This may not be useful info, but could reveal something regarding the new switch.
I have two MD22L's and replaced the original Volvo
5 position control switches several years ago and did not have any tach problems.
The original switches on my engines were 5 positions, left to right: Stop (fuel shut off), Off, On, Glow, Start. Spring return to On from Start; spring return to Off from Stop.
Not wanting to spend the way-too-high costs for replacement Volvo
switches, I procured industrial grade 3 position Cole-Herse switches (Off, On, Start, spring return to On) and two momentary contact Cole-Herse push buttons for the Glow and Stop functions. Analyzing the wiring from old to new was straightforward and everything worked as predicted. Cost was about 1/3 the cost of new Volvo switches. Rum money