Whine when charging TomTom

Hi all,

I spent the weekend I spent the weekend cleaning up the electrics on
my ER6-f, building something based around http://www.canyonchasers.net/shop/generic/relay.php.
The autocom and the hotgrips are working fine.

Fine that is, until I plug the USB power into the TomTom. With the
Autocom and the hotgrips running off of the battery, and the TomTom
plugged into the autocom, things work fine. Rider to Pillion comms are
working and the nice lady in the TomTom tells me where to go.

When I plug the USB cable into the TomTom to charge though, I get a
permanent whine. It starts out quite soft for around 2 seconds, then
gets louder. I've tried re-routing all of the autocom cables, but
nothing changes.

I was using the 'Hard wired 12v and 5v USB Cable kit' from
http://www.dogcamsport.co.uk/power_for_bullet_cam.htm to provide the
USB power for the TomTom. When the problem manifested, I went and
bought a spare TomTom car charger, stripped the insides out and
connected that straight to the battery (no fuses, no relay, nothing).
The whine is still there.

As it only happens when powering the TomTom, I'm confused. Any ideas
on where the problem lies or how to rectify it ? Without the TomTom
power, everything is clear and there is no unexpected noise.

Thanks in advance,

 

Hi mate,
at a guess it could be an earth loop - has it worked without the whine
(ever) with the USB plugged in, or is this a new installation? AFAIK the
Autocom will see (hear?) the noise from the earth loop as satnav speech,
fade down the audio and switch it through to the headphones (correct me if
I'm wrong), the 2 seconds fade-up is probably the fade-down on the other
audio?

What can happen is the earth wire in the Autocom audio lead and the earth in
the USB power lead complete a loop which picks up stray magnetic fields
(from alternator, ignition coils, etc.) and couples it into the audio - you
*could* try a one-legged lead for the audio (disconnect the braid shielding
at the tomtom end, maybe on a cheap lead from the Paaahhnd shop) and see if
it goes away - common practice with guitar effects loops off amps, where the
same thing can happen. Disadvantage is that if you then disconnect the USB
lead, your audio may disappear...

Assuming the Autocom's audio earth is good enough (should be, I'd have
thought, and the current to the tomtom should be well within its
capabilities), you could try disconnecting the earth (0 volts) side of the
USB lead instead, then the audio lead's earth would be used for both the
audio and charging connections

On the same sublect, the Autocomm "isolating leads" are about £25 each,
anyone know if they have an isolating transformer (big bulge in the cable,
I'd guess) or are they just one-legged leads? Anyone have one, and a meter
to buzz it out?

Dave H.

 

It's a brand new installation, so it's never worked before.

I would guess though, that the earth loop would have to be only in the
satnav power section of the thing, otherwise, why wouldn't the autocom
whine with satnav audio and now power?

Does this sound logical ?

Not sure I understand this. In fact, I'm absolutely sure I _don't_
understand this.

Would running the autocom straight to battery and the TomTom straight
to battery show whether this is the problem or not?

I assume that the autocom audio is good enough, as both autocom audio
and satnav via autocom audio is clean.

Would I need to hack a new cable for that? Or just snip the earth on
the USB lead?

I'd not heard about those autocom isolating leads - I might give them
a ring about those and find out - thanks

 

IIRC Autocom units seem to like their own leads.
I had an early one that utterly refuse to work properly without its family
member.

--
Greybeard

FLHR -03 UK (95 cu-in Stg 2. Big Boy 2!)

Garmin Zumo 550, To get me home!

ukrm@foxtails[dot]co[dot]uk

 

I picked up part number 1314 - isolating lead. Instant solution to the
problem.

I've now got my wiring all done up nicely, fused, relay off of the
tail light, etc. until I can bribe my step-dad into building it into a
box for me. Autocom works a treat and the GPS sounds great even on
charge.

Thanks for the pointers and advice !