.9 voltage drop to headlamp

[Gusable]

85 cal2. Very dim headlight.  H4 connector was melted on ground wire. Solder in new ceramic H4 connector. Meter on battery static 12.45v.  Key on at H4 green wire and ground at H4= 11.50v. Tested a stand alone ground added direct to battery didn’t change. It’s in the switch or something.  Main connectors behind headlight seem ok, cleaned pins. Your thoughts?

[DoubleGuzzi]

Your thoughts?

Clean light switch connections. The wiring diagram shows that particular green wire going through two connector blocks - likely candidates for voltage drops.
Add a fused relay, if not already one.

[Gusable]

Four pin headlight connector behind the bezel is just green with corrosion!

[michaell32]

Use something like this and follow the wire via the diagram testing various spots. This will tell you if it's a connection or switch causing the voltage drop.

[Gusable]

This four pen was corroded but I remember I checked it at the harness side and it was still low-voltage so I think I’m gonna run a relay

[John A]

Use a relay. Use the original switch and harness just to operate the relay.

[MikeP996]

The melted connector indicates that there is a circuit resistance issue.  Adding a relay/direct connection to the battery would bypass the problem.  Is the current headlight bulb on the bike the same wattage (or lower) as the original?  If it is higher, that could cause the overheated circuit.

FWIW, the way I look at issues that occur in an old vehicle - whether electrical, fuel, whatever, is by asking the question, "Was it a problem when the vehicle was new?"  IOW, were people complaining about it from the beginning and/or did the factory include a pertinent change at some point in production? If the answer is, "No," then I would first check to be sure the headlight bulb does not exceed the wattage of the original and then test the existing wiring/connectors to find where the voltage is going awry. 

OTOH... adding the relay/direct connection to the battery will ensure proper voltage to the headlight and, TBH, it might be easier than sorting out/correcting whatever is causing the problem with the original circuit. ;)

[faffi]

Use a relay. Use the original switch and harness just to operate the relay.

This. Did it on my KZ650 due to a similar voltage drop. Also fitted a relay for the ignition system after I replaced the points with Dynatek and Accel coils.

[Gusable]

There is a 55w halogen in it now. Cleaning connectors and I shoukd be closer to 12v key on Eng off. Looks pretty good light output. Going to do the relay for sure. Got some led “PAR36” aux bulbs coming and they’ll put most of the light down range. Going to be fine when all done. Thanks guys

[Antietam Classic Cycle]

Greg Bender sells a "relay solution" that I installed on my Convert. Worth every penny IMO.
https://www.thisoldtractor.com/for_sale_wiring_harness_tonti_california_ii.html#relay_harnesses

[Frenchfrog]

Relay's  here too...and a proper earth loop that goes back direct to the battery too