robt964

Hi Chaps,

I've a few questions to those who've fitted a kill switch AND are still using the original Motronic ECU.

Now, whilst not in use, I understand that the Motronic requires a permanent +ve connection to the battery to maintain some memory. This comes to pin 18 of the 55-way multiplug via fuse 35 (10amp). This also serves as the main supply when the unit is active (hence the large 10amp). The unit is then powered on and off by supplying a signal to pin 37

If a true kill switch were fitted ie total isolation from the battery, then this would be a major pain as whatever 'Adaptive' settings are held would frequently be lost when the switch is 'off'.

This first diagram illustrates the traditional wiring of kill switch. With a modern engine driven by an ECU, the switch contacts shown breaking the supply to the coils is redundant since killing the ECU kills everything.


This is how I think the wiring can be revised to maintain a supply voltage to the ECU once the switch is off.


So, in the normal 'on' position, the +ve supply to fuse 35 is made through switch Z (which will be closed) and by a very small mA 'delay/slow' fuse to the unswitched side. When the switch is moved to the closed position the battery is isolated and the ECU powers off as the signal pin 37 is now disconnected from the supply. Switch Z is now also open meaning that the supply to fuse 35 is now only possible via the low mA fuse. This fuse will be capable of supplying the small current required to maintain the memory but insufficient to run anything or cause an issue (it'd just blow).

Is this the correct approach with the Motronic? I'd rather not re-invent something here so if you're already done it, do you care to share?

robt964

So 65 'reads' and no replies

Come on, I know many of you have a kill switch fitted

StanUK951

That's 65 people who know less about electronics than you already do Rob, which includes me because I looked and moved on!

I have a kill switch that was wired in by PO. it kills power to everything so is a bit of a drag but a simple adaption with Scan Tool sorts it and I just live with it like this. I like your more elegant solution though.....

evanscott964

Ha Ha !
Afraid I can't help Rob I'm not running Motronic, is it imperative to have a constant supply what does it run?
Simon

StanUK951

Good point Simon, what about a small battery for the memory? Or is that a bit pointless given you have a big battery there already...?

evanscott964

Rob
Could you take an ECU supply wire, fused, from Battery side so it has its supply as per std setup. Then wire in the cut off as per the suggestion but use switch Z to cut the coils so it will cut engine when used but not supply to ECU
Also you need to be careful with your solution as the power will and does back feed through the Alternator if care not taken

kos11-12

Rob,
I think Garret376 did a writte up on this forum

robt964

Yes the alternator will feed back when the switch is flicked to 'off' when the engine is running, and that's the purpose of the large resistor, to dump the current and collapse the field in order to prevent the voltage rising. Hmm but I see what you mean, the motor will still run on as the ECU pin 18 is still being supplied and nothing introduced to cut it. I'll sleep on it....

kos11-12

https://rennlist.com/forums/964-1989...right-way.html

robt964

Thanks for the link Kos, he addresses the killing of the ECU in nice way but not the issue of supplying a small current when off. I think I will follow this route tho, and add to it evanscott's suggestion of keeping fuse 35 on the unswitched battery side. Thanks chaps

Jamie Summers

Rob,
RS has factory kill-switch, and I'm pretty sure there is no bypass to the Motronic, thus I'm really not sure how big an issue it is ??? Granted I'm running MoTeC, so can't really comment on Motronic specific issues, but I have no problems and car spends most of the time with the battery isolated.

Jamie

boxsey911

I don't have a kill switch but regularly disconnect my battery either to work on the car or when not in use over the winter. Car seems to run exactly the same each time I reconnect the battery. It's a nuisance having to reset the clock each time though!

ras62

The RS supplies the Motronic directly from the battery on a 1.5mm red wire via block E21 at the central electrics. The C2 motronic live is also fed from e21 via a link to e25. Disconnect the 1.5mm red at e25 and connect direct to the battery will effectively do the same job as the RS.

robt964

ras62 - thankyou for the confirmation. I'll do that.

robt964

Hi Steve, you're right. In all honesty I'm not sure how much of a tangeble issue it really is. We know that a car left disconnected for a period will loose its 'Adaptation' params. We know that these params can be benefical in maintaining a steady idle. I'm just trying to avoid a potential necessity to frequently use the scantool to put things right. Given the 'fix?' is just re-routing one wire and the RS model already does this, it's no biggy to make the mod if I'm already going to the trouble of fitting a kill switch.