Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

I cannot get the brake lights on my euro 88' variant sport to work right. When I hit the brakes, the brake light illuminates, but it kills spark, which is weird because I would expect that IF the bulb was dead.

To begin with, I don't have spark unless the blue wire from the stator is grounded in some way. From the stator, the blue is the brake light power, and the green is for the HT coil/spark.

This diagram is for a US model, but it matches for the most part.

I do not have the original tail light, I lost it and got a CEV knockoff puch/tomos light from treats, with no resistor...that couldn't be it, could it?

Please, somebody help me figure this out, I wouldn't dare ride without brake lights in NYC, and not being able to enjoy the bike I spent so much time on is proving depressing.... :/

Derbi-Variant-Sport-US.jpg

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

Bump, anybody?

Re: Derbi Variant brake light woes

I what is that symbol that's between the y/g from the stoplight switch and the ground?

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

> Nick Haber Wrote:

> -------------------------------------------------------

> I what is that symbol that's between the y/g from the stoplight switch

> and the ground?

You mean this?

20190722_163220.jpg

Pretty sure it's just a spade terminal/ground point? Theres a central 4 prong ground point where all the grounds converge up top behind the headlight..maybe its referring to that?

In any case, I don't have a kill switch, and there is no green wire from the ht coil going up to the bars. I just have the blue wire going up to the brake switches, and the yellow/green from the switch to ground

This whole thing is so frustrating mainly because the brake lights were working fine before I rebuilt the bike...only thing different is that I have a non OEM tail light now, and I wired an external condenser...I didnt do anything to change the harness from what it was..at least I can't remember changing anything.

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

Bump

Re: Derbi Variant brake light woes

So after looking at the diagram longer I'm pretty sure you're gonna have to get a 6.2 ohm resistor and just solder it in line on the stop light circuit. If you look at the circuit, without that resistor the whole node is connecting ground to the Y/G going to the coil.

With that resistor, you will have a voltage to the coil whereas right now the coil is going to be grounded. You'll then have to un-ground the blue wire wherever it is, cause I think what happens is instead of the stop light circuit (and therefore the ignition coil) grounding with the coil it's grounding wherever you have it which is letting the bike spark but not illuminate the tail light at the same time (since when you illuminate the tail light it connects ground back into the circuit and therefore straight into the coil).

I hope what I wrote makes at least some sense.

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

> Nick Haber Wrote:

> -------------------------------------------------------

> So after looking at the diagram longer I'm pretty sure you're gonna have

> to get a 6.2 ohm resistor and just solder it in line on the stop light

> circuit. If you look at the circuit, without that resistor the whole

> node is connecting ground to the Y/G going to the coil.

>

> With that resistor, you will have a voltage to the coil whereas right

> now the coil is going to be grounded. You'll then have to un-ground the

> blue wire wherever it is, cause I think what happens is instead of the

> stop light circuit (and therefore the ignition coil) grounding with the

> coil it's grounding wherever you have it which is letting the bike spark

> but not illuminate the tail light at the same time (since when you

> illuminate the tail light it connects ground back into the circuit and

> therefore straight into the coil).

>

> I hope what I wrote makes at least some sense.

At last, a glimmer of hope!! Thank you so much for the detailed explanation..that's exactly the kind of answer I needed. I'm going to get a resistor and give it a shot, and report back!

Re: Derbi Variant brake light woes

I'm gonna post my response to Pm's here for future reference for people who don't have good experience with electrical circuits. This is super shitty napkin math that doesn't account for some things, like the internal resistance of the coil but whatever.

6 ohm is probably close enough to work too if you can find those cheaper. By my shitty napkin math the rear bulb is rated at 6V 10W so it's rated for 1.667 A. With a 6.2 ohm resistor @ 6v you'll have 0.968 A but with a 6 ohm resistor you'll have 1.0 A. I don't see the .03 A making much of a difference to the coil, and 1A is well within spec of a 10W bulb.

