bad earth

[bobunit1]

This will probably sound stupid. What is a bad earth and how do you fix it.[/quote]

[markhanoi83]

[defblade]

An earth on vehivles is the path electricity takes to get "back" to the battery after going through the thing it's making work, eg a bulb. This is known as the negative side of the circuit. (It's not really this simple, but it's enough to work with).

It's normal for a short wire (or even the mounting plate) to carry the negative current to the metal frame of the bike, and the negative terminal of the battery to also be connected to the frame of the bike, so rather than running wires right back to the battery for every single electrical item, the frame carries it all.

This is why you always disconnect the negative side of the battery before the postive - otherwise, if you touch any bare metal (or often even painted metal) with the spanner/screwdriver you're using to undo the battery positive, you'll create a short circuit with lots of lovely sparks and possibly burns and death for you. ALWAYS DISCONNECT THE NEGATIVE FIRST!





A bad earth is when the current doesn't have a nice clean metal-to-metal contact at some point in the return journey to the battery, the electricity will then find other places to go causing all sorts off odd problems. To sort, take off the battery negative before you start fiddling, look at the part you've got trouble with, identify the positive and negative wires coming out of it (the negative will usually be black) and folow the black wire to a point where it's screwed/bolted into the frame. Undo this connection, use sandpaper or similar to clean the metal connector on the end of the wire and, depending on how it's connetced, carefully clean the mounting point (the connection is often made through the actual thread of a screw so don't go taking loads of paint off your frame, or it'll end up worse than you started! Clean the threads with wire wool). Then put it back together and give a little squirt of WD40 or smear some vaseline over it to protect it from further damage.
Sometimes there's no earth wire, and the component's mounting bolt goes through a metal plate in the unit (quite common with lights) and into the frame. Same thing to sort though - clean up the plates where the bolts go through and the threads as above.


HTH, turned into a bit of an essay!
____________________
Honda Varadero 125cc => Suzuki Bandit 650 33bhp => 77bhp =>
BMW K1200R Sport 163bhp => Aprilia Shiver GT 750 95bhp

[Robby]

The previous two posts are correct, but something else to note.

A bike has a lot of earth connections, and a lot of interconnections through the wiring loom. Sometimes a component is earthed through the frame, sometimes it connects back into a common earth on the wiring loom, sometimes both.

Electricity will always take the easiest path - usually the shortest route. If this path is blocked, either by a broken wire or a lot of corrosion, it will find another path. This other path is the problem, seeing as it will often mean interfering with another component.

This is one of the reasons why you'll often see the headlight getting brighter and dimmer in time with the indicators on an old bike. It isn't a case of insufficient power from the battery or alternator, but more that all of the connections are corroded to the point where the circuit can't carry enough current to run two indicators and one headlight at full brightness.