Tach jumps

[nibbes123]

Installed an electronic ignition on my 74 spider now the tach needle jumps up a little, when I rev the engine tach needle goes up with rpm when I let off on the gas pedal needle goes to to zero than back to 900 and starts to jump a little. Anyone have an idea what is going on, did not do this when I had the old dist. with dual points on.
Nick

[ga.spyder]

Nick the tach is energized by the coil.I would check all my connections in that area.I had a similar issue last week,and found a wire from my control module had a bare spot.It was grounding intermittently on my coil mount.My tach would go from a steady 2500 and jump to 6000!! All better now.Good luck

[donerehart]

Nick,
I put an electronic ignition system in my spider several weeks ago and the exact same thing happens, but only at idle...and very slightly every 3 or 4 seconds. It runs great, though...interesting...
Don

[nibbes123]

Will look for lose connection or bare wires grounding.
Thanks
Nick

[grittracing]

Also open your dizzy and check the screws to be tight and for bare wires just had a problem with a screw that had come out .my tack was jumping

[danaspider]

Just added a solid state coil, no oil, tach was erratic especially at idle, dropping to zero then jump up. Added ground from heats ink. Problem solved. Appears the new bolts I added for mounting to fender did not make a very good contact for the ground.

By the way engine ran fine.

When in doubt add and check grounds!

[nibbes123]

Thanks will add another ground and see if it stops jumping a real pain in the you know what.
Nick

[maytag]

my tach is acting erratically as well, so this is timely for me!
please let us know what you find out?

[4uall]

I found this video http://www.youtube.com/watch?v=-6j3glUtecY

Would anyone have any photos so I know what I am doing in our cars?

[4uall]

Apparently if you just wait until the next morning it corrects itself Still could use the photos, I will try later

[bradartigue]

This may help you:

http://www.mirafiori.com/faq/content/ta ... achfix.htm

Although in the article John speaks of the Marelliplex the FIAT-Marelli electronic distributor caused the same issue with all types when retrofitted to an earlier, formerly points-type, vehicle.

[4uall]

Brad, as always, many thanks. I was going to go with this type of solution. Can this be done regardless of the Tach's behaviour? Mine seems to corrected itself. Only did it for one day. I have made no changes to system

[bradartigue]

Brad, as always, many thanks. I was going to go with this type of solution. Can this be done regardless of the Tach's behaviour? Mine seems to corrected itself. Only did it for one day. I have made no changes to system

Sure, won't hurt anything. It is an odd service letter in that they call the plex out specifically, but the plex and the other distributors, coils, etc. are all exactly the same except for the advance mechanism. When the plex was sold it was an upgrade part for cars made prior to 1979, it was not intended for the later cars, which already had tachs with capacitance built in.

Which means there's a kicker, late model tachs stick and fail more often than early model, typically right on the 78/79 line. Might be a coincidence, might not, but I've rarely had failed 68-78 instruments at all (excluding the clock) and I have a box or two of failed 79 on stuff, even X1/9 stuff.

[giuliot60]

Hi Jay (or anyone else).

Did you ever install the tach fix?

Mine is till bouncing at idle even after replacing the dizzy pickup (previous one had wire damage inside the dizzy), module, and vacuum advance unit.

So I'd like to install either the capacitor (mirafiori.com) or resistor (YouTube), but wanted to know which way to go.

Thanks.

[dmwhiteoak]

Mine was jumping due to a loose connection at the starter solinoid. The power wire that connects to the back of the ignition switch had a film of oil at the connector. Pulled it off and cleaned it up and no more problems. The car ran fine just the tach jumping.