1975 Mk 3 Commando charging circuit problem

It's annoying when things don't work as they should, and I understand the urge to make them do so.

But apart from stubbornness is it really worth putting effort into this issue? All the warning light does is tell you that (a) the ignition is on, and (b) the engine is not turning. The second is obvious, and the first is easily checked.

Much more useful information is provided by one of Al Osborn's battery status monitors (usual disclaimer).

I have had the same problem after fitting a Trispark Mosfet regulator to my Mk3, after replacing 3 warning light assimilators I emailed Trispark, Steve Kelly replied that the OE style assimilator is not compatible with modern type regulators, in my case the warning light would work correctly when the battery was accepting a charge but when the battery was fully charged the Reg would do its job and stop the charge therefor the red light would stay on, I confirmed this by switching my lights on which placed a drain on the battery.

Have a look at this link. https://www.improvingclassicmotorcycles.com/products.htm

Works great on my Mk3 fitted with a Trispark, 3 phase Alt, Mosfet reg.

Binned the capacitor . modified ignition harness.

Fit a voltmeter and it will tell what's really going on.

Found the battery status monitor that Julian suggested works well and much easier to see the colour of the LED with a quick glance than looking at a voltmeter.

Check the charge voltage with a non 'cheap digital' . Cheap digital pick up ignition interference and readm rubbish. TGhen fit some form of voltmeter on  the bike, a digital/LED device (I sell the BSM) will give you confirmation in a moment. The original Assimilator is known to upset reg/ rects as the Assim. switches the lamp to earth via output from the alternator. Reg/Rects don't like this.    I can check Reg/Rects for the cost of the return postage.

Remove the bulb.

Either the unit is now broken or more likely there is a connection issue from the alternator to the unit so it is not seeing the alternator voltage.

Bin the WLA , even if it's new. I found the new ones do exactly what yours has done.

Replace it with a low voltage lamp driver from Al Osborn to power the red console light.

Also use a BSM, again from Al Osborn if you want to know what your charge system is doing. 

Beauty about doing this is that you can site the units in the head lamp shell. Saying that, whilst your at the electrics why not rewire the bike. Get rid of all the crappy connector blocks under the tank and all the redundant wiring and make a neat job of it.