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
