Instrument Dimmer for VW Polo 9N: Circuit Tricks

Hello everyone,

While I don't regret buying the base model, there are a few minor things that bother me.
For example, some models do not allow the brightness of the instruments to be adjusted, while the better versions have a dimmer: a double control, along with a brightness adjustment knob, integrated as a small addition in the dashboard (separate from the main light switch).
My base model car only has the headlight range adjustment knob in that compartment.

So, I bought the dual controller and installed it, but I can't seem to adjust the brightness.


Regarding the circuit:
The original headlight leveling system has 4 connections: 3 for the sensor potentiometer (one of which is ground), and a fourth connection for the incoming lighting voltage (wire 58d), whose circuit is closed through the ground pin of the sensor potentiometer.

The light intensity controller and dimmer have only one extra pin, which is for supplying power to the internal dimmer electronics (which contains an IC and a few surface-mount components) via the pilot wire. I had temporarily installed a corresponding cable, but as I said, nothing could be controlled.

According to the SLP (Service Literature Publication), the output of the instrument dimmer is located on cable 58d, which also supplies power to the control panel lighting (and other lighting such as the rear window switch, ashtray, and heating controls...).
And the power for this wire (58d) apparently comes from the novel body control module that integrates various functions such as turn signals, horn, windshield wipers and washer, rear window heater time limitation, CAN-K gateway, interior light control, etc.
(Brave New World: For example, if a relay, such as the turn signal relay, breaks in older cars, the replacement part might cost 20 euros - but with the 9N model, you might have to buy a new body control module, costing over 500 euros . But that's just a side note.)

All other connections of cable 58d are only consumers.


If you connect the combination regulator directly to 12 volts (as if the parking light is turned on), it can effectively control its own lighting. At its output (58d), you can measure a voltage of approximately 4 to (supply voltage - 1 volt).
However, it appears that the vehicle's control unit is constantly outputting the full battery voltage to the dimmer output on the vehicle's electrical system, so that all lights are always at full brightness, regardless of the dimmer setting (fortunately, the dimmer seems to be able to handle it...).

If I were to disconnect wire 58d from the body control module (according to the wiring diagram, it's the only connection to the instrument dimmer output!), the instrument dimmer might potentially power the other lights (if it has enough current capacity), but the body control module would no longer know which brightness commands to send to the instrument cluster via CAN.


Okay, now here are the questions for the experts in automotive electrical systems:

Can an output from the body control module (58d for switch illumination, etc.) simultaneously function as a sensor input that reads brightness commands from the instrument dimmer?

If so, how does it work, and is it possibly only necessary to appropriately code the vehicle's body control module so that the brightness control of the instrument lighting is activated?

I can email you the PDF specification sheet if needed (in case my circuit description is too unclear or unbelievable).

hello,

Unfortunately, I cannot help you with that.

But did I understand correctly that your Polo, when new, doesn't have the option to dim the instrument lighting? That can't be true! My girlfriend drives a Polo from 1991, a Fox model, and even that has that feature!

What exactly did VW do there?

I wish you much success with your project. In case of emergency, you can simply connect a dimmer (potentiometer) to the tachometer lighting.

Best regards.

BlauBaer wrote:

What exactly did VW do there?

Hi,

saved - and created a subtle incentive, instead of buying the base model, to opt for the Comfortline, which costs a whopping 1150 euros more .
The fact that the interior light doesn't have a reading light is the second, and last, thing that bothers me about this base.
But if the Comfortline light fits into the basic headliner, it will also be installed - and according to SLP, no electronics can interfere with that .

Quote:

I wish you much success with your project... In case of emergency, you can simply connect a dimmer (potentiometer) to the tachometer lighting.

No, that probably won't work. Because the instrument panel connector doesn't have a connection to pin 58d, but instead receives the desired brightness information via CAN from the vehicle's onboard control unit.

ulf wrote:

...For example, some models do not allow the brightness of the instruments to be adjusted, while the better versions have a dimmer: a double control, along with the brightness adjustment knob, as a small addition in the dashboard (separate from the light switch). My base model car only has the headlight range adjustment knob in that compartment. . .

