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Instrument Cluster Dimmer still too bright at minimum setting

2011 Silverado instrument panel dimmer won’t reduce below about (subjectively) 40% intensity and cannot be turned off - still way to bright for old eyes on dark mountain roads. Dealer says its characteristic of the model and has no suggestions. Scan-tool results for dimmer potentiometer output reads 2.7 volts (9%) at minimum rotation, and 3 volts (100%) at max intensity. Scan-tool results for LED PWM drive signal reads 0% at minimum and 100% at max. The pulse width modulator (PWM) appears to be an imbedded component of the body control module (BCM) so is not accessible. The dimmer potentiometer is a component part of the headlights combo switch so is likewise not directly accessible. Since I can’t get to the potentiometer voltage divider circuit, if I add a fixed resistor to the output leg of the potentiometer and reduce the dimmer signal voltage by another 0.1 volt (to 2.6 volts out to BCM/PWM) will I set a BCM DTC or raise the voltage on the LED drive pulse to a damaging level? Alternatively, if I place a spst switch in the dimmer output signal, will the PWM simply switch off, or try to further decrease the drive pulse width? Last shot-in-the-dark question before I drag a scope out and start tapping into wires, does anyone know or know how to find out what IC chip is used in the PWM?

I think I would just leave it alone. The pulse width is how long the injectors are opening to allow fuel into the cylinders. If it is really that bad, I think I might try some light window tint film on the instument glass so that it can be removed again.

The pulse width modulator in question is solely in the interior lighting control system and is not related to the engine control module.

You’re clearly not scared of making a car do what you want it to do. I like that.

I wouldn’t use a resistor because it may need to be of a fairly large wattage. It will also be difficult to experiment with what the value needs to be. If not all LEDs are on at the same time, you’ll likely see variation in light levels, which could be annoying.

You could check to see how they switch those LEDs. If the common to them all is positive, you could drop a simple voltage regulator in. It would basically bring the voltage slightly lower from what it is now. Most regulators can be adjusted fairly easily to any voltage with a potentiometer.
You may need to run it off 12V instead of the voltage they use because you may only need to drop a fraction of a volt. Most regulators can only deal with a one volt drop.
The voltage they use to drive the LEDs could be used to turn your regulator on/off with a relay so everything operates the way it should.

Actually I think Bing’s idea of covering the instruments with something is the best.

If you are determined to mess with the electronics.

“LED PWM drive signal reads 0% at minimum and 100% at max” Indicates to me that changing the resistance or opening the dimmer potentiometer circuit won’t do anything. At 0% the duty cycle is too high to dim the LEDs further. I think you have to find a way to modify the pulse width output from the PWM. A timer triggered from the output of the PWM would give you more control over the duty cycle. You will have to dig out the scope to be sure.

Or cover the cluster with a tinted film.

Total agreement with light window tint. Much simpler solution than removal of instrument cluster.

All commentary appreciated. WRT MTraveler’s comment, the fact that the scan-tool indicated a 0% to 100% swing in LED drive signal simply indicates that the scan-tool measured the expected pulse widths based on its internal calibration (GM software). Clearly, since the cluster did not dim below about 40%, the ACTUAL duty cycle of the PWM never approached 0% (ie zero pulse width).
WRT RemcoW observations, there are no incandescent lamps in the internal lighting scheme, so the dimmer pot really only provides the dimming reference signal to the BCM/PWM, and thus would appear to carry no significant current load. A power-off measurement of the dimmer resistance values (wiper-to-ground) during control rotation will give me the pot and voltage divider characteristics, so a back-of-the-envelop calculation should provide “safe” resistor values (Ohms & Watts) to achieve the desired voltage to the BCM/PWM. What do you think?

Regarding the one comment, saying limiting the voltage won’t work:
Limiting the voltage to the common will dim it further because PW modulated LEDs are often driven pretty hard: way more than the steady state forward current. You just don’t want a specified max forward current to be exceeded for fear that shortens the life of the LED.
With you driving it harder, you bias it harder as well. That means the voltage is actually lots higher than the normal 1.5ish volts. By lowering this bias, you limit the current and will definitely limit the light output without starving the LED.

The average current may be around 5mA per LED at some comfortable level -remember, you have a bunch of them. Adding that up will be more than a pot can deal with. You’re really not supposed to put a lot of current through a potentiometer. Sure, people do it but it is bad practice. If you went with an electronic approach to solving this, I’d at least make that current go through a transistor that you bias through a pot.
If you’re doing that, a three terminal regulator circuit would consist of a lot less parts and it has an internal reference so you don’t see voltage variations.

Maybe the aforementioned window tape is the best approach…

Do you have any trouble seeing the gauges during the day? Covering the instruments with a tinted film may make it difficult to read the instruments during the day. Don’t do anything too permanent until you try it out for a while. And make sure you can undo your change if you plan to sell the truck eventually. It could make the truck undesirable to a lot of people.

You could just add a cutout switch that would darken all of the instruments when you’re at a particularly bad stretch of road, then switch the panel lights back on when you get to an area you’re more comfortable with. You wouldn’t be able to see your speedometer obviously, but I’d guess you were pretty much going under the speed limit and taking it easy on a treacherous road anyway. Any warning lights for abnormal conditions, such as low oil pressure, would still illuminate.

I think your idea of adding a series resistor to the dimmer pot wiper lead will work fine. Like you say, you will have to figure out what resistance value will be practical to use. You could also install a 1k ohm pot it the line and see what that does and if it has enough adjustment for what you want to see. I would guess something in the half watt range would be ok to use since this is just a control for the driver circuit. If that has enough adjustment range then you can measure the resistance of the pot and install a fixed resistor in place of it. You could also add a bypass switch like Oblivion suggested, to bypass the resistor in case you want to go back to normal operation.

I had a car with a piece of photo film over a light that was too bright. I used a laundry marker to darken my radio lights on another car.

If not all LEDs are on at the same time, you’ll likely see variation in light levels, which could be annoying.

Especially if they are driven in strings which is a common means to split the loading.

Also, you can’t see the intensity variation in LEDs when they are well biased. Start dropping the drive to make them really dim and you’re going to see why they bin LEDs in analytical instrumentation. Their intensity is going to vary noticeably from device to device.

I recently bought one of those giant numerical clocks for the bedroom. Getting older and vision not what it used to be. There should have been a big sample bottle of geritol strapped to the side… Anyway, the consequence was the room was lit up like daylight at night, even on the dim setting. I opened it up and altered the drive to make them less noticeable. Guess what? The variation in intensity reared its ugly head and they look goofy. No biggie for a clock but in a car dash it might not be acceptable.

That being said, I’d go with the window tint film solution. As much fun and rewarding as it is going the electrical route, it’s far cheaper, easier and likely to provide a better overall solution to take the simple path.

I would find the lead that feeds power to the lamps and put a resistor in series with it.
I would start with a few 10’s of ohms, 5W.
Downside is this will reduce the maximum brightness too.

I put one of those “Happy Face” decals over the instument panel light. Works fine for me