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Help with removing a light bulb from motorcycle circuit?

  1. Jul 19, 2015 #1
    Hello there.

    I was looking for a place on the web where someone with an electrical understanding might be able to help me.

    I have a gas powered scooter which I wanted to make a change to the lighting circuit. My question is regarding the resistor in the circuit. With this circuit, while the engine is running, the headlamp as well as instrument lights are always on. The headlamp is a 12v 25w bulb. There is a 5w 6.7ohm resistor in that lighting circuit, which from what I understand, is there to protect the instrument lighting if the headlamp bulb has failed or is missing. The service manual says to check this resistor if you have an issue with the instrument lights burning out.

    I want to remove the headlamp completely from the circuit. Reason being is that I will create a dedicated new circuit for an LED headlamp which will have it's own +12 lead from the battery which will be switched through a relay and switch to turn the headlamp on, off, and high beam.

    In regards to this resistor in the circuit, would it make sense to replace the 5w resistor with a 25w resistor to simulate the load of the headlamp that was originally there? If that original resistor should be changed, would it be possible to use something less than a 25w resistor and still be safe? Something like 10w or 15w? I would rather convert the least watts to heat and have it wasted if the extra power could be used by the rest of the electrical system. I have had this scooter running before without the headlamp, and it eventually burnt out the instrument lights.

    I've attached a picture of the lighting circuit.


    Any help would greatly be appreciated!

    Thank you in advance.
  2. jcsd
  3. Jul 19, 2015 #2
  4. Jul 19, 2015 #3
    Correct. That is the auto choke. However, the auto choke no longer exists since the engine has a different carb on it now with a manual choke.

    The problem with your suggestion is that the whole point of doing what I am doing (aside from having an led headlamp) is to have the choice of the headlamp being off-on-high. Right now I am forced to have the headlamp on any time the engine is running. I want the ability to turn it off. My concern is that if I have the headlamp off with the engine running, is it going to again burn out the instrument bulbs, and from what I understand that resistor is there to prevent the bulbs from burning out. It is a 5w resistor that is being used with the headlamp on. The manual states if that resistor goes bad, it can cause the instrument lights to burn out. By removing that headlamp, it is removing 25w of resistance from the circuit which I would assume would be more than that 5w resistor can compensate for, thus requiring a larger resistor such as the 25w. The 25w would be equivalent to having the headlamp bulb still connected.

    I guess another option would be to leave that 5w in place and add a 10,15,or 25w resistor in place of the headlamp bulb. That would then provide the resistance where the headlamp used to be, plus the 5w that is already in place.
  5. Jul 19, 2015 #4
    From the diagram it does not seem there should be a problem. If the headlight burned out, same as being switched off, do you really think the resister will burn out? Try running it normally and feel the resister, guessing it will be warm to hot. Now disconnect the headlight and see if there is a large increase in heat.

    Think you are over thinking this. If not, hopefully others will get you going in the right direction.
  6. Jul 19, 2015 #5
    Are you saying that if disconnect the headlamp and run it on it's own circuit (as explained in the op) that I could just leave the 5w in place without having to goto a higher watt part or add one in place of the headlamp?

    This system just throws me off because it's not like a normal automotive 12v system where you can simply add or remove lighting as long as there is enough alternator output. The fact that it has this resistor as part of the lighting system is something new to me.
  7. Jul 19, 2015 #6
    I think you can simply disconnect the headlight and run it and nothing bad will happen. Also the circuit that has the headlight can be used for your new LED light with the addition of a switch in the circuit so that you can turn it on when you want it on. Why would you want it off, you are less likely to get hit?
  8. Jul 19, 2015 #7
    I will try it out. Worst case is the instrument bulbs burn out. I was going to put it on it's own circuit with a relay so it is switching +12 directly from the battery to the light with larger, newer wire instead of running the power through the original wiring where it also has to go through the regulator/rectifier etc. I haven't done any voltage tests on it, but I am wondering if there is any voltage drop there or anything.

    This comes from past experience with auto applications where I have found more so than not, voltage drops with load from the headlamp harnesses. I have been able to get more light output when running +12 from the battery through a relay with larger wire, to the lights than I have when simply running off of the factory wire harness.

