Tach reading half the rpm?

[boatdoc]

Tach reads about half the actual rpm of the motor.
1983 FXR , shovelhead primary, evo motor. Has Dyna coils on it.
Tried a spare tach , same thing.
Any ideas what i am missing here?
Probably something really simple.

[Tacocaster]

Helpful to know if it's a pre-existing or new aftermarket Tach but if new, it's likely not "optioned" properly - and the "spare" tach you used was from what?

Assuming you had a working meter before, did you change your ignition by any chance? Any recent work done by you, a friend or a dealer?

As I recall (hopefully correctly for '83 Evo's) HD uses a Pink wire from the Coil Pack to drive their Tachs. What colour is yours?

[boatdoc]

The bike belongs to a friend, history is unknown.
Tach & speedo assembly appears to be stock.
Spare tach was purchased by the owner because he thought the stock tach was the issue, both tachs are HD.
Yes this bike appears to have had many hands on it, not all work done is what I would call done well.
Yes the stock wire in the harness is pink. Another wire had been run from the coils to the tach and the stock wire is disconnected.
Tach has not worked since he bought the bike.

[xlfan]

You state Dyna coils (in more than one coil), that means that you have single-fire ignition, and only half of ignition signals are picked up by your tach, hence half tach reading.

[Tacocaster]

xlfan gave you the reason and now your friend has a decision to make:

1. Keep the Single Fire Ignition and ignore the lack of a tach (assuming bike runs good).

2. Dump the ignition for stock style and re-wire. You'll need a schematic and might find a butcher was a previous owner and even worse, might be tough to find new '83 OEM parts.

3. Find an aftermarket custom style Tach that supports S/F Ignitions.

I'll take door #1, please.


 

[aswracing]

Most single fire ignitions have a separate tach output. You simply reroute the tach's input wire to that discrete tach output of the ignition and it'll start working correctly.

But in the unlikely event your single fire ignition doesn't have that output, there's another solution as well, what's called a "single fire tach adapter". It's basically just a pair of diodes, that logically OR's the two primary wires on a single fire system while simultaneously isolating them from each other, thus creating a single signal that provides both primary signals for the purpose of driving a tach. Just google it, you'll find them.

[boatdoc]

Thank you everyone, makes perfect sense.
Aaron the single fire tach adapter is a great idea and even available here in Canada.

[FSG]

[Deye76]

Dyna ignitions, Ultima, and DTT have clear instructions for wiring the tach for single fire operation.
All the same procedure. The instructions can be had online. I am not good at wiring and have empathy with your trying to solve the problem, especially without a schematic.

[hrdtail78]

Some of the electronic ignition modules also have the option to run single or duel fire.  You can always put it in duel fire mode for testing tach purposes.  The wasted spark duel fire isn't the power killer some might think it is on a street bike.