bad switch box

does anyone know what the major cause to burn out a switch box, I have a 1990 135 HP mercury and 5 years ago I bought it with a bad switch box, I changed it and now 5 years later I burn another, I don't remember if its the same one, inside or outside, but what causes a switch bot to go. is there any test on other components that lead up to the switch box failure.

Thanks Bob

Depends on failure...usually bad bias circuit in opposite switchbox or bad coil causing inductive kickback...

faztbullet I guess my next step will be:

1) replace the bad switch box
2) test the voltage to all (6) coils
3) test the stator out put with a DVA tester
4) and last do a switch box bias test red lead on ground & black lead on the white/black sw.box terminal

if all this Is good I there any thing else I should check

Thanks bob I really don't want to keep replacing switch box's

Check the bias first as if bad it will damage the new box if hooked up....

faztbullet switch box stop circuit inner box 400 volts
outer box 400 volts
Stator low speed inner box 300 volts
outer box 230 volts
Stator high speed inner box 0 volts
outer box 0 volts
Switch box Bias 12 Volts

all checked at 900 to 1,000 RPM

Low speed is higher than spec. 195-265
high speed is 0 volts 120-160
bias was good at 12 volts

should I replace the stator next

Thanks Bob