wear in any underdash wiring I can access. None of the fuses are blown, and all fuses are of the correct rating. I swapped positions for same-rating fuses in case a defective fuse element was not burning out as designed. At this point, options include: A) Take it to my indy and let them spend a ton of money to not track down this issue. B) Ignore it; accept that driving at night is no longer an option. C) Ignore it; accept that knowing how fast I'm driving at night is no longer important. D) Borrow a cluster and see if it works as expected, or E) Buy a used cluster, verify that the backlights work, and try to graft the two units together so I have my mileage and the other cluster's lights, or F) Tear out the backlighting, install some LEDs, and be done with this mess. At this point, I'm leaning heavily toward F. I can build an LED harness in an hour, with stuff I already have on-hand, that will work for both day and night driving (auto-dimming, of course). I could steal power from elsewhere in the cluster, but I'll probably pull a fresh, fused line from the fuse box and use a tiny fuse to ensure I don't somehow do even more damage to the cluster. Assuming I just remove the incandescent bulbs and I don't use any permanent adhesives to situate the LEDs, this should all be reversible when I finally decide to ship the mess off to one of the speedo/instrument repair shops to get it done properly. You know, for my concours restoration.

...and...not so fast, Sparky. Spent some time last night installing some LEDs for the LCD displays. It's a lashup, but the dash is reassembled and I can make out the pertinent info on each screen. What I noticed last night, when I was sitting on the ground pondering my fate and absently turned on the headlights, is that the cluster briefly - for a couple hundred ms tops - lights up normally; it then powers down to nearly 0% and pulses. Here's the kicker: it pulses at a variable rate, and I can think of nothing in the electrical system to explain this. Since it ONLY happens when the lights are on, I'm going to redirect my attention to the Litronics, taillights, and all other HOLY ****, I bet it's my aftermarket blinking third taillight module...

I think you just answered your own question LOL

Check your turn signals in your litronics. The two wires leading to the bulb might be exposed and touching each other.

Overnight parts from Japan?

It wasn't the flashing stop module, but it WAS a short. The side marker originally damaged during the road debris incident wound up with an internal short in the connector. Triple-check all of the things you're sure are not the problem...