Wiring tachometer

[Gasman76]

https://imgur.com/a/KdyGxw7

link to pic of tachometer i'm using. one wire from IGN to green connection on ignition switch, one wire from COIL to points side of coil, and one wire from GRD to the ground wire behind ignition switch

[IXL2Relax]

Originally Posted by bustert

do not think the diode will help.

The idea of the diode is to isolate the tach input line from the coil power. The diode is 'pointed' toward the coil. That will ground the input line to the tach whenever the coil is grounded (energized). When not grounded, the coil line will not flow power back to the tach input because of the blocking diode.

I believe most tachs have a pull-up resistor on the input line or is floating at a 'high' condition, so that allows the grounding of the coil to trigger the tach operation.

But since directly connecting the tach causes the loss of the coil trigger signal, the tach line may be faulty - grounded.

The OP should use a meter on the ohms scale, connected on the input line & the ground line, to see if there is a significant short.


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[bustert]

i am familiar with splitting the trigger to interface with a non compatible system. take the MSD system. the older ones need diodes to split off the trigger to run a tach or other assy like shift.
however, i do not think that is the case here. that would be internal to this tach and that points to a failed tach.
sometimes a tach will not work because it is confused especially by an ac component so the capacitor shuts it to ground, remember the radio whine?
in the points system, the capacitor shunted this component to ground and stored the dc for feedback. this ac component was generated by the flyback of the ign coil which if left unchecked would prevent the coil from transferring most of its energy. on the electronic version, they have a diode which shunts this to ground.
it operated like a flyback diode in a crt circuit like television. without this diode, the scan line will not jump to the next line to be written instantly which blurs the picture due to lag time error.
looking at the schematic, it should work as he has it. the trigger on the fx came in on the pink wire on the negative side of the coil. power came in on the orange that feed the instrumentation.
the ? is, does he have a correct tach. mr. jb is correct for that year being ign triggered tach. the 74 had trigger from alternator, mine did.

'74 and '75 FXE tachs get their signal from the alternator, the
nature of which is a pulsed AC voltage.
'76 and later FXE tachs get their signal from the coil, which is
a pulsed DC.
HD part#92040-74 is tach for '74 and '75, uses pulsed AC.
HD part#92040-76 is tach for '76 and later, uses pulsed DC.
HD part#74510-70A is regulator for '75 and earlier (has extra wire).
HD part#74510-75 is regulator for '76 and later (no extra wire).

side note: try a diff ground, the tach was usually grounded from the shell to the ground point on the tree.

[canoli]

Originally Posted by Gasman76

https://imgur.com/a/KdyGxw7

link to pic of tachometer i'm using. one wire from IGN to green connection on ignition switch, one wire from COIL to points side of coil, and one wire from GRD to the ground wire behind ignition switch

Did you manage to get it working?