• Don't overlook our Welcome Package, it contains many links to important and helpful information about functions at VHT like posting pictures and sending PMs (private messages), as well as finding the parts you need.

    AD

Voltage drop battery to harness

450roo

Veteran Member
Joined
Nov 10, 2020
Total Posts
242
Total likes
26
Location
Adelaide, South Australia. Australia
I have a volt meter fitted between the black wire and green wire in the headlight. When I connect the meter directly to the battery terminals I get a reading of 12.86 volt, the same meter connected as said above to black wire and green I get a reading of 12.27 volt. Is it normal to have .5 volt drop through the harness?

I must add the machine runs, well starts easily, and the battery is fully charged after a ride, the battery holds the charge with minimal drop over weeks of standing.
 
Direct across battery is essentially no resistance.....
Connected through the loom you have the wire's length, any looseness or corrosion at the plug joints, switch terminals, etc.....With the key on, you may also have minor draws such as the neutral lamp or one coil energized.......
 
.5 volt drop isn't too bad, considering the age of the machine. Another way to measure voltage drop is to connect your meter to the battery positive with the opposite lead to the black wire. With the switch on, the resultant reading(actual voltage drop) should be pretty close to the numbers you noted above.
 
Assuming you have one, try it again with the run/stop switch in the stop position and the bike in gear. The draw should be about 0 (no coils, no lights); flipping it into neutral should turn on that one little bulb, and the drop should be very small. If you have a LED in the neutral light, the draw will be tiny, and the drop as well. This should indicate if you have a higher than normal contact resistance in the harness.
 
0.5 volt drop is generally considered max allowable before you need to investigate connections, grounds, etc. It's doubtful you will ever get zero volt drop with all the components in the circuit (and if you ever do, start looking for what you did wrong or 'the magic smoke' leaking out )
 
0.5 volt drop is generally considered max allowable before you need to investigate connections, grounds, etc. It's doubtful you will ever get zero volt drop with all the components in the circuit (and if you ever do, start looking for what you did wrong or 'the magic smoke' leaking out )
I've been told this stuff will work for leaking smoke if you put it in backwards since it's for Positive earth
wire-harness-smoke-Custom.jpg
 
You make light of my worry Jim!

Thanks guys, makes sense so I will worry no more.
Not trying to make light of the issue but you're within generally acceptable standards. Personally I like to see no more than a .2, preferably lower, on any power wire. But that means replacing every connector with new after stripping back the harness insulation to get clean copper which sometimes means adding additional wire because now it's too short. Then there's the switches that have to be disassembled to clean the contacts.
The Prince of Darkness is a long standing joke for anyone who has dealt with Lucas Electrics.
 
I was being facetious, Jim.
I grew up on British motorcycles, I have had several BSA's and AJS's and a James, have ridden with many on British machines as well. So I know the prince of darkness very well.
As for rewiring the Honda, it works well so I will leave it. I just wondered if the voltage drop was normal or an indication of troubles to come.
 
Why I suggested you repeat the measurement without a coil powered. One coil draws over 2 Amperes, so a 0.5V drop would not be excessive. I have one of those LED battery monitors, the kind with a sequence of red-amber-green, which is connected in the headlamp, and when idling, the first green flickers with the coils charging.
 
Tried with the coils disconnected.
At the battery terminals 12.73v at the black to green wire in the head light with 1 coil connected 11.9v with both points open 12.6v so I'm only losing 0.13v in the harness.
Thanks again for all the help!
 
Not to shaken up things, but as an engineer I did some measurements on my CB450 K0 harness, and I wrote the following on the other forum in 2014:

It seems odd, but every connector, especially on older bikes like this introduces an small power-loss. If the resistance of a connector would be 0,1 ohm (very realistic at these bikes / connectors, and in many cases more), the voltage drop over the connector is (at 1A) 1*0,1= 0,1 V, which results in a power loss of 0,1 W per connector. The amount of connectors is way over 30 (including switches), resulting in a total loss of 3W. In case of the generator to harness, the voltage is much higher then 12 V, the current too, resulting in a higher loss.

A moderate connector can have a 0,5 ohm resistance or higher, resulting in an overall 15W loss
 
Back
Top Bottom