How I did the math:

Using the rating of the stock tail light bulb (6v,10W) and the equation P=IV you can find current (I=P/V, or I=10/6=1.67A in this case). You can expect that to be the maximum current in that loop. So on a 6V bike with a 6.2 Ohm resistor, using V=IR (I=V/R, or I=6/6.2=0.968A) you can see that this falls well below the rating of the taillight (and by assumption, the rating for this loop of the circuit). You could plug and play resistance values if you have some resistors laying around to see what the current is gonna look like in that loop. I wouldn't stray too far from spec though, since like I said this is a rough estimate that doesn't account for things like the internal resistance of the ignition coil.(edited)

Re: Derbi Variant brake light woes

You're over thinking it Nick.

It has nothing to do with the missing resistor. Electricity takes the path of least resistance. That resistor provides a ground when the brake bulb burns out so the engine will keep running (a fail safe). That's why it's wired in parallel with the bulb. The resistor needs to have more resistance than the brake bulb so ground will be routed through the bulb and light it. What makes the bulb light? Breaking one or both of the brake switches wired in series going to ground. Which brings me to another point. That wiring diagram has a typo. The brake switches shown normally open should be normally closed. They come both ways btw. And the kicker?? The brake light works as its supposed to when Brandon pulls the levers.

Your problem is elsewhere and not a missing failsafe resistor for a burned out brake bulb. You've got wiring crossed somewhere. "A rebuild" includes a lot of disconnected wires. Don't understand why it would involve removing the taillight though. Are you positive you have the wires in the correct locations going to the tail light assembly? Tail light is marked with letters for each circuit.

There are many other peds that use this tail light with or without the resistor. Garelli added the resistor around 1980 to its models.

Just in case either of you have any doubts about what ive said:

Screenshot_2019-07-24-04-52-47.png

Re: Derbi Variant brake light woes

> live ɘvil Wrote:

> -------------------------------------------------------

> You're over thinking it Nick.

>

> It has nothing to do with the missing resistor. Electricity takes the

> path of least resistance. That resistor provides a ground when the brake

> bulb burns out so the engine will keep running (a fail safe).

You're right that I went overboard with my math and shit up there, and I wasn't paying attention and I thought the resistor was in series not parallel. But I think you pointing out that the resistor is in parallel is just proving my point that the resistor is the issue.

Here's a shitty paint drawing I did at work. Current will never flow through the brakelight if the resistor is not there and the wire is shorted.

EDIT:

Actually after drawing all this out and thinking about it you probably just removed the resistor and didn't short the wire, right? So that leg of the circuit doesn't exist. Your shit doesn't run unless you ground the blue wire. The original resistor was also probably grounded to the frame, where the tail light connected to the resistor/ground. You don't have a ground after your tail light bulb right now right? You're grounding it before you hit the bulb and therefore the tail light will not illuminate. If you don't have the blue wire grounded your coil isn't grounded and therefore it won't run.

DOUBLE EDIT: Wait does the tail light work? if so then I'm wrong since they ground in the same spot(edited)

brakelight circuit.png

Re: Derbi Variant brake light woes

> Nick Haber Wrote:

> -------------------------------------------------------

> > live ɘvil Wrote:

>

> > -------------------------------------------------------

>

> > You're over thinking it Nick.

>

> >

>

> > It has nothing to do with the missing resistor. Electricity takes the

>

> > path of least resistance. That resistor provides a ground when the

> brake

>

> > bulb burns out so the engine will keep running (a fail safe).

>

> You're right that I went overboard with my math and shit up there, and I

> wasn't paying attention and accounting for the resistor being in series

> not parallel. But I think you pointing out that the resistor is in

> parallel is just proving my point that the resistor is the issue.

>

> Here's a shitty paint drawing I did at work. Current will never flow

> through the brakelight if the resistor is not there and the wire is

> shorted.

Nope. You do know this is a.c. not d.c. right?

You're forgetting this tail light comes both ways. With a resistor and without. The difference is with the resistor the motor continues to run when bulb blows.

Without the resistor the motor dies when pulling brake lever and bulb is blown.

There are three possible paths to ground for the stator coil in order of the path of least resistance:

1. Brake light switches

2. Brake bulb

3. Resistor

Remove that resistor and the engine dies when the BRAKE BULB BURNS OUT.

The brake bulb is WORKING when he pulls the lever as stated above ^^^.(edited)

Re: Derbi Variant brake light woes

> live ɘvil Wrote:

> -------------------------------------------------------

> Nope. You do know this is a.c. not d.c. right?

