Do Not Sell My Personal Information Jump to content

Dashboard Light Dimmer not Working

Recommended Posts

Ever since I got my RX300, the dimmer on the dashboard has not worked. You can turn it and the lights don't dim.

So last weekend I got round to plugging techstream into the car to see if there was a setting that I could change. No luck, there isn't a setting.

However, you can see in the live data that when I turn the knob, the module responsible sees it. You can see the value change as you turn the potentiometer on the dash. So the module appears to be connected to the dimmer switch.

But I can't get the dashboard lamps to dim. I got annoyed with it last winter and put up with switching the screen off in the centre of the deash, but it would be good if I could just dim the lamps, especially the instrument binnacle..

The light sensor is working, because the headlamps turn on automatically, so if they are supposed to auto-dim, they should dim.

Does anyone have any ideas, or even details of which modules are responsible for getting the lamps to dim so I can have a poke around with techstream?

Link to comment
Share on other sites

Brouwers: On a car like the RX I don't think there is a direct connection, it's down to the modules to talk to each other in order to make it work. It looks like the module that takes the input from the pot is working because I can see the values change.

The problem is that when the headlamps come on, the instruments don't dim, so I need to know if the headlamps are controlled by a different module that isn't talking to the instrument module. If it isn't then I need to know why.

Without knowing if the same module controls everything, which module gets input from the light sensor for the automatic headlamps etc. It's hard to diagnose.

On a simpler car everything would be controlled by the body control module, but I suspect there are two different modules at play here. The bad news is there are no communication errors reported by techstream.

The strange thing is the owner's manual states at night there's an option on the nav display to go to "Day Mode", but my screen doesn't have that option. Conversly, the nav screen doesn't change when the lights come on. Again leading me to think that the module responsible for monitoring the outside brightness and/or the headlamps isn't passing the info to the module that controls dashboard brightness. Or if the dash brightness module has a daylight sensor connected to it, the sensor isn't working.

So I can happily twiddle the knob on the dashboard for the brightness, but that module isn't being told whether it's day or night.

I think I need a service and repair manual for the car. That way I can look at which module the dimmer connects to and where it gets it's information regarding day/night from.






  • Like 1
Link to comment
Share on other sites

14 hours ago, Herbie said:

Mark - I've just sent you a PM regarding this.


Herbie, many thanks for the info, that's so generous of you. I may need a day or two to digest it all! 

The strange thing is the exterior lights turn on and off automatically, so I just need to fathom if there is more than one light sensor. i.e. 1 for a/c another for lights and a third for interior lights. I've a lot of circuitry to pore over this weekend. Lol.

Or if it's all controlled by one module, what software setting I need to change. 😁

  • Like 2
Link to comment
Share on other sites

Well, just a bit of an update. After spending the weekend poring over the info Herbie gave me, I've hopefully come up with a diagnostic plan.

The Dimmer rheostat connects to the instrument panel module. The module dims the dash switch lights directly. It also sends signals to the other modules regarding the dimmer setting.

The thing that threw me for a while is there is also a dimmer cancel signal alongside the dimmer setting signal. More on that in a second.

The Nav Screen takes three signals regarding illumination:

The illumination signal to say the lights are on (and I assume makes it switch to night mode and dim the display) That comes from the network gateway module.

The dimmer signal to say how much to dim the display and switch illumination from the instrument module.

and finally the dimmer cancel signal from the instrument module.

I spent ages trawling through the details until I found the cancel signal is set by the switch that clicks on the dimmer rheostat. So turn the switch fully clockwise and the cancel signal is set to on.

I do know from my initial look with techstream the other week that the dimmer rheostat is working, because the values change from 0-255 in techstream. I now have to check the illuminate signal and the cancel signal to see if those values change to the appropriate values in techstream and then electrically… now I know what I'm looking for that is.

When I get chance to hook up the laptop to that car, I'll post an update.




  • Like 2
Link to comment
Share on other sites

17 hours ago, Silversalmon said:

That last reply makes me feel like Catweazle....

I'm one of those people that think the same when it comes to carburettors. It's all mechanical voodoo.

I'm a child of the digital age: sensors and computers I can understand. :-) Engine computers are simple: measure the amount and temperature of air going in to an engine and then squirt the correct amount of fuel in based on a table of values. Then check what comes out the exhaust pipe, tweak as required and repeat. 

Then manufacturers started thinking about adding lots of computers linked by network to control different body, transmission and engine bits and things went haywire...  I mean, is a computer really necessary to work the windows? What's wrong with good old switches?

