March 25th, 2011 | xs650 |
how do they work..inductive coils-for points and TCI units
- coils are an electrical transformer containing insulated primary and secondary winding circuits in a ratio of about 1:100-200 wound around a central iron core
- the primary circuit, heavier wire, starts at the +ve treminal and ends at the -ve terminal
- the secondary circuit, finer wire, sits inside the primary circuit, wrapped around the iron core
- 12v is fed to the primary winding creating a large (enhanced by the iron core) magnetic field surrounding the secondary windings
- the coil is now storing a large magnetic field
- when the +12v to the coil primary winding is turned off, by the points or TCI unit, the magnetic field inside the coil collapses
- this causes a “Back EMF” (Electro Motive Force) current in the primary wire of about 100-300volts which is now applied to both windings. When the coil collapses this rapidly changing magnetic field creates current in the secondary windings
- the primary-secondary winding ratio, say 1:100, produces a corresponding voltage output increase..if the ‘back EMF spike’ primary (low tension) wire is about 150v the secondary (high tension) wire is 100 x 150=15,000 volts
- the faster the power cutoff is in the primary, the faster the collapse, and the faster (more powerful) the spark is
- when the points open (instantly cutting off power to the coil) 15,000 volts goes to ground from the secondary winding via the spark plug
- the ‘back EMF spike’ is why we have condensors – these are actually capacitors, and absorb this spike, created by the magnetic field collapse as the points open and helps shape the coil collapse to produce the high power secondary collapse and slows the coil collapse just long enough for the points to get far enough apart so the ‘back EMF’ output won’t arc across the points and destroy them (sometimes in a matter of seconds)
- TCI units have a pickup coil, induced by a magnet placed on the rotor that replace the points…this signal is fed to the control unit and serves to collapse the primary winding induced magnetic field
- this is where the discussion arises between crank and cam based ignition systems, with the crank based system bypassing cam chain wear as a source of timing error
difference between “Induction” (TCI and points) and CDI systems
- TCI collapses an already charged coil by disconnecting it (TCI switches off briefly)…these systems use a higher resistance type coil and are known as an “induction” or “Kettering” ignition systems…when switched on these have a permanant 12V input
- CDI sends a brief high (200+) voltage pulse to an uncharged coil which acts like a transformer and multiplies it even higher…the step up is normally around 100:1…these systems tend to use low resistance or “racing” oils.
- Do Not Use a “racing” -or- low resistance type coil in an “induction” ignition (points or TCI) system..the low resistance coil will flow more current thru the TCI producing the “Hot Toaster” effect…it will work for awhile, but will eventually burn the points or TCI module out
testing an inductive coil
- best done at operating temps, this is because metal expands with temp and shorts or breaks are more likely to be discovered
- disconnect from the wiring loom, note which wire goes where…
- pull plug caps from the plugs
- if possible remove coils from the bike
- use an ohm meter to test the primary and secondary windings
- primary winding, Rx1 range, test across the +ve and -ve terminals, should read 3-5 ohms, zero or infinite readings show a compromised circuit
- secondary winding, Rx100 range, +ve to the coil output, where the plug lead goes, -ve on a primary winding terminal, should read between 6k and 15k ohms, lower or zero readings indicate a defective coil
- note that just because the coils test ok, it doesnt mean they are, intermittent defects are common and may not be seen at time of testing, the only failsafe way is to replace with a known good coil
testing for spark
points models
- this can be seen at the points, on points models, doesnt mean that there is spark at the plugs however
- remove the plug, reconnect to cap and lead, hold next to the cylinder and kick the motor over, spark should be fat and blue, about 6mm long
dual coils
- dont remove dual coil plug leads without earthing them to the cylinder…you will damage either the dual coil, the ignition system or both…there is no internal secondary winding earth, a disconnected-unearthed plug will cause the high tension voltage to seek an exit, generally by arcing through the primary winding… normally both plugs are needed to complete the circuit, if one isnt earthed the other wont fire
- make a spark tester like shown below
non points models
failure to observe these will result in system damage
- dont turn on unless both spark plugs are connected
- dont test for spark with one or both plugs disconnected
- dont disconnect battery with engine running
- do ensure that the battery cable connections are secure
- dont leave the ignition on when engine is not running
- make yourself a spark tester
- use the clamp to fix the tester to the cylinder cooling fins where it will be earthed, bending the electrode away will allow you to see the full spark
