Dimming wire voltage

[genebaby]

It may seem an odd question but has anyone connected any digital gauges or similar to the solid grey dimmer wire in the light switch assembly?

I have a gauge that dims when the lights go off but randomly fluctuates bright again at night.

Checking the voltage on that wire it seems it doesn't stay a constant 12v in my car, it started at 7v and finally went down below 3v, at which point the gauge started acting up.

This doesn't seem like normal behaviour.

[Umai Naa!!]

It's a PWM signal generated by the body control module.

It only sends out as much voltage as needed, for the component to function.

[DV52]

@genebaby: The Grey wire that terminates on pin#10 on the rotary light switch is what is known as a "Terminal 58" (T58 ) supply - which simply means in DIN 75552-speak that it's instrument illumination voltage source. On a mk7, the car's T58 rail is generated by the BCM on pin #62, socket C - this wire runs to multiple points inside the cabin to back-light the myriad LEDs inside various switches (including the rotary light switch)

As you may be aware, illumination of the the instrument cluster clocks and lot's of other switches via the T58 supply is controlled by a photo-transistor that sits between the "7" and "8" numbers on the rev-counter (see picture below)



Grab a torch and shine it into the area shown, the clocks should increase in intensity (as should a number of back-lit switches inside the cabin.

The job of the photo transistor is to modulate the value of the T58 voltage-rail according to the ambient light levels inside the cabin - so at least on a mk7, no there is nothing normal about a 12V T58 rail!!

Don

[genebaby]

Thanks for the information guys, this is very strange. The instructions for the gauge are to use that wire for the dimming function, which has always been problematic.

I did a quick multimeter test yesterday at a friends house and I was sure we saw 12v with the car having the lights on, using the dial, and minimal mv with it off. Typical behaviour.

At home last night at dusk so the car was in auto lights night mode the best I saw was that 7v to start and now I see it goes down from there, and the gauge seems ok until it goes below 4v. Who knows how low it might go, and what for? It certainly seemed to be on nothing but a downward spiral.

Here is a pic from yesterday when after maybe 10mins it had gone to 4v and I decided to take a photo and then go for a drive, but then it hit below 4vand I saw the fluctuations start.



So just now I went out again in good daylight and lo and behold, we are getting a full 12v, as I thought I remembered seeing yesterday in the day.



In night mode at dusk I could never see 12v on any setting on the dial.

I was seriously blaming the gauge but the company said I'm the only one they have heard having this problem and they blamed my wire, but if my wire is standard for a 7.5 you would think they would see this often etc.

[DV52]

@genebaby:The readings don't mean much unless you describe precisely where are you measuring the voltages - explain where both meter-leads are connected (please)

Remember, the T58 rail voltage is generated by the BCM (hex09 module) and it's the ONLY supply to the back-lit switches in the cabin. So when you are reading very low voltages, the rest of the native LEDs in various switches should be greatly dimmed too - if the readings are correct.

Also, one of the common errors with these type of tests is that folk tend to believe what's displayed on the meter screen. As has been said already - this is a Pulse Width Modulated Voltage - which means that the T58 voltage is riddled with a series of ON/OFF pulses. I'm not familiar with your Digitech meter - so I'm not sure how it reacts to the inherent PWM frequency when asked to read a DC Voltage.

Finally, I'm not sure why you blame the gauge - it won't impact on the T58 rail unless it's drawing a high level of electrical current. The T58 rail isn't fused in the traditional manner - rather, the total load is monitored by the BCM. If the gauge was drawing high amps, you would see an error message from the BCM saying so!

Finally, lastly: have you checked how the gauge is earthed? Maybe this might explain your fluctuating voltages -of course, I'm totally guessing?

Don

[genebaby]

Hi Don, the reading are done thusly: Negative probe on ground, positive is touching the dimming wire.

I'm not blaming the gauge, I was before I took these readings. It seems to handle the voltage being 4V and above, I did not beleive my car could be the cause. The gauge doesn't draw anything from that wire, it's just sensing the voltage for dimming functions.

As it stands, my car shows different behaviour in the day and the night, and if it was reversed it would actually be ideal, as it reads a solid 12V in daylight, and was 7V and going lower at night time.

The gauge otherwise connects straight into the OBD2 port for it's power and signal information that it displays, this fuction I'm after, which is the standard setup for many others with this gauge, is simply dimming.

