OE Ford Tachometer and Ignition Wire (Coil+) Function Question

[mjlan]

Can the existing coil+ (in a car with the factory Ford tach) power supply wire be used as the trigger for a relay without messing up the tach calibration?

I am not electrically savvy enough to convince myself that the change in electrical load on this wire from powering a coil to triggering a relay won't make the tach read weird.... 

Thanks,

-Matt

 

[tony-muscle]

I don't see any issues doing this.

 

[TheRktmn]

Yes you can. 

 

[midlife]

Yes, so long as the line is still connected to the + side of the coil.

 

[mjlan]

Ok thanks.

The OE tach needs to sense the coil primary circuit opening and closing to function then? Is there an issue for the tach if the wire is just triggering a relay? The tach would just sit on 0 RPM right?

The question was pertaining to a Duraspark conversion, so the relay would be feeding +12V to the DS1 coil, so the original coil feed wire would no longer sense the coil primary circuit...

 

[midlife]

The OEM Ford tach is a current sensing device, sensing the opening and the closing of the points.  If the wire in question is only acting as a relay, then the tach would receive no direct current fluctuations, as the relay is a high impedance (i.e. low current flow) device.

 

[Hemikiller]

The question was pertaining to a Duraspark conversion, so the relay would be feeding +12V to the DS1 coil, so the original coil feed wire would no longer sense the coil primary circuit...

The tachometer will not function set up this way. You can bypass the resistor wire and feed a full 12V through the tach to the DS1 coil.

IIRC, you can tap into the red /lt green wire at the ignition switch connector, and jump it to the tachometer connector. Disconnect the tach, turn the harness side 180° and connect only the (*going from memory here*)  male bullet on the tach side. Connect your jumper wire with a male bullet to the female on the tach connector. That'll give you a full 12V through the tach to feed the DS1 coil - no relay needed. 

 

[TheRktmn]

Unresisted (Full 12V) voltage will make the tach inaccurate (read high) and shorten the life of the tach.