>

> You're forgetting this tail light comes both ways. With a resistor and

> without. The difference is with the resistor the motor continues to run

> when bulb blows.

>

> Without the resistor the motor dies when pulling brake lever and bulb is

> blown.

>

> There are three possible paths to ground for the stator coil in order of

> the path of least resistance:

>

> 1. Brake light switches

>

> 2. Brake bulb

>

> 3. Resistor

>

> Remove that resistor and the engine dies when the BRAKE BULB BURNS OUT.

>

> The brake bulb is WORKING when he pulls the lever as stated above ^^^.

I mean, you're right that I was wrong, that's why I immediately edited it and realized that all of my previous logic was based upon my misreading of the wiring diagram.

In the article you posted: "If there is a poor ground under the rear fender, anytime the brakes are applied the engine will die"

Which is what I was trying to edit the above post to say. Check the brake light ground.

Re: Derbi Variant brake light woes

> Nick Haber Wrote:

> -------------------------------------------------------

> > live ɘvil Wrote:

>

> > -------------------------------------------------------

>

> > Nope. You do know this is a.c. not d.c. right?

>

> >

>

> > You're forgetting this tail light comes both ways. With a resistor and

>

> > without. The difference is with the resistor the motor continues to

> run

>

> > when bulb blows.

>

> >

>

> > Without the resistor the motor dies when pulling brake lever and bulb

> is

>

> > blown.

>

> >

>

> > There are three possible paths to ground for the stator coil in order

> of

>

> > the path of least resistance:

>

> >

>

> > 1. Brake light switches

>

> >

>

> > 2. Brake bulb

>

> >

>

> > 3. Resistor

>

> >

>

> > Remove that resistor and the engine dies when the BRAKE BULB BURNS

> OUT.

>

> >

>

> > The brake bulb is WORKING when he pulls the lever as stated above ^^^.

>

> I mean, you're right that I was wrong, that's why I immediately edited

> it and realized that all of my previous logic was based upon my

> misreading of the wiring diagram.

>

> In the article you posted: "If there is a poor ground under the rear

> fender, anytime the brakes are applied the engine will die"

>

> Which is what I was trying to edit the above post to say. Check the

> brake light ground.

Thats right and wrong.

You keep forgetting the brake light is working. Where is it getting its ground from?

Re: Derbi Variant brake light woes

> live ɘvil Wrote:

> -------------------------------------------------------

> Thats right and wrong.

>

> You keep forgetting the brake light is working. Where is it getting its

> ground from?

It's getting its ground from the bad ground on the fender. When you hit the brake light switch the ground is no longer in the handlebars, but is instead in the tail light. The bulb is grounding but the coil can't ground through it too because the ground is bad.

Instead of grounding the blue wire to get spark, ground the Y/G and see what happens.

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

I appreciate all the input! I have even less of an understanding of the fundamentals of electricity, so you can imagine how frustrating this is.

A few things to clarify:

-The running light works, and it shares a ground with the brake light

-when the brakes are applied, the brake light comes on, but it also kills the bike at the same time

-if I splice in a ground into the blue wire directly off the stator, I can squeeze the brakes and not kill spark..but the brake light does not come on.

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

Tonight I'm going to remove the fender and inspect the junction in the harness, hopefully I find my answer there

Re: Derbi Variant brake light woes

> Brandon Weiss (Detective brandon to you) Wrote:

> -------------------------------------------------------

> Tonight I'm going to remove the fender and inspect the junction in the

> harness, hopefully I find my answer there

Remove tail light and see if spade connections are crossed.

99% chance its something you put back wrong somewhere on bike.

Re: Derbi Variant brake light woes

I have a bag of those secret tail light resistors. Holler if you decide ya want one.

20190529_103417.jpg

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

> live ɘvil Wrote:

> -------------------------------------------------------

> > Brandon Weiss (Detective brandon to you) Wrote:

>

> > -------------------------------------------------------

>

> > Tonight I'm going to remove the fender and inspect the junction in the

>

> > harness, hopefully I find my answer there

>

> Remove tail light and see if spade connections are crossed.

