Jawa ignition testing

From Moped Wiki
Jump to: navigation, search


Before You Begin

Like any other moped that's been neglected there is probably about a 50/50 chance that the ignition is working. Often in the case of Jawas, people just shrug it off and assume it's the thyristor. However blind parts swapping is an extremely inefficient diagnosis technique. So leave superstition and rumors alone and approach this without assumption. You will require a basic understanding of electronics and a multi-meter that has a Diode setting


Basic Operation

This ignition is simple in operation, I'm going to explain this in simplistic terms to give a basic understanding of the operation of this system. There are 3 coils at work here, an HT coil, a charging coil, and an impulse coil. It also includes the infamous Thyristor. The charging coil which is the narrow coil found at the 1 o'clock position on the stator provides power to the system via the red wire to the "G" terminal on the thyristor. The thyristor has a large capacitor which is charged through the "G" terminal. The "1" terminal on the Thyristor runs to ground (-). The "I" terminal runs to the impulse coil which is located on the under side of the stator directly behind the charge coil. The rotor of flywheel has a single magnet that corresponds to this and induces a voltage that runs in to the "I" terminal on the thyristor switching it "on". At this time the Capacitor inside the thyristor is "dumped" through the "15" terminal to the HT coil causing it to fire. (it's bit more complicated but this should give you the gist of what's going on)


Diagnosis

First off check for the obvious stuff: missing wires, missing components, badly damaged parts, etc. It must also have an appropriate HT coil. This system requires that the primary winding resistance of it is less than 1 ohm. Most moped coils are between 3 and 6 ohms, THESE WILL NOT WORK. This is not to say that there are not other coils for bikes with CDI ignition that will work but if you see a coil other than the aluminum can type get a hold of an original coil before you continue.


HT Coil

There should be less than 1 ohm resistance between the "1" and "15" terminal There should be approximately 6000 ohm resistance between "1" and the spark plug wire


Thyristor

The bench test of this requires that you put your multi-meter in diode mode as it needs to supply a small voltage to the circuit to test it. You check the resistance between the "I" and "1" terminals on the thyristor. You should get a reading between 50 and 400 ohms. You then reverse the connection and should get a similar reading though not necessarily the same. I've seen them run higher and still work but you should not get a reading of 0 resistance or an open circuit reading.

If you need to replace the thyristor, please see the Jawa Thyristor page.


See also: