" I have 1976 Merc 454's w
" I have 1976 Merc 454's with what is apparently Mercury's earliest Thunderbolt ignition package.
The tachometers are non-standard: They only have two terminals, one going to the "tach" terminal on the ignition, and the other to ground. The tach pin is labelled "(-)", and the grounded pin is labelled "(+)". S-W made the tachs, but I can find no part number. The actual signal is a DC milli-amp level that is proportional to RPM. I have one bad tach and want to replace it.
I discovered that the coil drive on this ignition is also bogus: One leg of the primary is grounded (not +12 as is normal), and the other leg is driven with negative pulses by the ignition electronics (not switched to ground as is normal). The input circuits on new, normal tachometers from Teleflex and others have a diode that prevents this negative pulse from registering.
I think my options are:
1) Find someone who may have some of the old tachometers lying around from a take-out. Anyone?
2) Make a little circuit board that inverts the coil pulse for use by new tachometers.
3) Retrofit the engines with newer Thunderbolt and replace all four tachs. I hope I would not need new distributors if I did that.
Can anyone out there help? "
" I have 1976 Merc 454's with what is apparently Mercury's earliest Thunderbolt ignition package.
The tachometers are non-standard: They only have two terminals, one going to the "tach" terminal on the ignition, and the other to ground. The tach pin is labelled "(-)", and the grounded pin is labelled "(+)". S-W made the tachs, but I can find no part number. The actual signal is a DC milli-amp level that is proportional to RPM. I have one bad tach and want to replace it.
I discovered that the coil drive on this ignition is also bogus: One leg of the primary is grounded (not +12 as is normal), and the other leg is driven with negative pulses by the ignition electronics (not switched to ground as is normal). The input circuits on new, normal tachometers from Teleflex and others have a diode that prevents this negative pulse from registering.
I think my options are:
1) Find someone who may have some of the old tachometers lying around from a take-out. Anyone?
2) Make a little circuit board that inverts the coil pulse for use by new tachometers.
3) Retrofit the engines with newer Thunderbolt and replace all four tachs. I hope I would not need new distributors if I did that.
Can anyone out there help? "