Hi, so I have this issue that I can't solve for anything. My 93 LS400 has no backlight on any buttons (AC, Radio, Heated Seats, LCD and Gear Selector). If headlights are off the LCDs have backlight. If any funcion is active, the active light illuminates (little green square). If I turn on my headlights, the rear light failure yellow light illuminates on the dashboard and I have no internal lights, LCDs turn off (should only dim) but the active lights stay on and dims a bit. I just repaired the dashboard and ECU, and I cant remember if the car had this problem before fixing the dashboard as I just got it and it was the first thing I've done to it. I tested some of the backlight bulbs and they are working.

 

Did you forget to plug in the dimmer dial?

 

It is plugged and working on all of the dashboard.

 

To be clear, would this accurately describe the issue you're having:

1) When headlights are on:
- LCD displays for climate and radio are not backlit
- Tail light failure warning light turns on in Instrument Cluster
- All other illumination (radio, climate control, heated seats if applicable) are not backlit
- Dimmer switch does nothing
- Instrument Cluster backlighting does not change with dimmer switch but is on

2) When headlights are off:
- LCD displays are backlit
- All other illumination is turned off
- Instrument Cluster backlighting is on

I would suggest to read and test as per this:
https://www.clublexus.com/forums/ls-...l#post11624337

You may have a bad rheostat light control / dimmer switch. Or something may be wrong with the wiring

Also check your trunk wiring harness for any breaks - it's caused weird illumination issues on the cluster before.

Additionally, you might want to ask for help and have someone check for outages in all of your tail lights when you turn them on or hold the brake.

I have a feeling that the tail light failure indicator is a separate issue. It could be a flaky bulb or bad ground in the tail light circuit.

I once had a tail light bulb work intermittently and throw this failure lamp light because the ground wire for that socket was not making good contact in the socket itself.

 

You're almost right. I'll copy your writing to acuratelly describe what is going on:

1) When headlights are on:
- LCD displays for climate and radio are not backlit
- Tail light failure warning light turns on in Instrument Cluster
- All other illumination (radio, climate control, heated seats if applicable) are not backlit
- Active button light is dimly lit
- Instrument Cluster backlighting is on
- Dimmer switch controls instrument cluster backlighting (white lights)
- Glovebox light turns on when glovebox door is open

2) When headlights are off:
- LCD displays are backlit
- Tail light failure warning light does not show up in Instrument Cluster
- All other illumination is turned off
- Active button light is fully lit
- Instrument Cluster backlighting is on
- Dimmer switch controls instrument cluster backlighting (white lights)
- Glovebox light does not turn on

I've changed all my rear light bulbs, both brakes, turn and reverse. All of it are the same brand. The tail light failure warning light was contantly on after I used the brake once. I had a brake light spent so I changed all of the bulbs to match. All of the exterior lights are working as intend, I tested multiple times.

 

I just opened up my rheostat and now I'm almost 100% its the problem. it has a thin wire that runs directly to the instrument cluster backlight (it was modded to LED) and there are a bunch of wires not connected to anything

https://1drv.ms/i/s!AowSbC8D35kShdFQ...DivsA?e=1BFB24
https://1drv.ms/i/s!AowSbC8D35kShdFS...vyUPA?e=DJMTGF
https://1drv.ms/i/s!AowSbC8D35kShdFR...poTsQ?e=GEdSDp

 

[Reattaching your images as post attachments]

Based on what you described, the issue here sounds like it is because your instrument cluster was modified.

Since we are not dealing with a factory setup, it will be harder to tell you exactly what is wrong.

I see that your cluster was messed with in the past, and you fixed some of the issues like the backlighting recently:
https://www.clublexus.com/forums/ls-...l#post11647268

The fact that the rheostat light control is not wired up to the rest of the circuit is definitely a main contributor to your backlighting not working.

Also, what do you mean by the "Active button" light? Do you have a picture? Are you talking about the Security light?

 

I'll keep working on this dash and rheostat. I think I'm in the right track. Yesterday I rewired all the circuits to what appears to be the correct wiring from what I've gathered from the service manual. Unfortunately it doesn't go in depth enough to know all the wirings and its connections but at least I'm getting the correct continuity from the connector terminals now. After what I did when I turn on the headlights all the cluster needles turn off, along with the outside temp and odometer.

The "Active button light" is the small square in some buttons to indicate its on. As per the attached pictures.

 

I want to give an update to my issue as my problem is not solved. I fixed all the wiring that were messed with on my car to the original configuration. That I am sure as I downloaded the service repair manual and the electrical wiring diagrams. I've done all the testings and everything is giving me the correct values. However I still cant get any backlights on my central console. In the following image, those are exactly the lights that don't illuminate when turning on the headlight.



I did all the following tests and they all work as specified:








Is there a way to bypass the rheostat light control volume and/or the rheostat light control?