[jluetjen]

1) Unplug the switch from the loom. If you push down on the microswitch with your finger (so that it clicks), can you see the switch open or close on an ohm-meter? If it doesn't -- it's dead and needs to be repaired or replaced.

2) If the micro-switch works, have you confirmed that it is adjusted correctly so that closing the throttles trips the switch?

3) If one and 2 both passed, confirm that the throttles are resting on the stops when they are closed. In my case someone in the car's past had adjusted the linkages so that throttles wouldn't completely close in order to keep the car's idle at 1200 RPM. The result was lots of pops and bangs out the exhaust on over run.

PS: What does any of this have to do with the tach signal. That's a completely different circuit.