Power to light for starter button CB350

[ducatiboy]

Hi folks,

I have a CB350 that I have built a wiring loom for from scratch. Everything works just fine BUT I have changed the starter motor button to this:

The red button is backlit by a light. I want the light to light up when the key is switched on but to then switch off when the engine starts. Where do I pick up power from that will switch off with the engine running? Sort of like a generator dash light which the cb350 did not have so is not on the wiring diagram. Can anybody help?

 

[Flugtechnik]

I don't think there is anything like that on these old bikes. My first guess would be to use some kind of relay that gets switched off when there is power coming from the alternator.

 

[Sonreir]

I don't think there is anything like that on these old bikes. My first guess would be to use some kind of relay that gets switched off when there is power coming from the alternator.

Exactly what I would do.

Use a normally closed relay and run one end of the contacts to a black wire. The other end gets wired to the light in the start button.

If you get a relay that can be run from A/C, you can wire it directly to a yellow wire and then you'll be good to go. So long as the alternator is producing current, the relay remains open and the starter light is off.

 

[ducatiboy]

I remember my old 500 pantah had a light like the one I'm after. With the engine running it would be off but sometimes when idling very low it would start to flicker on. Obviously it is was somehow wired up to the alternator to switch off when the alternator was generating current. The other option I was thinking of was a oil pressure switch wired into the start button. As soon as the engine starts there is oil pressure (hopefully!) which would switch the light off. Bit more complicated to do this tho as it would mean tapping into an oil passage somewhere...

Just wrote the above paragraph and posted before reading your reply Sonreir. That sounds perfect, I'll give it a try. Thx

 

[ducatiboy]

Sonreir, I read your comment quickly just as I was about to go out. Now that Ive read it a little more closely I'm not sure I understand. By running one end of a normally closed relay to a black do you mean to earth or to a 12V positive? This may seem like a silly question but I have been using the oldmanhonda diagram (http://oldmanhonda.com/MC/wiring350K4.html) which has black as positive. I am more used to thinking of black as earth.

Is this what you mean? Black (positive) goes to relay which is normally closed. Relay goes to light, light goes to earth. So when key is turned on the black lead powers the light via the closed relay. Yellow from alternator is also connected to relay. Yellow delivers AC current to relay when engine starts, opens relay, breaks circuit, light goes off. So essential the relay needs three wires to it. One from power (after ignition switch), one from alternator and one to light.

Is this right????? ??? ??? ??? Sorry for the dopey electrical questions. Electrons are not my friends...

 

[JohnnyC]

I'd use a tacho switching relay , as used in LPG systems on cars that arms/ disarms when a tacho signal (at coil -) is sensed or lost. That way no glow at idle which happens with most charge indicator lamps on bikes.

 

[Sonreir]

Sonreir, I read your comment quickly just as I was about to go out. Now that Ive read it a little more closely I'm not sure I understand. By running one end of a normally closed relay to a black do you mean to earth or to a 12V positive? This may seem like a silly question but I have been using the oldmanhonda diagram (http://oldmanhonda.com/MC/wiring350K4.html) which has black as positive. I am more used to thinking of black as earth.

Is this what you mean? Black (positive) goes to relay which is normally closed. Relay goes to light, light goes to earth. So when key is turned on the black lead powers the light via the closed relay. Yellow from alternator is also connected to relay. Yellow delivers AC current to relay when engine starts, opens relay, breaks circuit, light goes off. So essential the relay needs three wires to it. One from power (after ignition switch), one from alternator and one to light.

Is this right????? ??? ??? ??? Sorry for the dopey electrical questions. Electrons are not my friends...

That's all pretty much correct. A relay is a switch that requires power to turn on or off. In this case, you're switching the black wire (+12V) to the starter button light. The power to operate the relay comes from the yellow wire from the stator. Because the relay is normally closed, it will allow power to run through its switching contacts until it received power from the yellow wire. As soon as the engine starts turning, the light on the start button will go out.

 

[ducatiboy]

Great thx Sonreir. Message received and understood BTW my son says your avatar is "so cool". He's 13 so he'd know.

Johnny, using a tacho switching relay is a great idea. So instead of using a yellow wire from the alternator to open the relay and break the circuit, I'd splice a wire into one of the coil inputs (i.e. +12V) and run it to the relay. Engine turns over, creates signal simultaneously to coil and relay, relay opens, breaks circuit and boom ...engine light off! Additional advantage of no flickering on at idle and should also switch off while engine is turning over using the starter motor itself. Any disadvantage? Won't interfere with the signal to the coil at all?

Thx for your help guys! The DTT community is awesome 8) 8) 8)

 

[JohnnyC]

No interference as its designed to be wired to coil - terminal ( just like a tachometer). The ones I use ( can't recall the brand I just buy em as I need em) can also simply have the pickup wire wrapped around a HT lead and still operate perfectly.