26 Mar 2012

ZFE too smart for its own good?

During my visit to BMW Battersea for my first service yesterday I had an interesting chat with one of the technicians who warned me about powering the TomTom cradle via the GPS connector. After doing a bit of digging I found that, because the it has a transformer integrated to step down the voltage from 12v to 5v, the cradle draws approximately 12mA just being connected to power. This increases to 70mA when the TomTom unit is clipped in with the battery charged, and 300mA when charging.

This is all fine when the bike is running of course, but the technician mentioned that in certain circumstances the slight draw can “fool” the ZFE into thinking a trickle charger is attached and wake up the CANbus circuit which will then slowly run down the battery.

I tested this by keeping the charging TomTom clipped in when switching off the engine. The TomTom would shut down as expected when the CANbus cut power after ~30 seconds, but about a minute later it would start up again, fully powered.

Looking around on several forums, a solution is to unclip the TomTom before switching off the engine. Doing this, it seems to stay inactive (clipping the TomTom back in after a few minutes doesn't start it back up). Without a multimeter to test the circuit  (I'm not willing to lick the contacts on the cradle) I can't tell if this is consistent. For now I've taken to disconnecting the lead from the cradle after parking the bike to not take any chances. I don't fancy the idea of the bike autonomously deciding to keep circuits live on a whim.

Has anyone else experienced anything like this? Any electricians among us? I presume someone knowledgeable in vehicle electrics could sort this by splicing in a diode to keep the voltage from seeping backwards (which presumably is what tricks the ZFE into thinking a charger is attached) or a transistor/relay to switch off the connection automatically when the draw drops down towards 12 mA. I suppose an alternative would be to splice in a switch or circuit breaker but this would defeat the purpose of permanently attached the cradle to a switched power source.

Unfortunately I'm not an electrician so I'm a bit at a loss with this, and I don't want to mess about adding bits into the wiring on a wish and a prayer... Any feedback would be gratefully received...