Replacement Speedo

[Triton Thrasher]

My concern being that if a GPS speedo needs to be negatively earthed, then I might break it. I can always polarise the dynamo to negative earth to make things easy and fit a negative earth regulator.

A GPS speedo has + and - leads. On a positive earth bike, the speedo - lead is connected to a switched live feed from battery -.

The speedo + lead (probably red) is connected to battery +.  That can be achieved by connecting it to the engine, frame, handlebar or the headlight common return terminal,  so long as whichever you use has good continuity to battery-. 

[Rex]

You could, but it seems a faff when you should just convert the bike to the relevant (pos or neg) earth. The twenty quid jobs do work (for a time anyway!) and fit in the headlamp shell of the later bikes so look the part.
I'd do anything rather than have a modern looking device (like a push-bike speedo) on the bike. Just looks out-of-place.

[Triton Thrasher]

You could, but it seems a faff when you should just convert the bike to the relevant (pos or neg) earth. The twenty quid jobs do work (for a time anyway!) and fit in the headlamp shell of the later bikes so look the part.
I'd do anything rather than have a modern looking device (like a push-bike speedo) on the bike. Just looks out-of-place.

Connecting a GPS speedo to a + earth bike is exactly the same “faff” as connecting it to a - earth bike.

You connect + to + and - to -.

A polarity-sensitive LED headlight with dip, main and common return terminals is another matter.

And a cheap magnetic speedo of the right ratio, that fits your cable, is very probably good enough.

[Greybeard]

As I said, the polarity of the device can be independent of the bikes wiring. HOWEVER, if the device case is internally grounded you will may have a problem as it will could short out negative to bike positive/earth. If the gizmo is insulated from it's internal connection then I think it should be possible to use a NEG earth device on a POS earth machine.

[Triton Thrasher]

I’ve only used one GPS speedo. It has a plastic case and has leads for connecting to positive and negative of a power supply.

All the other ones on EBay look similar. 

Using the speedo casing as a connection to negative would seem like a strange thing for a manufacturer to do.

[Greybeard]

I just went searching for information and found this, which may help to explain what I'm on about. Similar problem.
https://www.mgexp.com/phorum/read.php?1,860081

[Greybeard]

Using the speedo casing as a connection to negative would seem like a strange thing for a manufacturer to do.

Well, in their design for a modern vehicle negative is equal to earth/ground so why not use the case as part of the return feed? I'm just putting forward a possible scenario; I don't have experience of doing this.

[Triton Thrasher]

I just went searching for information and found this, which may help to explain what I'm on about. Similar problem.
https://www.mgexp.com/phorum/read.php?1,860081

A fag lighter socket has to be wired for the same polarity as all the handy electronic devices you stick in it.  He can have it that way in his Morris, so long as he doesn’t mount the socket directly contacting a steel panel.  Hope someone told him that.

[Greybeard]

A fag lighter socket has to be wired for the same polarity as all the handy electronic devices you stick in it...

He's connecting it up so he can use modern, negative earth devices so he will feed it the opposite of normal; he will connect Neg to the metal outer of the socket and Pos to the centre connector, (After re-reading this I've swapped those around).

He can have it that way in his Morris, so long as he doesn’t mount the socket directly contacting a steel panel.  Hope someone told him that.

They did.
As I said on page 1:

....as long as the device....is insulated from the machine you can feed it with + and ground it with -.

[Triton Thrasher]

Well, in their design for a modern vehicle negative is equal to earth/ground so why not use the case as part of the return feed? I'm just putting forward a possible scenario; I don't have experience of doing this.

Earthing through the case makes sense for some things.  Original old bike speedo illumination used the case as a return path, although loss of good continuity was quite likely.

The makers of these GPS speedos just connect all the wires from the printed circuit to a multi pin plug in the base.  There’s a separate coaxial to the sensor,  but it’s not electrically connected to any part of the bike.

[Greybeard]

i dont think you can confuse a battery with oposite flows of current by doing that, and i want one if you can, its a new one on me

If you consider removing the battery from the bike and using it to power an electrical device on the bench you would not be bothered about Negative or Positive earth would you; you'd put the Pos to Pos on the device and Neg to Neg? So, my thinking is as long as the device is insulated from the machine you can do the same thing on the bike.

[berger]

greybeard if the battery on the bike is earthed -  and you put an independent led which is+ earth acroos the battery it will not work because the battery is already polerised

[Triton Thrasher]

greybeard if the battery on the bike is earthed -  and you put an independent led which is+ earth acroos the battery it will not work because the battery is already polerised

  Connect a 6 V LED between the battery terminals the right way round and it will light.  One terminal being connected to the frame doesn’t change that.

[muskrat]

Insulate the instrument (GPS) run its + wire to + on the battery and it's - wire to - on the battery, leave the bike wires as is. So long as the instrument is isolated it will work.
Cheers

[duTch]

 My first experience with using a extarnal device was when I was 17, bought a ewe-beaut Pioneer stereo and jury-rigged it into my old lady's Austin 1800..no-go, so switched wires and was ok (didn't realise was +ve earth) but blew a channel ....(Space Oddity on one channel is not so good )

 I agree with mainly the last two comments that so long as this kind of speedo is electrically isolated from the frame it'll work, and in the case of the LED light, depending on the if base is isolated like a P43t, the wires should be able to connect as as normal, as the dipping wires only run through the switch and don't earth, and horn doesn't (generally) matter- but well leave that out- and lighting, these should be debated in another arg-thread