Voltage measurements vary widely

I have been trying to get my 1978 Honda CT90 K9 set up for long distance riding (Bellingham WA - Los Angeles CA) this spring. The problem I am having trouble addressing involves the 6V electrical system. I have a silicon rectifier that produces sufficient DC voltage, but I either have lower voltage for the ignition coil or lower voltage for my LED tail light, while having too much for recharging the battery (resulting in a loss of water in the flooded lead acid battery after long rides). I bought a new flooded lead acid battery in Aug 2023 and it seems to function well and accept and hold a charge (typically 6.2-6.5V) before I start the bike in the morning without needing to use a charger.

As far as I can tell my bike has original electrical components, except:
-6V LED gauge lights and tail/brake light
-New flasher relay that also worked for 12V turn signals (to eliminate the flasher relay as the source of the lower voltage, I removed it and there was no change to my measurements).
(Headlight and turn signal bulbs are original, and I have made no changes to the generator or ignition components).

This is confusing to me, so let me tell you what I have been measuring. For quite some time I have had a multimeter measuring the voltage at the battery, but I recently bought a small digital voltage gauge from China. As the voltage gauge does not have a power switch I wanted it to be connected to the power that turns off with the key (the black wires inside the bucket). What I did not expect is that the voltage of these wires was much lower that at the battery. As examples, when the bike is not running and the key is on the battery is 6.4V and the black wire is 3.7-4V. when the motor is running the battery is 7.2V and the black wire is 4.7V.

I have tested that there is no resistance in the wires between the battery terminal and the black wire when the key is on, and the key switch is effective is disconnecting all power when the key is off.

Is this degree of lower voltage common?


Well-Known Member
Try putting a regular incandescent lamp in the tail light rather than an LED. That will consume more wattage at high RPM and possibly prevent the boiling of your battery electrolyte. Since these bikes don't have a standard voltage regulator, they run a "balanced" electrical system - watts produced at an average RPM = power consumed by the lamps most of the time.

It's not perfect . . . .

Thanks for your message, Rick, I agree that I need a non-LED tail light but for a different reason. The LED is not lighting at all when the headlight is on because the voltage going to the tail light is below 4.5V!

I am trying to understand whether it is typical for these 6V electrical systems to have voltage on the black wires that is below 5V, when the voltage at the battery is above 7V? It seems that simultaneously I have too little power to light while I have too much to charge the battery without overheating. Or the balance is so fine that I am not sure how to manage it.

I tried a rectifier-regulator that was designed to keep the voltage below 7.5V at the battery, but it ended up draining the battery when I was riding in the dark, so that the bike would stall when I was waiting to turn with the turn signal on.

Is this a problem with my bike not working as it was designed, or a design weakness of the small Hondas?


Well-Known Member
Somewhat of a weakness in the design of small Hondas but we can't really argue that point very hard when Honda successfully sold MILLIONS of this style of bike around the world.

The voltage drop differences you are seeing around the bike could be attributed to some of the tiny copper strands inside your wires being broken inside the insulation - typically at the connectors.



New Member
No, it is not normal to have a 2-3 volt drop across a few feet of copper wire. Voltage drop is the result of current x resistance. The copper wire itself has very little resistance by itself. So you are consuming a large current draw (short), or you have more resistance than you think. Run a temporary external jumper wire from the battery to the black wire junction in the bucket. If that solves the voltage problem, start examining the original wiring path. Switch contacts, cable connectors, frayed/broken wire strands, etc.
Thanks Old Guy, I will be trying your suggestion and trying to see why the voltage drops. I did check the resistance of the wires and the fuse, key and kill switches and all of it showed no resistance using a multimeter, but it may be that the problem only arises when their is more current flowing. The real challenge with old bikes is that some aspects deteriorate over time, even with little miles ridden, but those deteriorations can be hard to locate.
I have to post again, because finally I am having happier results! OldGuy's comments and suggestions were very helpful. I do not think I was checking resistance carefully enough.

I double checked my voltage measurements with my friend who recently retired from teaching electronics at the local technical college, after he looked over the wiring diagram and did not expect we would see the drop in voltage I described: 6.4V at the battery, and 4.2V in the bucket. We checked at many different points and confirmed that we were seeing a 2.5-3V drop over only a couple feet with connectors and the key switch. We connected a wire directly from the battery to the connectors in the headlight bucket and got much better results. Eventually, we replaced with wire from the battery to the key switch, and then doubled the wire from the key switch to the connectors in the headlight bucket, so I did not have to replace my whole loom. We found that I was still loosing 0.6V just for going through the key switch, but with the other wires replaced or supplemented I am now getting enough volts where I need them so the bike is not stalling at stop lights if the turn signals are on (when it voltage would fall below 3.8V). It seems that it is also allowing the voltage at the battery to be lower (and hopefully not boiling the acid on highway riding), because it is more efficiently moving them to be used by lighting and ignition through the connectors in the bucket.

My conclusion is that I would recommend anyone rehabilitating a 1970s Honda, to check the voltage not just at the battery, but also at the connectors where power is being consumed, as the age and neglect of these bikes can significantly lower the conductivity of the switches, connectors and wires, hiding problems. My bike now starts much more reliably, because there is more voltage getting to the coil, so I expect a brighter spark. but also less power is lost to deteriorated wires and connectors, so more to power the marginal 6V lighting system.

Thanks to Oldguy and others on the forum for helping me better understand the challenges of these bikes. I never guessed that the problem with my 45-year-old motorcycle was that its electrical system was old and deteriorated - who could have guessed! So many of its other parts have kept going.