Bouncy lagging tach

[AcampoDave]

Pretty much like the title says. It bounces up incrementally as I wind up the motor. It also reads low most of the time. I've swapped the tach (Autometer) with another identical version. I've swapped the coil, I've tried a different ground. Tomorrow I'm going to swap the trigger wire to the HEI and I'm wondering what guage wire I should use for that as well as the 12V hot. I think it's all wired with 18 guage now. I can't think of anything left to try besides that, but if anyone has a suggestion please let me know. Thanks.... oh and I'm sure the tach itself is set for 8 cylinders too.

[Willshook]

Is the HEI getting a clean, direct 12V?

Does it steady if you turn on the headlights?

[raggedjim]

Maybe this would help?

https://www.youtube.com/watch?v=-6j3glUtecY

[AcampoDave]

Thanks for the replies, here's an update: I swapped the 18 guage sender wire from the HEI with a 14 and nothing changed. (The HEI is powered with a 12 gauge wire right from "ign hot" on the fuse block and the motor runs great so I believe my power wire to be strong.) I also doubled down on my ground by adding another wire to a firewall location which now compliments the original ground at the dash, still no change. I tried with the headlights on too and again no change. Really all that's left is adding the resistor so I'll get one next time i order from Amazon. I think my pigtails and plug at the distributor is good, (it's an Accell) . I looked it over when I change the sender wire .

The needle never bounces above my actual rpm. What it does instead is lag towards the lower side and bounce to catch up with the real rpm under acceleration. At a cruise, It also just reads plain low like I have an imaginary overdrive at highway speeds.

[AcampoDave]

I messed with the connector at the cap, it worked good, for a few minutes, then back to the same . Then I removed the Accel connector and put 2 separates for batt and tach.....nope. Lastly I gave up, put my original coil back in and it worked right again. I drove a while and it kept working. I read on line that I need dielectric grease on that rubber washer below the coil and I don't have any and its dry, so I guess I'll put some on there another day, but it's fixed for now and I don't know why.

[HO455]

You might check the 12v power to the coil for AC power. Do the test just like you would for 12 DC but with your meter on AC volts. If you read AC volts then your alternator diodes are failing and allowing AC voltage to pass. The AC voltage will cause strange tachometer issues.