Golf mk7: Auto Start/Stop disable switch - who wants to help?

[DV52]

A while ago now, I hosted a short discussion of whether it would be possible to automatically disable the S/S facility on a mk7 when the ignition is turned-on. As those that have this annoying facility will already know, the car doesn't remember the position of the S/S button after the ignition is turned-off and the driver must press the centre console button every time that the ignition is turned-on to re-disable S/S.

I know that it is possible to completely disable S/S with a VCDS tweak, but I was keen to find a mechanism whereby S/S would be automatically disengaged but the driver could re-engaged S/S if he/she so desired at a later stage in the journey.

Introduction
A quick look at the wiring diagram for the S/S button on the centre console (see extract below) suggests that the factory method for the S/S disabling facility is very rudimentary..

As is clearly evident from the wiring diagram, apart from the LED circuits for illuminating the S/S button (this illumination occurs both when S/S switch is pressed and when the dash lights are turned-on), the actual S/S disabling device appears to be a simple single-pole, single-throw, momentary switch. It can't be any simpler than that!.

What appears to happen is that the S/S switch shorts to earth the signal from PIN# 60 on the C socket of the BCM (i.e. J519 in the diagram). The shorting mechanism is done through PIN#4 on the centre console switch module:

Preliminary design - Timing considerations
So, according to the wiring diagram above, all that needs to happen to automatically disable S/S every time that the ignition switch is turned-on is to design a circuit that automatically triggers when "terminal 15" is energised so that it momentarily shorts-to-earth PIN#4 on the centre console switch. Doesn't sound difficult- I hear you thinking (and you are correct in your thinking).

But, I learned long ago that it pays to be conservative when designing circuits for applications about which little is known. There are (at least) two problem here related to the fact that I can find no information about the characteristics of PIN# 60 on socket C of the BCM:

1. I'm not sure how quickly the signal from this pin appears after the ignition is turned-on.
2. More importantly perhaps, I don't know the electrical characteristics of the circuit inside the BCM that produces the signal at PIN# 60, So I'm blind to the ability of this pin to either source, or to sink voltage traces from external switching sources.

In respect of issue 2, I believe it prudent to provide compete electrical isolation between any external switching device and PIN# 60.

With regard to issue 1, I believe that there needs to be a "delay period" after the ignition is turned-on before the S/S kill signal is sent to PIN#4 on the centre console switch. The objective of this period is to allow the car's circuits (including in particular, the BCM) to stabilise. For the sake of a number, I have chosen 5 seconds as this period (intuitive guess). I've also (intuitively) decided to make the duration of the S/S kill signal 2 seconds albeit I have noticed when I press the centre console button on my car that it's actually the downward press (i.e. negative edge of the switching pulse) that activates the disabling command.

So, using a conservative design approach and based on my intuitive guesses, the Auto S/S disabling device needs to have a timing cycle similar that that shown in the diagram below

Preliminary Design - Circuit configuration
The overriding objective in designing an Auto S/S switch is a low component count. There doesn't appear to be much space in the cavity behind the centre console switches, so I'm keen to avoid the need to jam a heap of electronic components into this area. After a few initial designs, and a bit of research, I was able to locate a monostable oscillator that was available from a well known internet provider (the oscillator cost less than $15 AUD delivered). The oscillator can be self triggered at start-up and it has two variable resistors to provide the timing periods for the initial wait (T1) and for the period of the S/S kill signal (T2). All that was needed in addition to the oscillator was a DIL style relay to provide the electrical isolation that I mentioned earlier.

Once the device is fitted (and assuming that it operates as designed), the driver is able to re-energise the S/S facility as early as 7 seconds after the ignition is turned-on, if desired. Else, the S/S facility will remained turned-off and it will be turned-off again at the next ignition cycle.

The full circuit arrangement for the complete device is shown below.

As can be seen, installation of the device requires the connection of just three wires. I've taken the Terminal 15 supply (i.e. switched +ve 12Volt) for the device from the neighbouring PIN# 10 on the same switch module that houses the VW S/S disable switch (read post #14 of this thread for a discussion on this matter). Hence the three "take-off' points are from the same switch module - making the task of installation and splicing the new connections easier (only access needed is to the switch model in the centre console). If the vehicle does not have a TCS/ESP button on the centre console, attach the +12Volt (T15) wire to the ciggy lighter power supply wire (which will be Back/Yellow as well)

Please note that the wiring arrangement shown in the schematic above will NOT work if fuse# 40 has been altered to make power to the ciggy socket a permanent 12Volt supply (i.e. the auto S/S disable switch needs a Terminal 15, switched power source to trigger the monostable oscillator and fuse# 40 provides this supply to Pin# 10 on the centre console module).

The physical arrangement for the circuit above is shown in the picture below:

Components
The Auto S/S kill switch requires the four items shown in the picture below:

Other than the Timer (Mono), the items should be readily available at any decent electronics store. The Mono can be sourced from a number of Hong Kong based suppliers as listed below:

eBay

Whilst the mono is designed to operate in a number of modes, it is used in this application as a simple delayed one-shot pulse generator with self triggering occurring at start-up. In this configuration, the mono is used as-is without any additional soldering links necessary to the device itself.

Construction

1. Start by completely cutting-off the 4 legs of the relay on pin numbers 9, 8, 13 & 14 as closely as possible up against the relay package (All the legs on this side of the relay are superfluous and they become a shorting hazard if they remain intact)
2. Solder the diodes to the pins of the timer as per the circuit diagram. Make sure that the diodes are a snug fit against the Mono and it’s very important to ensure that the diode is correctly oriented so that the “silver band” on the diodes face the Vcc and V-out pins - connecting the diodes in the opposite direction will destroy the Mono!
3. Solder a short wire across the Earth and Trigger pins of the Mono
4. Place a piece of double sided tape to the rear of the Mono (see pic below) and mount the relay onto the tape so that it is oriented with the legs facing the Mono pins (notch to LHS as per "back view" pic below)
5. Solder the Earth and V-out pins on the Mono to the relay and attach the three wires that will connect to the centre console switch
6. Once the components are soldered together, place the switch on a test bench and adjust the potentiometers so that T1=5sec and T2=2 secs. Hook-up a multimeter (on ohms scale) to the relay out wire to "see" the pulse, or use an LED and 240 ohm series resistor connected to +12Volts.
7. Slip the soldered components into a sleeve of heat shrink tubing to protect the switch

Way forward- Who can help?
Having gone this far, and having constructed an initial functioning unit that works well on the test bench, I'm looking for someone who is willing to install and to trial the device on his/her car. The value proposition for anyone interested in helping-out is that I will provide the test unit for free which they can keep EDIT: Forum colleague Mattaus has kindly offered to help out - many thanks
Cheers
Don

POSTCRIPT: I have designed and built an alternative version of the Auto S/S kill switch that uses a memory module instead of the Mono timer described in this post . Details of the alternative device can be found HERE.

There is also a more sophisticated version of the design for the switch HERE