>

> 99% chance its something you put back wrong somewhere on bike.

I removed the tail light and checked, everything was in order. Looks like I'm going to have to make a new wiring harness from scratch......sigh

Re: Derbi Variant brake light woes

> Brandon Weiss (Detective brandon to you) Wrote:

> -------------------------------------------------------

> > live ɘvil Wrote:

>

> > -------------------------------------------------------

>

> > > Brandon Weiss (Detective brandon to you) Wrote:

>

> >

>

> > > -------------------------------------------------------

>

> >

>

> > > Tonight I'm going to remove the fender and inspect the junction in

> the

>

> >

>

> > > harness, hopefully I find my answer there

>

> >

>

> > Remove tail light and see if spade connections are crossed.

>

> >

>

> > 99% chance its something you put back wrong somewhere on bike.

>

> I removed the tail light and checked, everything was in order. Looks

> like I'm going to have to make a new wiring harness from

> scratch......sigh

That's a bit overboard I think, surely it'd be easier to check check all the wires in the stop light/ignition circuit

Re: Derbi Variant brake light woes

> Nick Haber Wrote:

> -------------------------------------------------------

> > Brandon Weiss (Detective brandon to you) Wrote:

>

> > -------------------------------------------------------

>

> > > live ɘvil Wrote:

>

> >

>

> > > -------------------------------------------------------

>

> >

>

> > > > Brandon Weiss (Detective brandon to you) Wrote:

>

> >

>

> > >

>

> >

>

> > > > -------------------------------------------------------

>

> >

>

> > >

>

> >

>

> > > > Tonight I'm going to remove the fender and inspect the junction in

>

> > the

>

> >

>

> > >

>

> >

>

> > > > harness, hopefully I find my answer there

>

> >

>

> > >

>

> >

>

> > > Remove tail light and see if spade connections are crossed.

>

> >

>

> > >

>

> >

>

> > > 99% chance its something you put back wrong somewhere on bike.

>

> >

>

> > I removed the tail light and checked, everything was in order. Looks

>

> > like I'm going to have to make a new wiring harness from

>

> > scratch......sigh

>

> That's a bit overboard I think, surely it'd be easier to check check all

> the wires in the stop light/ignition circuit

I agree,

Creating a new wiring harness would be a last resort I believe.

There are still a couple of variables to consider:

1.) You have a European model and are referring to a U.S. wiring schematic.

2.) There's the Zener Diode to consider in all of this. That diode could be used for a couple of reasons.

3.) The wiring schematic shows normally open brake switches instead of normally closed brake switches. Both types were widely used and even look the same on the outside. One has to test on bike or with a multimeter.

More detective work is needed.

The most important clues are:

No spark until blue wire is grounded.

Brake light works when engine dies.(edited)

Re: Derbi Variant brake light woes

Screenshot_2019-07-24-21-10-56.png

Re: Derbi Variant brake light woes

Brandon Weiss (Detective brandon to you) /

>

> > That's a bit overboard I think, surely it'd be easier to check check

> all

>

> > the wires in the stop light/ignition circuit

>

> I agree,

>

> Creating a new wiring harness would be a last resort I believe.

>

> There are still a couple of variables to consider:

>

> 1.) You have a European model and are referring to a U.S. wiring

> schematic.

>

> 2.) There's the Zener Diode to consider in all of this. That diode could

> be used for a couple of reasons.

>

> More detective work is needed.

The brake lights functioned perfectly before i took the bike apart (and i mean fully apart, completely deconstructed)

So, theoretically it has everything it needs to function. No Zener......

The only things that i can think of that are different from before i took the bike apart are

-different tail light

-external condenser

Everything is put back the way it was as far as connections/terminals, so that ONLY leaves me with the possibility that somewhere inside the harness of wires, a place that i cannot see without slicing it apart, is something funky, like two wires with broken insulation or some shit, that broke while fishing it back through the frame somehow.... There is really only one way for all the wires to be plugged into each other, and after approximately 10hrs of staring and prodding and poking at this harness, and moping, the last resort is starting to look like the logical option.

The only thing i have left to try is the resistor, which today i learned is probably not it.(edited)

Re: Derbi Variant brake light woes

Remove brake bulb and try again to see what happens.

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