That can't be true!

Even our basic LUPO model already has a brightness control for the dashboard lighting.

Does the Polo already have PWM control, or is it still using the conventional voltage divider control?

Hi Ulf,

Have you checked if you can access this amazing engine control unit via diagnostics?

Perhaps it's possible to adjust a basic brightness level with it, which would also partially solve your problem (if I remember correctly, you want it to be as dark as possible).

Because it appears that this specific power source supplies the entire system, it should be able to provide sufficient power, and it is likely controllable by the controller, which means that implementing a PWM (Pulse Width Modulation) would be a relatively simple task.

Are you absolutely sure that the two circuits (yours and the one with the control) are identical, and that it's not the case that the output of the "dimmer" goes to a DIFFERENT connection on the control unit, which receives information about the desired brightness, and that yours is connected to +12V?

I highly doubt that an output can also be an input... that would be a rather risky circuit design.

To get a better understanding, I would try to determine the maximum load that the "dimmer" can handle. You can roughly calculate how much current the lighting needs, and then you can slowly increase the load with a potentiometer while observing the output voltage and stop when it starts to drop.

If the dimmer has enough "power" to operate independently and supply power to the grid, then it might be possible to switch it in the way you mentioned. Otherwise, it would only provide input power to the other device (which makes more sense, considering that the device exists). Alternatively, you could try powering up the device and looking for the datasheets of the components; that should also reveal the power consumption.

Best regards,

Jan.

@ bertil

He definitely has a PWM (Pulse Width Modulation) system, because you can't properly control LEDs any other way, especially when they are part of a cluster.

@ulf

In my Octavia plans, there's a 'interior lighting amplifier' marked. It redistributes the 58B signal from the controller to a few more lights.
I can check later this afternoon to see if that part is actually installed and what it's doing.
Perhaps this will provide some clues.
However, to my surprise, I've noticed that the vehicle wiring harnesses are absolutely specific to the equipment installed. In my Octavia, not a single cable is redundant. There's no extra cabling like you used to see. So, it's possible that you're just unlucky.
Unfortunately, I don't have any Fabia-specific plans, as that model also uses the same body control module.

CU Gremlin.

Gremlin wrote:

Unfortunately, I don't have any Fabia plans, as it also uses the body control module...

Hi Ralf,

No surprise, it's the same base... If it helps, I'll email you the Polo-SLP files.

Jan6K wrote:

Have you checked if you can access this amazing engine control unit via diagnostics? Perhaps it's possible to adjust a basic brightness with it, which would also partially solve your problem (if I remember correctly, you want it to be as dark as possible).

Hi Jan,

AFAIK, that part is diagnosable. However, for those settings, I would need the channels to be selected again, etc.

Quote:

Are you absolutely sure that the two circuits (yours, and the one with regulation) are identical, and that it's not the case that the output of the "dimmer" goes to a DIFFERENT connection of the control unit, which receives information about how bright you want it, and that yours is connected to 2?

According to SLP, that is the case. I have the plan for the instrument dimmer, but in my car, the only thing missing is the power supply for the dimmer from the parking light circuit.
The cable configuration in the connector of the vehicle's onboard control unit matches the plan exactly with Dimmer 1.

Currently, I suspect that the aforementioned input 58d of the vehicle's body control module (BCM) is also an output that can be switched to 12 volts via coding. In this case, the input would receive these 12 volts as the input for the (missing) dimmer, and the lighting would be set to maximum.

When the 12-volt power is turned off, the input reacts to the voltage supplied by the dimmer (which would then power the lights for the switches, ashtray, and heater) and converts it into commands via CAN, for example, for the instrument cluster.

Hello Ulf,

As I understand it, pin T6c/2 on the engine control unit (ECU) for the vehicle's electrical system is always an input, and the constant 12V power doesn't come *out* of it, but rather *goes in*.

