Hi all - I had my first test ride today (did about 4 miles) on my 1975 DT400B Twin Shock.
I’ve got a few issues that I need to sort out. I’m hoping you can help me…
The first is that when I came back, my battery was flat. I’ve fitted the full Rex’s Speed Shop 12v electronic ignition system. I had no problems fitting it as my bike does not seem to have had any modifications to the wiring harness. It was plug and play.
So, any suggestions as to why the battery (which is new) isn’t charging and how can I test the charging system to find out where the problem is?
Before the ride, all the lights, horn, indicators etc worked fine.
Many thanks…..

You are in the UK?  With any luck, Rex's might help you sort it out.  From what I've seen you might have to be patient. 

Not sure what they changed in the charging system but a basic test is voltmeter across battery with engine running and revved should show voltage higher than static battery voltage...  something in the 13 to 15 volt range?  You'd have to ask Rex's.

Could just be your battery failed.  You should charge it off the bike before testing anyway... 

 

Also could be a blown fuse? Broken wire? Have you checked the actual battery voltage at the battery? 

4 miles is not enough for a battery to die running just the tail, signals, and horn if it was in good condition... 

I agree with MarkT. 4 miles should not drain that much off a good battery unless you have a serious draw somewhere. As for checking the charge, a decent quick check is, as said earlier, to connect your voltmeter across the battery terminals, read what the battery voltage is, start the bike and read again. You would expect to see a about a 2 volt increase (roughly).

Pull your battery and check it out real well, maybe put on a charger. Double-check all your connections for the new system. Basically go over your work, and satisfy yourself that it was not something missed in the upgrade, before starting deeper troubleshooting. And by all means contact Rex's before tearing things down too much.

Just been looking at the systm online. Looks flash & hope you get it operating as it should. Do you just fit 12volt bulbs & battery & stuff & away ya go. Does your DT run lights on all the time--like are they DC powered including headlight. Or is headlight off the mag but regulated to a maximum brightness. Looks fast anyway.
www.rexs-speedshop.com/product/dt400-d-e...th-12-volt-lighting/

I had this issue 2 times after fitting the Rex full kit. unplug the connector from the engine on the r hand side, start bike and check for ac voltage, 1 probe on earth and the other on each wire, you should get a good ac voltage on each wire, I cannot remember what the voltage was but there is a chart on Rex site and a trouble shoot guide if this happens.
I had ac voltage so I knew my fault lay within the bike. My harness is not that brilliant and the first fault when I lost all power, a flat battery and no lights turned up to be 1 of the light wires in the headlamp was shorting to the shell, once sorted and the fush changed ( fuse never blew) chinese 1 I got all power back but the indicator relay supplied with the Rex kit was stuck on all the time, no flash. I opened this up and found the mosfet transistor had blown, and N channel fet will work, so a rummage on some old PCBs I found 1 and fitted it, now the flasher works again.
the second time I had a failure, lights worked but nothing else, this was a dirty earth, once cleaned up all worked again and still is

Mine does that if I'm riding with the lights on. Doesn't charge the battery and eventually I lose my tail/brake light. As soon as I kill the lights the tail light comes back and the battery begins to charge.

I figured I may be running too many watts on the headlamp bulb, might be sucking up all the juice and there's not enough left to charge. Haven't spent any time looking into it though.

I think they are 2 seperate circuits, light circuit and dc charge circuit, so you may be running your lights on the charge circuit not the ac circuit

when I got my Rex 12-volt kit for my twin shock like I had the same issue. the earlier bike is two separate circuits for charging when the lights are on and off, and the Rex kit was missing the extra green wire for charging. I had to splice in the one extra wire and then everything was good with the kit otherwise.   here's a picture you can check your 4 pin connector plug to see if you have a missing wire. 

I bet that's my issue. I have only two spade connectors on the stator side connector and three on the chassis harness. Nothing going to the green/red wire. Thanks.