My TY175 lost spark so I thought I'd check the primary and secondary winding resistances of the ignition coil. The manual states 6k ohm for secondary winding resistance (between HT lead and coil earth) and 4 to 5 ohm for primary winding resistance (between small black wire and coil earth). The secondary resistance was fine, but I've run into problems measuring the primary.

To start with, I put the red lead on the thin black wire and black lead on the coil earth and the meter showed an open circuit, but when I swapped the leads over it read about 2.5 Mohm resistance. Strange, conducts one way but not the other - methinks maybe there's a diode in there. Sure enough, the ty175 manual shows a diode inside the coil to prevent the motor running backwards.

I read up a bit on testing diodes and found that modern digital meters only put out a small test voltage to measure resistance, and that voltage is not enough to make a diode "forward biased" (i.e. conductive). Most silicon based diodes need >0.7 volts to begin conducting and my meter only puts out 0.5 volts to measure resistance. Hence, when trying to measure the primary coil resistance with a digital multi meter, all I'm measuring is the high resistance of the diode while it is in a nonconducting state. Apparently older analogue meters use higher test voltages to measure resistance so would work in this case.

Has anyone else come across this?

Cheers,

Ade

Hi Adrian

Do you want me to bring the coil off the 250 to the meeting tomorrow night for you to check the readings?