China tach doesnt work

Found a zener diode that measured 13k both ways! It's a W8 so it's a 5.1v so I'll try and replace if I can find one. Is the voltage that critical? Kinda makes sense as I injected my square wave after this diode (seems to be part of the conditioning/protection chain: 2 diodes, 3 resistors and 3 capacitors) and i got some action on the meter. Its the diode above the right hand screw lug above the resistor and I'm injecting right into the NPN.

BZT52C5V1
"W8"
5.1 max
4.8 min
5.4 nominal