OK I have a situation here that is proving a little bit more troublesome than I thought it would be. I have a 1967 sportster and I am trying to put a electric tach on it. A simple job right? I have used several different tachs but to no avail. When I start the bike everything is fine but after ten minutes or so it starts behaving erratically and the needle starts bouncing all over. When it returns to idle it works ok again. Now these bikes came with a mechanical tach but surely an electric one should work. Once i bring the rpm up it starts to bounce whether riding or standing still.

I know that i must be close to having it work and wonder what I am missing. I do have several tachs that I have tried and they all do the same thing to varying degrees.

 

How do you have it hooked up ? Are you coming off the coil ?

 

Yes 1 wire to trigger side of tach another power wire from the other side of the coil to + side of gauge. I have the 2 wires running side by side in a loom and also wonder if there might be interference between the 2. It seems accurate in the low rpm but not the higher.

 

I'm thinking the one wire you're using for power should come off a different power source and not off the coil.

 

yep mr. mad is correct. run power from another source.

even though you think there is a steady 12vdc there, think about it, the coil is switched to ground through the module so when the module calls for spark, it shuts down current, another issue is there will be back flow emf as the magnetic field collapses and the coil rings.

 

Thanks fellas I was wondering about my power supply source and if it wasn't the culprit. I will move it to the ignition switch. They do run the power for brake light switch from the coil+ side also which I have done as well. Do you think there is an issue running the two wires together in the loom up the frame? It's strange because it works great initially and then goes sideways.

 

Try relocating the power wire like I suggested, see what it does, and we'll take it from there.

 

I don't know why I never tried that in my troubleshooting took for granted my power source was good I guess. So now I have to wait to get home at the end of March to change things around. I will post here the results. Thanks for all the suggestions

 

We gotta wait a whole month to see if we were right, holy crap. Just don't forget about us.

 

It's too wet and cold for me at home right now so I have to suffer through another month of hot sun and cold beer but I'll be back Adios