You'll love the DiagnoseDan YouTube channel then. All he ever seems to talk about is CanBus problems..

  • Like 1
Link to comment
Share on other sites

  • 2 weeks later...

The Dimmer is now Fixed!

As I hate it when people post up faults and never post a resolution and how they fixed the fault, here's how I fixed it.

Thanks to Herbie's information, I could see that the Tail Cancel signal (the signal that allows or denies the dimmer to work) is set by the dimmer switch (independent of the dimmer resistor value). The dimmer switch connects into the instrument panel, which is a control module all of it's own. The Tail cancel signal is promoted to all other modules from there.

I fired up Techstream and went into the instrument module live data. I could see that the variable dimmer resistor changed from 0-255 as before. But, the tail cancel signal didn't change as the dimmer switch clicked. So there was something amiss with the switch in the variable resistor. So, I pulled the dimmer/Odo panel out of the dash for more investigation.

Luckily the dimmer unit comes apart easily with two screws at the back.

Looking at the front part, you can see the white circular thing that is turned by the dimmer knob. Not the missing quadrant (the chunk out of the side). Ignore the metal strip, that's just the stop so you can't keep turning the knob.


Now I looked at the circuit board that screws to the back of the unit. You can see the variable resistor that the shaft of the dimmer knob locates into, but at the side of it is a microswitch (the thing in the middle of the board that sits up from the board). That microswitch runs along the outer edge of the white circular disc. It's switched on and off by the missing quadrant. Looking at my switch, initially it was missing the brown lever on top of the switch. Noticing there were holes for a hinged lever, but no lever, I turned the front part upside down and shook it. Lo and behold, the brown lever dropped out! So I refitted the lever to the microswitch (it's a bit fiddly as it's absolutely tiny!), as you can see in this picture:


I plugged the bare circuit board back into the wiring, switched the car's ignition on. Then covered the light sensor on the dash. the lights came on but no dimmer. So I pressed the switch.

Success! the dash lights and Nav display dimmed! A cheap fix, with no parts needed!

I screwed the dimmer/Odo unit back together, refitted it and turning the dimmer switch clicks the microswitch and dims the dash. Now I can enjoy a less dazzling drive in the dark.

No idea why the brown lever had come out of the microswitch. I'll keep an eye on it and if it happens again, I might need a replacement dimmer/Odo unit. But for now, I'm a happy bunny.

  • Like 3
Link to comment
Share on other sites

Great to see that it's sorted Mark, excellent  :thumbsup:

Also good to see proper fault-finding skills at work instead of "I've replaced x, y and z but it's still faulty" and just throwing parts and money at it willy-nilly.

  • Like 1
Link to comment
Share on other sites

Herbie, I don't like throwing money at problems, I don't have a lot of it to throw about!

But this was a good little exercise. The thing that threw me initially was the fact that you had the dimmer signal and the T/Cancel signal was entirely separate. But then looking on lots of pages of the info you sent me, you could see the t/Cancel signal was promoted to the other units. The eureka moment was then understanding that the t/cancel signal overrode all the other inputs to modules (light sensor, headlights on/off, etc).

So the problem did boil down to that pesky little switch in the dimmer/Odo panel. Luckily Techstream can tell you the state of the tail/Cancel switch without pulling the dash apart. That's really handy! I could see the fault before I started pulling things apart.

Thanks very much for the info you sent, I couldn't have done this without it.

With your info and Techstream, it boiled down to a 15 minute diagnostic check to prove the problem. It just took  couple of nights looking through the circuit info and the module signals  to pin down what I should be looking for! Shows you how complicated these cars are. And then getting time away from Christmas shopping to hook everything up and fix the problem. :-)

  • Like 2
Link to comment
Share on other sites

On ‎12‎/‎10‎/‎2019 at 1:37 PM, stringbender said:

Amazing work Mark. You must be well chuffed with yourself.


Yes, I'm pretty happy. Some people might think it's overkill to use Techstream and spend a couple of days reading the manuals to find the fault is just a teeny switch, but these cars are so complicated you need it all to see where the fault really lies. Without it, you don't know if it's the dimmer switch, the dimmer resistor, the light sensor, the wiring, or the modules themselves. All the above are involved in the simple decision to dim the instruments and the nav display. Plus those signals go through about three different modules. Without finding the fault first you could be dismantling half the car, swapping modules, sensors and switches out and still not find it. 

Plus I've learned a fair bit about the car whilst looking through the info.

  • Like 2
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...