Flappy, Bouncy, Wacky Tachometer?!, Tachometer bounces back and forth while driving at constant speed Options

[vjb206]

Hi folks! First time/long time for me here in the Garage. I just purchased a '74 1.8 in Ohio and it died on me in PA on the way home to NJ. Right before it died I noticed this bouncy/flappy tach thing happening.

(IMG:http://www.914world.com/bbs2/uploads_offsite/media.giphy.com-23760-1677337178.1.gif)

As you can see my speed is constant, my throttle inputs steady – just cruising. However the tach is going nuts.

When the car died (in a rest area off I-80 in PA) it would turn over but not start. We ran some rudimentary diagnostics and remediation measures:

- Heard clicking/popping noise coming from driver's side of engine bay at speed, this was accompanied by the flappy tach phenomenon
- Generator light came on at some point while driving
- When flappy tach/popping noises would happen, I'd notice the car losing power for a split second
- Electrical systems stopped working during my drive (door open buzzer stopped working mid-way through the trip)
- Replaced the battery with a brand new one, no dice
- Power at the coil
- No spark at the plugs

Car is at a shop in PA - they will get to diagnosing it over the next few days. I'd prefer to fix myself, but I can't get it home from where it is without basic drivability.

Has anyone ever had this happen? Is the flappy tach a symptom of a larger electrical issue?


I found the post on adding the 2200uf capacitor to stop bouncing when changing RPMs rapidly, but I don't think this is that issue. Especially since the bouncy tach and the car dying are correlated (I think).


Would love to tap into the collective wisdom of the platform to help point me in the right direction.

[vjb206]

So the wacky, bouncy tach (and red light) pictured in the 1st post were a result of a bad voltage regulator (and my points were burnt - not sure if/how the two were related). I replaced the voltage regulator and points and now the tach is just a "little" bouncy now upon acceleration.

The P.O. wired the tach to a "Tach Adapt" unit, and that sends a signal to the tach itself. When I removed the Tach Adapt unit the dashboard fuse blew. So I re-attached it and am living with a mildly bouncy tach ATM.

I am currently building a 2270 to put in the car, so my next steps are, when the 2270 is ready for install (after break-in and leak-down on test stand):
- Get all new: coil, plugs, distributor leads
- Install 123 ignition instead of existing dizzy
- Keep tach adapt
- See if bounce persists
- If so, remove tach adapt to see if that's the cause
- If fuse blows... then short is somewhere not in/on the motor. Procedure will be to replace relay board, look for shorts in the wiring harness, look for shorts in the dash, wire in a resistor to the tach, etc.

TBH, if the tach is always mildly bouncy and I can't reasonably fix, I'll just live with it and chuckle a bit when I accelerate ¯\_(ツ)_/¯ .