1996 odometer and gears digital haywiring

1996 Riviera - at dusk/night time - both the digital odometer/trip monitor and the digital gear range (inside of the instrument cluster) go haywire (looks like back to the future!) because/as soon as the twilight sentinel turns on the headlamps and dash lights.

Turning the twilight sentinel off (part of the headlamp switch) turns off the headlamps and dash lights - and the digitals go back to normal - steady - no 'blipping - no haywiring.

During the day - when cardboard is placed over the ambient photocell - 10 seconds later the headlamps and dash lights come on - but - the digital aspects of cluster go haywire. Take the cardboard off the photocell - and 10 seconds later the headlamps & dash lights turn off and everything is normal.
Searched a bunch of the past threads and didn't find anyone's car that shared this exact same problem. Only thing I did was clean photocell with alcohol/q-tip but still have digital 'haywiring.'

Here is a link to a youtube short - this 1999 has the same digital haywiring problem - only with my 1996 the headlamps do not flash on and off - luckily they both stay on while the dash is going berzerk...

https://www.youtube.com/shorts/LtBs1-dq4-8

Thanks to the forum members for any tips.

start by checking all your fuses

Great tip - I appreciate this and you very much. Yes, all fuses under back seat fuse blocks -and- all fuses at drivers dash side panel are good.

I do have the FSM and gone through it - only this problem doesn't appear to be part of the trouble shooting diagnosis.

Here is a vid I took yesterday...

https://www.youtube.com/shorts/8Uwhq3aAZDA

The headlamp switch controls the duration of the lights-off delay for nightlighting when you park the car at night. If it is changing your twilight sentinel behavior you probably need to replace the switch.

I think there may be something amiss in your instrument cluster. When did it start doing this?

Great reply, thanks for that, have been living with it for a few weeks - just annoying is all. Here is a little solid state dude that may be the culprit -or- may be the PCM itself - will keep you most excellent guys informed as it progresses - was still hoping some had seen this a dozen times!!

VaRiviera wrote:

Great reply, thanks for that, have been living with it for a few weeks - just annoying is all. Here is a little solid state dude that may be the culprit  -or- may be the PCM itself - will keep you most excellent guys informed as it progresses - was still hoping some had seen this a dozen times!!

Was any work done on the car just before the cluster started acting up? any work at all?

VaRiviera wrote:

Great reply, thanks for that, have been living with it for a few weeks - just annoying is all. Here is a little solid state dude that may be the culprit  -or- may be the PCM itself - will keep you most excellent guys informed as it progresses - was still hoping some had seen this a dozen times!!

I've seen that on the Climate Control, which is a 'classic problem' with the vacuum-fluorescent displays because they used an underrated resistor.  I've never seen it on the PRNDL or Odometer, though they ARE also vac-fluro displays.  I've gone crazy trying to find the actual dashboard circuits in my manual, but I can't find ANY circuits downstream from the Dimmer Module...

No work has been done in or around the dash/cluster - this Riv only has 24,000+ miles and appears to be 'unmolested.'

I like AlbertJ asked if anyone had been anywhere up there by the cluster wiring/etc - that could have touched or disturbed the wrong thing.

The odometer and probably the PRD123 digitals look in wiring diagram to have 28 year old solid state capacitors - either or both may be failing/defective. In that case remove cluster and send out for repair to expert -or- home fix with new capacitor(s.)

Also - these solid state capacitor circuits feed into ground 'distributions' -  a bad ground is never good. Tearing apart the dash is time consuming/labor intensive - for now its just a night time aggravation - during the day all is normal/well - but with the chance the capacitors are leaking I may have to decide to go in and start the cluster removal atleast for a visual inspection. Thanks men for taking time out on this one.

don't assume the capacitors are bad; if they were the displays would behave the same day and night...

Consider getting a good used cluster ($40 - $100 on eBay) and swapping it in to see if the behavior continues. If that fixed the problem, check for cold or broken solder joints (and burn marks) on your current cluster especially where the PRNDL and odometer displays share power or ground. If i does not fix the problem, check the lighting circuit.

I suspect you have an intermittent in the headlight dimmer but it's so quick that the only way to see it is on the fluorescent displays, the incandescent bulbs don't switch on-off fast enough to show the problem.