Coil test

[Andyf]

I’ve seen many comments that relate to Coil testing but most refer to LT coils or resort to ‘try one that works’. Has anyone seen or does anyone know if they can be tested without special tools?

Thanks in anticipation

Andy

[MickYork]

A "coil" is a length of (usually) copper wire coiled round a central core. The copper wire is insulated so it doesn't touch itself when it's wound. The coil can be tested using a resistance meter (ohm meter). This will test the resistance of the copper wire. If there is a break in the coil it will read, infinity or open circuit if there is a poor connection at the terminal ends, or a "slight" break, this will give a higher than expected reading. If the insulation on the wire has broken down and the wire touches another part of the coil or in the case of a lambretta, part of the metal frame, this will give a lower than expected reading.

Testing is ok but when the coil gets warm/hot the copper can expand and give create faults that cannot be measured when cold.

So, I would suggest, if when cold the readings do not match the expected readings (Sticky's book) then the coil is probably duff.
If the readings are ok and are inside the usual parameters....this does not prove the coil is good

Hope this helps

[coaster]

Good explanation from Mick above but if testing a pickup coil the resistance reading can be misleading as ther are a couple of resistors inside them and the meter will not see a break in the coil part. You need a thing called a Reluctance meter to do it properly. That said, if you pickup reads less than 95 ohs its probably toast

[Andyf]

Thanks gents - very helpful. I’ve spent most of today stuck in sticky’s book (!) but must have missed it.

[Andyf]

Just realised I missed the HT bit off the title. Sorry about that. Answers still relevant so thanks.