The missing volts

My bike lost 2 volts between battery and headlamp when I got it. Since then I have replaced:

Alternator
Rectifier
Regulator
The whole wiring harness (used)
Replaced or cleaned and adjusted every contact in the wiring harness
Every connector was soldered in place
Fitted new fues box
Fitted a brand new NOS ignition switch

My bike still drops 2 volts to the headlamp, coils etc.

Battery is new of the season and holds 12.7v after being parked for a week.
There is 12.70v between the battery poles as well as between + on the starter solenoid and frame.
Putting on the ignition drops 1v to 11.7v.
I then have 9.6v at the coils.
With the ignition off, I have 10.8v at the fuses, both going in and out.
Disconnection the fuses, I still have 10.8v at the 4-pin connector.

Hence the loss is somewhere in the harness. Any hint where I should start looking? As I see it, I probably have to measure the resistance through every wire and connector. If that fails to isolate the issue, I can either live with it as is, keep it as is but add a relay for the lights and coils, or build a new harness from scratch.

Unless somebody have a brilliant idea where to look that can save me tons of time and effort?

Tried more than one testing instrument? Some Digital multimeters will react to the magnetic field from the bikes alternator, giving wrong readings. Most Analog(the kind with the moving needle and several scales) are shielded from magnetic fields.

Would it matter with the engine not running? Does something happen when the ignition is switched on? And why would it matter whether I connect to the bettery or the ignition coils or the fuses since they're all very close and the instrument sits in the same spot, near the battery? Not saying you're not right, I just fail to see how a static alternator 3 ft from the instrument can upset the readings enough to matter.

It sometimes happens when the engine is running, as the magnetic field varies. Tried using the meter with longer test leads, positioning it further from the magnetic source?

The various loads in your electrical system will cause the voltage drops you see.

That's just Kirchhoff's Voltage Law in action.

He was a drinking buddy of Ohm. Legend has it that they were playing poker one night and bet each other that they could come up with their own laws governing electricity.

The best way to test this stuff is not with a volt meter but with an ohm meter. You are going to get a voltage drop no matter what. That's normal. If you suspect that the voltage drop is excessive, it's going to be because of a higher than normal resistance reading through a connection, through a wire or switch or through a load. Chasing stray volts is not the way to go.

Bone up on proper techniques for using a meter to test motorcycle circuits. There are quite a few videos on YouTube that deal with this specifically.

So basically this:

faffi wrote: As I see it, I probably have to measure the resistance through every wire and connector.

Clean/confirm your grounds (frame, engine, head light shell etc...) as well... You'd be surprised what a corroded/oxidized ground connection can do...

The resistance can be fine, but not enough circuit there, as in to small of wire to carry the load. This can also be seen when a wire gets a Hair Cut/ lost strands at the connecting end.
Voltage drop is how to find the Problem. If you suspect the Power side of a circuit, jump around it with a Direct (Fused) wire from Battery to Headlight with the same gauge wire. Recheck, do the same with the ground, then both. If the drop goes away on either one, then that side is an issue.

I have wired a separate new circuit for my coils through a relay. The headlight circuit is about 1-1 1/2 volts low and could use a new circuit directly from the batt/relay. Good info on Wired George's site.
You're working with 30 year old wiring and technology, it needs to be in perfect condition to stay ahead of or keep up with the current draw.

This is like doing voltage checks on 3 phase equipment and the rookie goes on a quest to find those missing volts on one of the legs because his meter reads 120v, 120v, and 118v.

Start the bike, now you have extra volts! What to do?

:laugh:

So less worry and a fix for the coils, perhaps.... we'll see. All connection, including earth, are as close to spotless as possible for something 35 years old, so other than a full replacement with new wires I may not get much further.

If you are looking for voltage drops, just measure them directly.

Red meter lead on the battery's positive post, then just use the black meter lead along the path and the meter will tell you, directly, how much voltage drop you have at each point in the path, relative to the battery voltage. Simple.

As soon as you come across a large drop, you know where to start fixing.