bouncing tachometer

[tttz]

Hi guys and gals,

i have noticed that my tachometer "bounces" once it gets up around 4000 rpm. what i mean by "bounces" is that the needle will suddenly drop down to 2000, then "bounce" back to 3000, then drop to 2000 or 1000 or all the way down to 0, then "bounce" up to 4000. The bouncing is random and occurs only when the engine is near 4000 rpm or more. Occasionally it will remain steady. is this common? Can anyone point me the right direction to troubleshoot and repair this?

i have a little experience with cars but no clue how the tach works.

i have a stock '71 240, with petronix ignitor and petronix 40,000 volt flamethrower coil.

Thanks for the assistance,

Ty

[2ManyZs]

Sounds as if you may have a loose connection, probably at or around the coil. If it dropping all the way to zero, it sounds like the connection is broken. Double check all your wiring connections at( and around) the coil and replace any terminals that are loose or damaged. Not dropping to zero all the time is a good indication of a loose terminal that is not quite losing all, but most of it's ability to conduct the proper voltage to the tach.

[tttz]

i'll check out the connections at the coil, and see if that helps. i'm looking forward to testing it out on my way home from work!

Thanks,

Ty

[tttz]

i briefly checked over the connections at the coil and everything seems to be connected okay. i played with the conncections that are friction fit, but i didn't take it all the way apart and re-connect everything.

Also, my fan belts had been squeaking so i tightened them up.

Unfortunately, the tach is still bounching, although, it seems to be less, and i haven't noticed any drops down to zero. it also seems to hold steady to about 4300 rpm before the bouncing starts.

is there another area i can check? Since i don't know how the tach gets its reading, any info on how it works would also be helpful.

thanks,

Ty

[Victor Laury]

If it's not an electrical problem, You may need a new tach. Datsun tach and speedo gauges use a dampening oil between two plates in a chamber. The viscosity of the oil transfers the spin from the signal plate to the needle plate, smoothing out the needle's travel. Somtimes the oil "Disappears" (I don't know how or where it goes), and the gauge gets very "Jumpy" indeed.

[RCA123]

I had the same problem. The 240z tach does not work well with electronic ignition. MSD makes an adapter, or you can, as I did, switch to 280z tach. I found one on Ebay for a reasonable amount. I opened up both tachs and switched the face plate and the housing so the mounts would work right. It works fine.

[tttz]

Thanks 2many, Victor, and RCA123,

sounds like i may need to replace the tach, or at least switch pieces like RCA suggests. I'm out of cash to spend on the car for now, so i'll have to deal with a bouncing needle.

Any other suggestions or insights about tachometers/speedometers are welcome!

Thanks,

Ty

[jkube]

Try checking the battery connections. I had the same thing happen and found the tach stopped bouncing once I cleaned the terminals (and negative cable on the opposite end of the battry terminal) and retightened the clamps/connections.

The bad connections are probably limiting current flow. The 240z tach reads electrical flow (current) off the positive wire to the coil.

The squeaking belts could be the alternator working overtime and may be related to your tach problems