Hey All
1976 DT400c
Got a bike a year ago with non functioning headlight / gauge lights
Replaced the Lighting coil with a NOS unit and ta da,
The headlight came on but has always been very dim.
Took a voltage reading at the yellow wire plug next to the air box while running
and it was 2.5v with light switch off and 1.5v with light on (both at idle) which explains the low lumens.
Im pretty sure I soldered 2 wires on the new coil as they came off the old non working lighting coil.
Now I see on the attached wiring diagram that maybe the two lighting coils are somehow attached?
Is this correct or are they both just grounded to the timing plate. See the red arrow pointed at the yellow colored connection on the attachment for what I'm referring to.

Question 2: How do I check OHMs (or whatever) on that NOS lighting coil to see if I got a defective one?

Question 3: On that second thinner lighting coil (piggy backed on the CDI power coil), is that there to charge the battery when the light switch is in the off position (daytime charging?) or am I getting something wrong here?
Appreciate all the knowledge on this site folks

My guess is that the coil is good, first,  it is making some voltage, and second, I have never seen a bad one (only damaged ones). 

I would also guess that there is a possibility that you bought the wrong (or mis-identified) coil. 

I would also guess that it has been wired incorrectly. If you take out the old one and put in a new one, there should be no guessing as to what wire does what.  It should be plug and play unless you are not following the factory Stator wiring. 

Maybe post some photos of the actual stator. 

Shots From weeks agoi think I’ll need to open her up and get some better photos and the backside toothx

Howmany wires do you have coming of it as lighting has a few with differen voltages at revs if you tested unloaded just to see. But my first thought & especialy if only one wire--is ithe windings look finer trhan lighting--like a points model DT250 magneto source coil. Hope i'm wrong but hows it compare with old.

Just opened up the patient and took some photos
Is that thin coil on the far right side (Lighting coil 2) is that supposed to have the wires around it to create power?
As mentioned the whole bike runs fine. flashers / rear lights off battery are fine
Just the headlight and the gauge lights & Hi beam are all super dim

Let me know what you're seeing and thanks as usual
This forum is stellar
 

One more note:

Looks like my green/red wire has an exposed connection (maybe grounding out?)
Is that wire sheathing fabric stuff available in after market?
Does that OEM type insulation have a certain name?

I believe it is called fiberglass wire insulation or sheathing, I’ve seen it on Amazon under those key words.

A better shot with the wires spread out

The fibreglass sleeving is available in various sizes . Vintage Connections or DEET  should be able to help .

Issue Solved
Bad Lighting coil from the start
Purchased a coil back in December 2021 to replace a non working one
Claimed it was a NOS but only put out 3v at rev so... ordered yet another coil also labeled NOS and this one looks the part
You can see the difference between them,The first one probably was re-wound but it's been too long to challenge or bitch about
(The crap lighting coil has been to Daytona and Barber)

So with this now fixed and the 6v stock bulb back bright and the battery now being charged
I also got a test HID bulb and test LED 6v bulb for this bike and a glass enclosure
www.octanelighting.com/5-3-4-motorcycle-...t-headlamp-each.html
The LED pulses at idle but is so damn bright and very nice looking at night. The HID is a nice stock look but a brighter option
but all of these bulb options are thankfully brighter than the bad 3v coil option which was like a small candle when riding at night with sun glasses on!
Thanks forum