I got 2 problems

I have black greease building up on my firewall and down pipe, There is a cv boot close by that i assumed must be leaking
How do i replace the boot (I have already loosened axel nut)
Will i need to drain transaxle?

My tachometer wasnt working when i started the car today, how should i diagnose the problem with my voltmeter? I dont know where to probe

I have a basic haynes repair manual (1984-1991 civic and crx models) But im still new to automotive repair
any help is greatly appreciated