Technical D97 Short Circuiting Coupling?

[boguing]

Here's another for @koalar.

Sequence of events.

Fitted a Pioneer double din head unit.

The dimming function made my gps app too dark to use in anything but full dark, and was a nuisance when driving with lights on due to poor visibility etc.

I cut the (I think yellow) wire at the head unit, extended each side and fitted a simple on/off switch. Worked perfectly, overriding exactly when I wanted.

I noticed that I had no illumination on the heater controls. Replaced the bulbs. And there was light.

Next day. No heater control illumination. Came on here and @koalar implied that there's a link between radio backlight and these controls.

Sure enough. Switch my switch and they light up.

This is where it gets really odd. They light up when my switch is off. Sometimes. If I switch it on and off a few times they'll stay off, and I have to switch the column light switch off and on to restore the heater panel lights. That implies there's something electronic involved.

Next. The head unit doesn't dim whatever I do.

Take the bulbs out of the heater panel. Still the same. This is really strange, because it's now restored to where I was at the beginning of the week when the switch was doing what I wanted. And the head still won't dim. (Except, see next para).

Two possible confusions. Maybe the bulbs weren't blown, and I just hadn't noticed the effect that my switch introduced. The 'dead' bulbs are in a reasonably full wheely bin.

A setting on my 'phone or Android Auto has changed? Can't see anything that I can control on either.

I'm a bit stumped. I'd assumed that the light circuit supplied a constant voltage to the head unit to control its dimming. From the circuit diagrams I see that the wire itself comes from the BCU, and that means it could just be a pulse. But if that's the case it couldn't be providing constant current to the bulbs in the heater panel.

Also in the diagrams the D49 short circuit coupling features in almost everything to do with the dashboard. What is it?

 

[koalar]

D49

One controlled wire from the body computer coming in
Splits to operate several devices at once, radio light, heater light steering wheel buttons if fitted


The body computer can be fussy, it's possible it's detected the wrong current and switches off the power

If you have access to a scanner that can connect to the body computer it should tell you what wrong

Otherwise put the original radio back temporary, I suspect it will the function fine, if it does I will have to have a think how we can get the new unit to match

 

[boguing]

That makes sense.

I don't think I ever saw the original radio, it had a DAB unit when I bought it. Pretty sure that it didn't dim. But also fairly sure that the heater controls weren't lit.

I'll stick MES on it for a look.

Still completely baffled by the heater lights coming on when the yellow is open circuit. May be even more electronics involved.

I feel that I'm definitely in your debt!

 

[boguing]

Sorted. When I was fitting the switch to disable the car's dim signal to the head unit (orange/white, not yellow as I mistakenly remembered) I had a choice - cut the car loom or the stereo loom. I chose the car one because the stereo is probably going to be around longer that the car.....

It's still a mystery that it behaved as it did, but I'd make a small bet that the the car's orange/white connects to something that comes back out of the plug and goes on to the heater panel.

Shifted the joint to the radio loom, reconnected the cut in the car loom and it all works as it should, and I still have control of the dimming.

 

[koalar]

Cool glad it's all fixed