It's magic!
Retro or flatpacks...there is still the need that the input to a tach be dialed down. ...No way that kind of voltage is going to be running around in a tach.
with this voltage unbuffered through a bad resistor, you'd get the magic smoke....Here is the wonky VDO analog tach reverse engineered theory for nerds:
"...
The input circuit consists of a 150K Ohm resistor R1 connected to AC coupling cap C1 connected to a 10K Ohm base current limiting resistor R2 connected to the base of Q1. D1 protects the base of Q1 from going negative, which would hurt Q1. When the input voltage pulses high, C1 initially looks like a short circuit until it starts to charge, so the current going into the base is determined by the input voltage across R1 & R2, a total of 160K Ohms. Using Ohm’s Law, we see that, in order to get the 1mA base current we need to turn Q1 on, we need a (trigger) voltage of 160K * 0.001 = 160V........All this led me to wonder how the tachometer input could get such high input voltages, I thought about inductive loads (coils) causing voltage spikes when they are switched off, and that is what actually happens. A quick internet search led me to
https://www.picoauto.com/library/automotive-guided-tests/primary-voltage/. Figure 5 and its corresponding paragraph give a good explanation of the phenomenon:
“The vertical line at the centre of the trace, called the 'induced voltage', is above 200 volts. The induced voltage is produced by a process called magnetic induction. At the point of ignition, the coil's earth circuit is removed and the magnetic field or flux collapses across the coil's windings. This in turn induces an average voltage between 150 to 350 volts (Figure 5). The coil's High Tension (HT) output is proportional to the induced voltage. The height of the induced voltage is sometimes referred to as the primary peak volts.”..."
