It's magic!
If the tach works with a higher 'spark' value ignition box, your R1 resistor (first off input) may have gone south, giving it a higher resistance than designed. This schematic (guy found that R1 resistance decrease = lower coil voltage to trigger) is based on an older Volvo Tach made by VDO and used the kettering points system trigger and its 150V-300V induced coil voltage to trigger the tach. The 100:1 windings of the coil increase this induced voltage to >15,000V @ high tension line to distrubutor. He says the R1 resistor is not needed for ignition box derived tach triggers (MSD, 'tach out' on CD boxes):
"...low voltage tachometer signals such as those coming from electronic ignition units or EFI systems... If you want to adapt one of these VDO tachometers to work with those systems, you can simply solder a wire across R1, shorting it out. Never do that for a high voltage points/condenser system, but it will work fine with low (5-12V) tachometer signal systems..."
If you have a multimeter and a soldering iron, I bet you can trouble shoot about any through hole mount component on that tach's small PCB.
If the tach works with a higher 'spark' value ignition box, your R1 resistor (first off input) may have gone south, giving it a higher resistance than designed. This schematic (guy found that R1 resistance decrease = lower coil voltage to trigger) is based on an older Volvo Tach made by VDO and used the kettering points system trigger and its 150V-300V induced coil voltage to trigger the tach. The 100:1 windings of the coil increase this induced voltage to >15,000V @ high tension line to distrubutor. He says the R1 resistor is not needed for ignition box derived tach triggers (MSD, 'tach out' on CD boxes):
"...low voltage tachometer signals such as those coming from electronic ignition units or EFI systems... If you want to adapt one of these VDO tachometers to work with those systems, you can simply solder a wire across R1, shorting it out. Never do that for a high voltage points/condenser system, but it will work fine with low (5-12V) tachometer signal systems..."
If you have a multimeter and a soldering iron, I bet you can trouble shoot about any through hole mount component on that tach's small PCB.
