Speedometer & Tach Lights Turn Off While Driving

[Ecto1]

This morning while I was driving the car the lights on the Speedometer & Tach turned off for about 10 minutes. No idea why. Nothing else in the car stopped working and all the displays were on. The lights just shut off and then magically turned back on by themselves. Has anyone seen this problem before?

Thanks.

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[JB60]

Maybe the auto dim setting , dash lights change from dark to light in sun , cloudy to shade

 

[Ecto1]

Maybe the auto dim setting , dash lights change from dark to light in sun , cloudy to shade

I don't think this was a dimming of the dash lights, they just completely turned off. The daylight outside the car didn't change during this episode. I've never seen these lights turn off before when I've driven the car regardless of the light present outside the car. When they're not on its VERY hard to read the gauges.

I'm not sure if the Speedo & Tach lighting even have an auto dim function. There's a manual dimmer for the dash and interior lights above the headlight switch. I suspect that the headlight switch and manual dimmer are an integrated unit. So if this is what is controlling the Speedo & Tach lighting perhaps this is where the problem lies but I'm not sure. I was hoping someone has already seen this problem and can point me in the right direction.

I haven't started pouring through the wiring diagrams yet but I suspect I'll have to. I'm hoping I don't have to remove the Dash again in order to resolve this.

 

[JB60]

Maybe scan for fault code hope you find out Mine switched from bright to dim then back this morning they do look they are off

 

[Ecto1]

After reviewing the Service Manual & wiring diagrams it appears that the Speedo & Tach lights are controlled by the BCM. The "manual" dimmer buttons on the Headlight Switch aren't really dimmers. They're dimmer requests to the BCM. Maybe the BCM will adjust the lights in response to the dimmer buttons, maybe not. Plus there's plenty that the BCM uses to determine what intensity the dash lights should be. This could be hard to diagnose. I think I need a CAM Bus analyzer that can decode Mustang CAM Bus commands. I can't afford one even if I could find one.

Plus there's 2 types of lighting control. One is controlled by CAM Bus messages between the various computers. The other is called "non-network" lighting. I believe this is a current driver that can vary it's output to brighten/dim the LEDs in components that don't have a CAM Bus interface. I don't know which category the Speedo & Tach lighting falls into. The Service Manual isn't very clear. I couldn't find a comprehensive IPC diagram in the wiring section. There are bits & pieces of the IPC in various other circuits in the wiring diagrams but I haven't figured out how the IPC lighting works. I believe that the IPC receives CAM Bus signals because it has it's own section in the ForScan menus. But I haven't yet determined if the IPC also receives the non-network lighting signal. The Steering Wheel buttons do receive this signal as well as other buttons around the steering column.

So far there are no DTCs when I scan with ForScan. I've also run stand-alone self test on the IPC and the BCM but these both return no faults present. Given that there's plenty of other inputs that the BCM uses to control the lighting I may have to run self-test on ALL the computers in the car. Even then it may not present any faults. So far this condition has only happened once and it may not set a fault unless it occurs so many times in a particular number of "driving cycles."

Guess I'll have to cool my jets until it happens often enough.

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