I suspect that in your configuration, a bridge is located between module 58 and 58d, because, as you correctly wrote, the dimmer is normally located there. Unfortunately, I haven't been able to find any SLP modules without a dimmer. It might be due to the older version of ElsaWin I have access to; in any case, the ASZ (Antenna System Control) is not selectable for the Polo in my system either.

My suspicion about the bridge's location is: somewhere near the (non-existent) dimmer.

'Just as a test, disconnect terminal T6c/2 from the control unit for the vehicle's electrical system. I bet your lights will still work (except maybe those on the instrument cluster). The cable thicknesses alone (thin at the control unit, thick at the switch) suggest that the current isn't coming FROM the control unit!'

But without an SLP (Smart Lighting Profile) and without a dimmer, all you'll be left with is: find the bridge between 58 and 58d.

I'm sorry I couldn't be of more help.

Regards,
Private.

PrivatBereich wrote:

Just for testing, could you disconnect terminal T6c/2 from the control unit for the vehicle's electrical system?

I bet your lights will still work (except maybe those on the instrument cluster). }
I just did that (before I read your answer) - and that's exactly what it is: a bridge/splice in the cable harness between the parking light wire and the instrument lighting.

Consequently, the instrument cluster remains dark because the lighting input of the body control module (BCM) is no longer receiving voltage. This voltage is converted into a signal on the CAN bus and interpreted by the instrument cluster (IC) as a command for the brightness of the instrument illumination.

So, I will redirect the "standby light" output of the light switch to the (currently unused) power input of the dimmer.
At its output (previously the input to the headlight leveling system's lighting), the dimmable voltage appears. Everything works fine, except that the license plate illumination is also dimmed.
Okay, so I'm going to disconnect those wires from terminal 58 / 58d and reroute them to the parking light output of the light switch; then it should work.


Thank you for your help!

I also once looked for the infamous 'lighting amplifier' in my Octavia.

That thing actually exists. And it made me laugh so hard I almost died.
According to SLP, it powers the light bulbs for the seat heating control and the heating actuator.
Since I have Climatronic, it only controls the seat heating regulator, and nothing else.
'But why are the lights for the ashtrays and cigarette lighters directly connected to the dimmer? They are also light bulbs. Everything else is LED... I guess it wasn't worth it for those few watts.'
-> joke number

Sure, here is the translation of the text from German to English:

'Cu Gremlin'

That's all for now regarding amplifiers. *g*

Best regards, WarLord.

Gremlin wrote:

but why are the lights for the ashtrays and cigarette lighters directly connected to the dimmer? They are also light bulbs. The rest are LEDs... probably they couldn't find LEDs for those few watts...

Hi Ralf,

While re-wiring my cable harness for dimmer operation, I initially misjudged the position of the cable splice. This resulted in the license plate lights also being connected to the dimmer, consuming approximately 10 watts.
However, the dimmer handled it without any problems, even without a turbocharger (the Polo-SLP doesn't have one).
So, I had to reattach the license plate light .

The dimmer actually uses PWM. The period duration is constant at 13 milliseconds, and the duty cycle is adjustable between approximately 20% and 90%.

Interestingly, the reaction to voltages above approximately 14 volts is as follows: even with the maximum brightness setting, the duty cycle is reduced again.

Okay...

With my luxury equipment , half of the interior is illuminated by default.

If I ever feel like it, I might take that piece of junk apart and see what it's capable of.

Cu Gremlin.

Thank you for your help!

You're welcome! It's the least I can do, considering all the technical articles and especially the diagnostic connector diagram you provided!

I'm glad it's working now, and that you're converting into a mini-comfort junkie .

Regards,
Private.

PrivatBereich wrote:

... and yet, you still end up converting to a mini-comfort junkie .

Especially the fact that he's already gaining weight again... even if it's only 50g here .

.... and yet, you still end up converting to a mini-comfort junkie


Especially the fact that he's already gaining weight again... even if it's only 50g here .

ohoh !!!