I am in the process of restoring a beat-up 1985 Honda CB650SC Nighthawk. Her name is "Drama Queen," which is very descriptive, because there has been nothing but drama in trying to get this bike safe and roadworthy. I love her, but she can be very difficult. And she never washes! I am new to this forum, so please excuse me if this question has been asked before.
I just read the excellent discussion herein entitled "turn signal relay question" from 2009. I was going to post my question as a response there, but the forum suggested I create a new thread, so here goes:
Everything about this bike is stock. It's not in good shape, but I have tremendous respect for the original designers, so the fact that the bike has not been modified (even though it is in terrible shape) is one of the main reasons I bought her. In the course of repairing the turn signals, I encountered what I thought was a weird symptom: The signal on the left side would often blink fast – about double the normal rate. The contacts in the turn signal switch are intermittent in many permutations, so complicated diagnosing the cause. Eventually my son noticed and insisted that it was because one of the wires to the front left signal bulb was an intermittent open circuit.
I insisted that that was impossible, because for virtually every thermal blinker circuit I know, the load determines the rate of the heating of the bimetallic strip which serves as an interrupter, and so an open circuit (light out) could not cause a faster rate of blinking. I humored him, thinking that maybe there was some kind of weird short circuit situation with the front lamp in which the rear light still received enough current to blink, and the excess load from the short caused it to blink faster.
Lo and behold, my son removed the front bulb during one of the circumstances when everything was working right, and the blinking speed doubled before our eyes. Eventually we removed the front bulb from the right blinker, and verified that this circuit speeds up its blinking rate when one of the bulbs, which are essentially wired in parallel, is removed, DECREASING the load. I have subsequently checked the wiring diagrams on-line, and in Clymer, and there is nothing unusual about the circuit. It is precisely as I expected. If I may revert to Neanderthal graphics, reduced to its underlying simplicity, it looks like this:
Battery+ ---> turn signal relay ---> switch ---> bulb ---> –Battery
When working properly, the "bulb," above, is really 2 bulbs IN PARALLEL. You can verify my reduction to the underlying simple circuit by referring to:
http://diagramonwiring.blogspot.com/2012/06/honda-cb650sc-nighthawk-motorcycle.html(This on-line schematic is the best I have found so far. It is much better than Clymer, for example, which is littered with errors. The "rectifier" in the top middle is drawn in the wrong sense, but, other than that, this diagram to be very correct.)
The "turn signal relay" is a black-box, but it is usually a normally-closed bimetal switch with its own heater circuit in series, which causes it to open, interrupting the circuit, and causing the "OFF" portion of the blinkers' duty cycle, a là:
---> {heater ---> bimetal switch} ---> (where the heater is in physical proximity to the bimetal switch)
Hence the more detailed Neanderthal version is:
(bulb)
Battery+ ---> {heater--->bimetal switch} ---> switch ---> bulb ---> –Battery
There are only 3 conductors going into the turn signal relay. They are +12V POWER, GROUND, and BLINKING LAMP OUT.
With only these 3 signals, how can these components, or reasonable variations thereof, be arranged to produce the common result that I am sure most of you familiar with Nighthawks have already seen for yourselves whenever a turn signal bulb burns out??? I have tried to my wits' end to solve this, and it doesn't seem possible. This is really puzzling to me. I'm sure there must just be some simple thing I have missed, being a neophyte WRT this type of motorcycle, but I just don't see it.
Please, would someone brighter than me, and with more experience with this model of motorcycle, tell me what I am missing? How can this simple thermal blinker circuit "speed up" when a significant portion of the load is removed???
Thank you.
Love those universal Japanese motorcycles!