    As far as having the ability to turn it off,aybe I am more likely to be killed, but I would like to be ae to turn it off during the day because:

    Less strain on the charging system
    Less wear and tear on my bulb
    I don't like the way it looks with the headlamp on during the day. Along the lines of day-time running lamps on cars. I hate them. The lights are a distraction from the visual appearance of the car or bike during the day when people are looking at it.
    Last edited: Jul 19, 2015
  9. Jul 19, 2015 #8
    If your LED replacement is rated for a similar voltage as the original light there should be no problem at all.
    You will get more light with less current being necessary and any other components in the circuit will not be upset about that.
  10. Jul 20, 2015 #9


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    Why do you say the resistor is part of the lighting system. The schematic does not show that. Are you saying the schematic is incorrect? Can you post a link to a correct schematic.

    As drawn, one would tend to think that removing or replacing the lamps with anything would be fine.

    That could only be the case if the regulator system was not able to regulate to 12V without a minimum load. That is always a possibility. In that case you would want to determine the minimum load required to keep the unit in regulation. But, the statement is confusing since it would likely burn out the headlights also.

    You can remove the instrument lights (so they won't burn out) and measure the voltage when you remove the resistor or headlamp. (be sure to rev the engine)

    I've burnt out all my bulbs by removing the battery.
  11. Jul 20, 2015 #10


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    Me too! That was on an 'old' BSA Bantam and it was 'old' even when I bought it in1966. The electrics were rubbish design at the time and I never did figure out what was wrong.
    But even modern cars alternators will be damaged by operating without a battery so removing a battery doesn't make sense. I should consider checking for good continuity in all the circuits if there is an actual fault.
    You can protect your LEDs if you use an extra voltage regulator just in that particular circuit.
  12. Jul 20, 2015 #11
    OK, here are some shots of the manual. This first page talks about the resistor and bulbs burning out


    And here are other pages on the system


    I believe that is everything. That first note a about the resistor and lighting is where I'm getting my info from.
  13. Jul 20, 2015 #12
    It is not clear how the regulator/rectifier works. This could be an issue with LED circuit design. If the voltage changes with changing alternator voltage (which seems likely) a simple resistor as a current source might be a problem (or might not). But if you have an LED driver circuit, it could be a bigger problem. I would test the new circuit under load first in place of the light bulb, but if you are not happy with that, try it on the battery.

    In either case, running a switch with an off position seems dangerous. The manufacturer didn't include one for a reason. I can see how it would make you look really cool at your funeral, but is that really what you want?
  14. Jul 20, 2015 #13
    There isn't an led driver circuit. This system consists of a 25w halogen bulb, 2 194 bulbs, 2 921 bulbs. The led headlamp I am using will not be part of this circuit.

    I simply want to eliminate the 25w bulb, and either leave it disconnected, or install a resistor in its place , or replace the 5w resistor with a larger resistor if needed in order to prevent the two 194 instrument lights from burning out.

    Take the headlamp out of the equation. If it looks like it needs resistance from the headlamp to operate properly is my main goal or question.

    Why Honda Originally wanted a headlamp on all the time is not the question. Essential the headlamp is no longer. Just need to know if a resistor needs to be installed to replace the headlamp.
  15. Jul 21, 2015 #14


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    Based on the statement that a bad connection on the resistor can cause the instrument lamps to burn out, and looking at how the headlamp is connected to the same circuit, I'd say that you need to maintain a minimum load for the regulator to work. But, that's a guess. The only way to know for sure is to measure the voltage at the instrument lamps when you connect/disconnect loads.

    You also have to worry about too much additional load burning out the regulator. Maybe the new resistor plus the LEDs will be too much. If you switch the load resistor out when you switch on the LEDs (and they draw enough power to maintain regulation), you also need to worry about the short interval while switching during which there is no load. The voltage will spike.

    You probably need to characterize the regulator for min/max loads and load response times. MY guess is you need the resistor, and need to characterize.

    Maybe you can find some information about that regulator. (like specs for an aftermarket replacement)
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