Can you, or someone else check your dimming wire voltage at night/dusk. This is either standard, which doesn't explain why there is nobody else with this issue, or my car is not behaving normally.

[DV52]

@genebaby: Alas I won't have access to my car for a while (lent it to a friend)- but I did check the voltage on my "Virtual" mk7 - which is the spaghetti mess in the picture below:



So, forgive the techno-babble that follows - but there just ain't any other way that I can explain this stuff: As I said in my earlier response, the T58 rail on a mk7 is generated by the BCM (pin #62/Socket C) and it's value is modulated by the photo-transistor in the hex17 module (I suspect that the dimming signal is communicated to the BCM as a CAN message - but I'm guessing). And... as has been pointed out by @Umai Naa!! - the method that is used for dimming is PWM.

With this as background information, here is a picture of the T58 voltage trace on pin #62/Socket C of the BCM on my test-bench - Alas, I had to use an old-fashion Cathode Ray Oscilloscope - it's all that I had in my workshop when I took the picture:



Notice the characteristic PWM waveform - it certainly isn't anything like a DC voltage!!

If you count the vertical axis lines on the screen -the ON pulse has an amplitude of 13.8 Volts (which is the value that I have set on the power-supply for my test bench) and the "Duty Cycle" of the waveform is about 30% - which means that the T58 voltage is energized for less than a third of the total time.

Now a good, laboratory grade multi-meter should read about 13.8/0.3= 4.6 volts if set on DC volt-scale - but "domestic" multi-meters often don't like seeing the PWM frequency when asked to read DC Volts. (incidentally, the PWM frequency for the T58 rail appears to be about 800 Hz from the screen shot - count the horizontal axis lines).

Finally, I also had a look at what information was available on the hex17 module about the T58 dimming function. At least for the dashboard module on my test-bench, it is possible to select Terminal 58 status in Live Data - like this:


I'm not sure what "A" means in the screen - but if I shine a torch into the photo-transistor, the value of "sensor for detecting light........." drops dramatically. So clearly the number in this metric is responsible for the dimming function (I guess).

Finally lastly, there is also information about the T58 rail in Live data in the BCM (not surprisingly). For example, if you select Interior Lighting, this is the result:


So, hopefully with this avalanche of information - you can better understand your readings

Don

[genebaby]

Thanks Don. I've been waiting to test on a friend's Mk7 Golf but in the meantime I realised we have an AW Polo.

Before dusk the dimmed setting gave a solid 12V. After dusk it still gave a solid 12V.

The differences. It's not a Mk7 Golf and it doesn't have the gauge attached, but it does show the expected readings at the expected times.

IE, it's a dimming wire and it gives 12V when dimmed and other gauges can use this to provide a dimming function. I've got this in my Falcon and the company that makes the gauge uses this wire for every gauge they produce.

I suspect the Mk7 will behave like the Polo, as being at 7V and steadily dropping does not seem like healthy behaviour, unless the gauge, which is only looking for a voltage signal, is causing the issue. But also only when dimmed at dusk/night in auto mode. It is fine if you force dimming during the day.

I think worrying about the PWM nature of the BCM controlling this is overthinking things. Clearly any minute fluctations are not going to be seen by anythting outside of an Oscilloscope and it won't worry any gauges hanging off it.

So it's still down to my car having a strange issue in auto lights dimmed mode, or the gauge is somehow affecting things.

[genebaby]

Ok, I have done the same test on another Mk7 Gold and it behaves as any auto electrician would expect, a solid 12V when dimmed, in the auto mode at dusk, like our Polo.

Went over to my car to show him and she showed a quick 8V and then dropped to 7V sharply and went down slowly but surely from there. We left it for a while while we talked and now I see that the P3 Gauge starts misbehaving at the very low 4V range. A few minutes drive home but the voltage was already nice and low and it was flashing bright and dim a lot.

To be sure we not only disengaged the dimming wire from the gauge circuit we also unplugged the power completely, to simulate not having the gauge in the car and the situation did not change.

Such a strange issue, but I guess I'll be seeing VW now about it as it's definitely not the way this should be.

PWM and BCM control be damned, that doesn't matter, it's 0V or 12V, like every multimeter will register and now I know that.

[DV52]

PWM and BCM control be damned, that doesn't matter, it's 0V or 12V, like every multimeter will register and now I know that.

haha..... must be wonderful to be finally proven that you were correct from the outset - well done!!

Hope the VW dealer agrees with your conclusion

Don