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Headlight dims when rev engine>?

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by duff_boy, Apr 22, 2008.

  1. Hey guys,
    I just noticed last night as i was leaving work, that the headlight dims when i rev the engine. Its bright at idle and as soon as the revs increase it seems to dim a noticeable amount(it was noticeable in the darkness of the car park anyhow). Weird i thought - on most bikes its usually the other way around, dim at idle and when the alternator speeds up it brightens.

    Now im not having any trouble with flat batteries/bike not starting etc. Would there be a problem with the charging system causing this? If so, how can i fix it?
  2. I'd say you need to get the alternator checked. It may be providing enough to keep the system charged OK but not enough to compensate for the extra drain of lights etc..

    Have you added any mods lately? - light bar, power outlet, etc?
  3. Haven't added anything at all.
  4. At a guess it sounds like there's something shorting out slightly which becomes more of an issue when the revs (and current from the alternator) increase. I'd be suspecting the voltage regulator or the connection between it and the alternator as the likely culprits if the bike seems to be running fine otherwise. Might be worth taking it to an auto electrician, many will do a quick alternator/circuit check for free.
  5. sounds like a weird failure of the regulator or a partial short somewhere.
  6. Partial short could be the go, I had one on my indicator that showed this simpton. Also could be the sign of a battery on the way out.

  7. Most probably a poor wiring connection to the headlamp, or the switch. Vibration from the engine revving is affecting the quality of the poor contact, wherever it is.

    It could even be a poor contact in the switchblock.. Most bikes pass the full headlight current throught the switchgear. Is it the same on both high and low? If so, it could be an earth or power feed to the switchgear which is not the best.

    You could have a poor earth (return) from the headlamp. Either use a multimeter if you know how or a test lamp if you don't.

    The clip or earth on the test lamp should go to a good earth point - beware, some bikes do not use the frame as earth, and you need to find a wire (often green or black or with a green or black stripe) which is the earth point. Check what colour wire is attached to the negative of the battery.

    Just touch the tip of the tester to the battery first, so you know how bright it should be, and then move around the wiring for the lights. Once you find where the lamp lights poorly you just backtrack until it lights well.

    Mind you, a multimeter would be better, especially when checking earth faults.


    Trevor G

    PS It is unlikely to be a reg/rectifier fault if the bike is starting well, and turning over as rapidly as normal. Start with the simplest and easiest checks first, the headlight wiring.

  8. Wiring seems fine - i haven't checked the continuity with a multimeter though. I thought it might have something to do with the indicators and flasher relay ive recently installed, so i tried removing the indicator fuse, no change.

    When i recently had the bike apart to check valve clearances i had to remove the radiator and thermostat housing. I merely just had to disconnect a few wiring plugs for temp senders, fan wiring etc and i reconnected them correctly afaik when i put it all back together.

    However, since then the radiator fan doesn't seem to engage at all. I think this may have something to do with it. If i run the engine up to temp with bike stationary the fan just doesn't come on at all. I know the thermostats opening correctly because you can feel the bottom radiator hose heat up, and its definitely getting hott because the radiator is hot enough to burn your hand. But the fan doesn't engage, and the temp light doesn't come on either.

    I might pull the tank back off and have a good look over the wiring, but afaik all the connectors are unique and you can't plug the wrong ones together.
  9. I'd say, seeing as lights dim as alternator speed increases, your alternator is not putting out any, or very little, current. As engine revs, coils etc draw more current, dimming lights.
    It's probably a charging system fault. What voltage do you have at the battery A: at idle and B: at 3000 rpm?

    Regards, Andrew.
  10. Could be:-
    worn out brushes in Alternator!
    voltage regulator cactus!
    poor frame earth!
    poor connection from alternator to battery via plug!
    poor ignition connection from main loom (battery feed to fusebox)
    corroded battery connections (high resistance - your enemy)
    alternator has poor winding and breaking down under load

    or a combination of any of the above!!!!!!!

    As Typhoon suggested check your Voltage at the battery under condition A and B, as a starting point!

    Hope it helps
  11. What method are you using to evaluate the wiring - just because you cannot see any breaks it is "fine"?

    Interesting check, but it won't solve anything. Even if the indicators were stuck in the on position (not flashing) they would not draw enough current to dim the headlights.

    Now we're getting somewhere. ;-)

    Is it a coincidence that the lights started playing up at that time?

    Or did they start playing up a little later?

    Either way, if you don't exercise care when unplugging you can easily stress or fracture the wire inside the plug. You need to make sure you are not pulling on the wire itself, or its insulation, but only on the plastic plug housing when disconnecting those brutes. ;-)

    Probably not a problem at all. The fan is only supposed to come on when the coolant overheats, around the 100C or more, mark. The fan uses a separate sensor embedded in the radiator, as distinct from the temp gauge sender which is normally mounted in the cylinder head.

    When you find the fan sensor you test it like this:

    1) If a single wire is connected, remove it and ground the wire while the ignition is on. The fan should run. If it doesn't you either have a faulty fan or faulty wiring to the fan or sensor.

    2) If 2 wires are on the sensor, just connect them together - the fan should run with the ignition on.

    The temp light comes on when the coolant at the top of the cylinder head where it is positioned exceeds the prescribed temperature. It does not work in conjunction with the radiator/cooling fan sensor.

    Does the temp light come on with the other warning lights when you turn on the ignition? It might not be coming on (well, hopefully it will never come on) because the bike is just not overheating.

    You seem to be worrying yourself silly over nothing...unless you know you were a little rough with the wiring during the tank job.

    I think you need to reread my earlier post and have a good think.

    Then answer this question: has anyone (intelligent, anyway ;-) suggested the impossible, that you had plugged the wrong plugs into each other?

    If you had, wouldn't that mean that the headlights would come on when you turned on the blinkers, and so on? Or the horn would blow when you applied the front brake?

    Electricity might be a puzzle, but it behaves in a very logical way.

    Your original problem is headlight brightness varying as you rev the bike.

    That is due to vibration, somewhere. It might be something loose inside the regulator or alternator area, but I doubt it. Here's how you can check:

    1) Do the blinkers blink at the normal rate, whether the engine is running, running at idle or running fast? Do the blinker lights vary in brightness like the headlights do?

    2) With the lights on and the engine running, what happens when you blow the horn and operate one of the brakes - do the lights dim even more? Do they stop fluctuating when you add this extra load to the system?

    3) Does the stop light vary in brightness along with the headlight? If not, it is defintely not the reg/rectifier.

    I have had reg/rec and alternator/generator problems on both cars and motorcycles, and I have also had wiring faults. This is the pattern I have always observed, but this might not be the same in your case:

    4) A faulty charging system will produce a constant but dulling light. Eventually the battery won't retain enough charge to start the engine properly.

    5) A wiring fault (bad connection/broken wire, damaged terminal) will frequently produce a fluctuating output from lights, horn, anything which draws a lot of current.

    A wiring fault can be in the wire which feeds power to the device or the wire which returns "power" to the battery. The latter are called "earth" wires - some go to the frame (and can suffer from loose bolts or screws or rust where they attach to the frame) and some go all the way back, by wire to the battery terminals.

    6) Turn on the lights with the engine not running - if you carefully wriggle the wiring loom, especially the wires near the headlight, does the light output vary?

    All the best

    Trevor G