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Got scooter out of hibernation and found the rear stop/tail light blows before I even w=switch on the lights. Is this down to just a poor earth ?

If it sounds like you have a short circuit somewhere, time to get the meter out and start testing the circuits.

jonno wrote:Got scooter out of hibernation and found the rear stop/tail light blows before I even w=switch on the lights. Is this down to just a poor earth ?

Generally in most cases thats what the issue is.... Easy way to test is to add another earth line to the light unit

sounds like damp causing a short if it was a bad earth the light just wouldn't come on

Check the whole light unit and the switch for the brake light, a good start. You could disconnect the feed to the rear light and see if there is a dead short.

Bulb not blowing now,but the lights are very dim.If I put on the main beam though the lights are very bright.

Maybe damp/dirt in switches. Give them a good clean and if damp, a blast from a hair dryer.

Just to clear up a bit of confusion in the suggestions given, a short circuit will NOT cause bulbs to blow. My money would be on a corroded contact in the lamp unit, either the bulb contacts or the bullet connectors. It could also be in the headset or the brake light switch. Poor earth somewhere is also worth checking out as already mentioned. The dim light issue also points towards a bad earth.

coaster wrote:Just to clear up a bit of confusion in the suggestions given, a short circuit will NOT cause bulbs to blow. My money would be on a corroded contact in the lamp unit, either the bulb contacts or the bullet connectors. It could also be in the headset or the brake light switch. Poor earth somewhere is also worth checking out as already mentioned. The dim light issue also points towards a bad earth.

He says it blows bulbs without the lights being switched on...........makes me think the switch must be part of the cause

MickYork wrote:

coaster wrote:Just to clear up a bit of confusion in the suggestions given, a short circuit will NOT cause bulbs to blow. My money would be on a corroded contact in the lamp unit, either the bulb contacts or the bullet connectors. It could also be in the headset or the brake light switch. Poor earth somewhere is also worth checking out as already mentioned. The dim light issue also points towards a bad earth.

He says it blows bulbs without the lights being switched on...........makes me think the switch must be part of the cause

Hmm, yes that is confusing, I assumed he just meant the brake light blew when he operated the brake. Either way, bad connection or poor earth somewhere.

I cleaned all the connections in the rear light and earth.That sorted the bulb blowing. Will go over the other connections tomoz.

Unfortunately I have to disagree on a short blowing lamps, one of the reasons lamps blow is because of a rapid change in current in a an inductive circuit, the current change can be caused by either the ‘supply’ or ‘return’ part of the circuit.
The initial diagnosis was the lamps are blowing before being switched on. If that were the case then power has to be coming from somewhere, ie a short

As a lamp is an inductor it will try and keep the current flowing by increasing voltage, the voltage is a result of the magnetic field in the lit lamp.
E=Ldi/dt. Where E is the induced voltage L = inductance and di/dt is the rate of change of current.
As the resistance increases, the current is changed, change of current causes the induced voltage to increase. If the rate of change of current is large enough then the induced voltage will be enough to blow the lamp.
If you clean and check all of your connections and as the others have advised to check out the switch

https://www.electronics-tutorials.ws/in ... uctor.html

Meds wrote:..................... If that were the case then power has to be coming from somewhere, ie a short

Bit confused as to how a short can produce power tbh I THINK what the tutorial was referring to Back EMF? but I'm not sure but thinking about it, if you applied a short across the regulated lighting feed, the shunt in the regulator would see that as additional load and increase the voltage to compensate. When the short is removed, maybe the it would take a few milliseconds to get the voltage back down??

Anyway, seems poor connections were to blame

I was thinking that the short circuit was bypassing either the brake light switch or lighting switch allowing the lamp to light when the scooter was started, rapid breaking of the short would cause lamp to blow. I wasn’t thinking about a dead short from live to chassis.
Overvoltages during indicative load switching is a big issue in high power and/or highly inductive loads, most modern motor control devices have snubber circuits to protect the switching devices. This effect happens in our low tec circuits but our switches are over speck’d and can deal with it. If you google snubber circuits there’s plenty decent explanations on why the overvoltages occur.

I see where you were coming from now Meds but the term Snubbing takes me back. It is or at least was used in the motorised systems used to move points on railway lines. The motor drives the points blades part way across and then a a snubbing circuit cuts in, disconnects the motor feed and then makes the motor act as a generator driving into a load to slow it down to prevent damage when the point blade hits the stock rail.

Sorted,I hope.Im sure the bulbs going was the damp. Noticed the headlight bulb had gone(dip) and the dim light was the sidelight (which comes on with headlight).Thats another story .Turns out the headlight holder,the bit that runs up the side of the new bulb base wasnt connecting sufficiently, on the new bulb.Its made me check all connections/earths which is a good thing.

Good news.

Bad news. Brake light keeps blowing at anything above tickover/low revs.Press brake pedal and increase the revs,mimicking road use and you can see the filament go.Obviously too much power going to the brake light.Using 12v/21/5w.

Check your regulator.

have you connected the back light and brake light the wrong way round.