How can I test if my CDI lighting coil is bad?

Hey all, long time no see...

Anyway long story short this is not a moped problem but a Lambretta issue, I'm posting it here just the old knobs on the Lambretta forum just told me to get new tires and look up a page on Facebook for electrical help...

Basically my issue is I have a CDI stator (BGM brand) that's running great where it counts, getting good spark, kill works, everything is grounded correctly, etc. there are 4 leads coming of this AC stator like most basic CDI's 3 go to the CDI box (ground, pickup signal, LT coil) and the last one is lighting, it's yellow.

Basically my lights don't work, the voltage I'm getting off the yellow lead from the stator is only about 2v. this is the same if I go right from the yellow lead or after the regulator, or elsewhere in the wiring loom (ie at the light). I know it's producing some power since the horn works. Can anyone tell me a surefire way to test the output of the stator lighting coil with a multimeter, see the image, this is my multimeter setting and I get that reading no matter where I measure from the stator yellow lead all the way to the headlight. Am I measuring wrong?

Sorry for off moped-topic but I feel like it's all the same stuff. In the past, I've always just hooked the yellow CDI leads on mopeds and pit bikes up to lights and a regulator and boom bright lights, any ideas? THX!

IMG_2480 copy.jpg

Re: How can I test if my CDI lighting coil is bad?

Dirty30 Dillon /

Post up some full pics, I love my Lambretta.

Having never worked with the BGM CDI, I guess the first question is: is this brand new? If so, I would suspect that the coil for lighting is bad. An easy way to test this is check resistance between the yellow lead and ground. Really high resistance or lack of cont. would indicate bad coil. If it's new, I would contact manu/retailer to get info on replacement.

I would also check the ground on the regulator/rectifier to make sure it's just not poor contact with stator ground, as your paint looks very new/nice, it could just be a painted over contact.

Barring any of this, I guess the first question is, have you test voltage while at operating rpm? You should see a fluctuation in output with RPM at the yellow wire direct out of the stator. If not, I would highly suspect stator being bad. If if fluctuates up to a decent nominal voltage there, but not after reg, than reg may have issues, i.e. grounding or failed.

Re: How can I test if my CDI lighting coil is bad?

Elliot! Hey man!

Is there a yellow wire with a red line also?

DC Stator: 2 Yellow wires (Thick Yellow and Yellow/Red tracer wire) goes to the two yellows on the rectifier.

AC Stator: (Thick Yellow goes to regulator. Yellow/Red tracer wire goes to earth)

"This stator can also be used as an AC stator by using the Yellow with (red) tracer wire to earth on the frame. Thicker Yellow goes to the regulator (AC) or to the Bgm Regulator/Rectifier AC/DC.

Black is an extra earth.

Red, White & Green to CDI"

Re: How can I test if my CDI lighting coil is bad?

Thanks for the great descriptive, and actionable reply! This is why the Moped Army community rocks.

So I dug around some more and isolated the issue, I was getting a solid 12v+ off the yellow when isolated entirely from the regulator but when plugged in it dropped to 6v then 2v if the light switch was turned on, something was draining it off.

Turns out I plugged the black tail light power into the ground terminal in the tail light housing and that was sucking the juice the whole time.

Lights are working great now! Thanks!! here's a pic of where I'm at on the bike, its almost complete, Li125 riverside running a 190cc top and 30mm OKO with a big box BGM clubman. Now its time to add all the pretty bits.


Re: How can I test if my CDI lighting coil is bad?

Dirty30 Dillon /

Hell yeah, good looks.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account