Author Topic: The strange green slug like resistor on the early Rd125!  (Read 211 times)

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Offline Barnett

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The strange green slug like resistor on the early Rd125!
« on: February 10, 2018, 11:56:56 PM »
Thought I would post this here to maybe help clear up an obscure mystery that despite hunting everywhere and searching the net and Yamaha service manuals I could either find no answers or other mystified questioners. This is closely linked to Foy's post a year ago on early rd125 wiring looms.
Am at the stage of wiring up the as3 rd125 and could not figure out what the resistor that lives under the battery box is for, it appears only on some of the early rd125's and on the europa in particular.

Some of these bikes were fitted with a 4 position key switch, (counting off as position 1), and it is my belief that these were destined for mainland Europe and possibly Canada. The lighting is available via the handlebar lights on switch both in key position 2 and 3 and the last position 4 whereby you can withdraw the key, this last position being common to other switches with only 3 positions.
This threw me a bit as I thought pos1 off, pos2 ignition and everything else but no lights, pos3 as 2 but with pilot light on and full lighting via switch available, pos4 lighting only for parking.
The lighting function in position 2 is far dimmer than position 3. Everything is dimmer, including rear lamp and meter illumination lamps. Thought it was a wiring fault.

On inspection of wiring diagram for the europa, (which has mistakes in it, warning!),  Which shows the green slug resistor it appears the lighting circuit at the left handlebar switch has two power feeds, one being the 'normal' feed if I can put it that way which is activated when the key switch is in position 3 , the other the 'dim' feed, (Key pos 2). I.e. Two supplies wired in parallel.  There is no mention of why this should be in the books I have or the owners handbook.

On the post where foy commented, martin gave me the clue suggesting daytime running lamps.

So the first feed to the lighting position 2 is via the green resistor under the battery box. This resistor is of the order of 2-3 ohms and gets quite warm, (power resistor), cutting the voltage roughly in half and thereby dimming the entire lighting circuit, pilot, main and dip, meter lamps and tail lamp.
Switching to the 3rd position on the main key switch instantly brightens all lamps up as it is now feeding from the battery and not via the resistor. Interestingly the feed via the resistor is still in circuit, it is not switched out by going from pos 2 to pos 3, as it dosn't need to be as the lamps ignore this feed so to speak as its restricted and suck the juice from the unrestricted circuit instead.

So to those that worry about the resistor as theirs is broken either replace with any power resistor (60 watts say) at 3 ohms or so, or higher if you want dimmer daytime or city lamps, or just unplug it at the resistor end, red wire or red with yellow trace. You will just lose the daytime running dim lamp feature on position 2, everything else will be as normal and work fine.

One wiring diagram shows all the wiring in place for the feature but no resistor, so you may have a four position switch but no city or daytime lamp feature anyway!

As to why a uk bike has this feature I have no idea as apart from the pilot lamp at the front and tail lamp there was no requirement for city lights in the uk at the time.

Was this a requirement in France or Holland? Who knows.

Sorry for long post and I expect many eyes have glazed over but may be useful to someone like me that was searching for an answer on this topic!

Below is a pic of the little rd progress to date!
« Last Edit: February 11, 2018, 12:01:53 AM by Barnett »
Yamaha 1974 RD125 (AS3) in bits!
Kawasaki 1977 z650 B1 being ridden
Yamaha 1972/3 U.S. early RD350 just acquired

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Re: The strange green slug like resistor on the early Rd125!
« Reply #1 on: February 15, 2018, 09:01:05 PM »
Good info!  Thanks for posting.  ;)