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.

[Steve]

My first guess would be the points or condenser.

[brant]

Usually it’s the ignition points/cons.

Does the car have points?
Have you serviced them frequently?

[emerygt350]

Yeah, what do you have for ignition?

[vjb206]

You all are awesome, thank you so much for the quick replies!

Great insight re: points... I just bought the car on Wednesday and was on my way home, so I hadn't had the opportunity to take a look and notate the condition of things like the points, etc. I should have. Lesson learned.

Would bad/failing points be responsible for the Generator light being on in the dash too?


I'd still like to better understand if it's likely just an issue localized to the dizzy before I head out, thus wanting to see if that single failure could be the cause of all of the symptoms:
- bucking
- generator light on in dash
- popping/snapping sound from engine compartment
- seemingly drain on power for in-cabin things (like buzzer, wiper speed looking sluggish, etc.)
- not starting

It's currently parked somewhere that's a 3.5 hour drive from my home so troubleshooting it will be tough. I have a 123 Ignition dizzy that I might bring out and would like to "pack" whatever I think I might need just to get me back on the road to get it home.

You think the popping/snapping sound could be the Voltage Regulator failing as well?

[Superhawk996]

Given the generator light and other electrical issues you may have something shorting out the alternator or B+ wiring.

There have been instances where the alternator cover shorts B+ To ground. Either intermittently (may be your case) or a hard short which usually melts or arcs to something that is grounded. A very large short will destroy the alternator and/or the battery.

When that short happens all voltage (including to coil) goes to 0v, coil doesn’t fire and you get momentary drop out of voltage to other thing lights, Tach, etc.

[emerygt350]

What superhawk said. Something is shorting causing the miss and the jumping tack. Be careful, you don't want to go up in flames.

[emerygt350]

And the 123 is great but don't put it in there till the short is gone. You will likely destroy it.

[ConeDodger]

Short.

[Jezibel]

- generator light on in dash

Just a peripheral thought on this. I recently had the alternator (red) light light come on when I started the car up from cold condition. After a couple of minutes of warming up, the ALT light went out by itself. I finally figured out that this was being caused by the alternator's V-BELT being very slightly loose, so an adjustment of the V-BELT tension was required. Sure enough, after the adjustment, the ALT light no longer came on at start-up. This odd situation can also be caused (I am told) by the V-BELT being a bit old. Good excuse for replacing it with a new one.

During warm-up, after a couple of minutes a sudden goosing of the throttle can cause a slightly worn (or old) belt to sudden start gripping the pulley normally...another indication that a new belt and subsequent adjustment are worth considering.

This may or may not have anything to do with the greater concerns you expressed here, but it's still worth knowing about.

[fiacra]

@vjb206

It has been a while since you started this thread. What was the diagnosis/solution here?

[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)_/¯ .

[vjb206]

And to @Superhawk996 's point, I will also look into the alternator. I'm planning on using a different alternator in the 2270, so I can perform bench inspections sooner than later. The inspection procedure for these alternators is daunting for me, so I've kind of been putting it off :-)

[brant]

did you use a dwell meter when replacing the points?

they can wear in a lot when they are new.
if it has a few hundred miles on it since they were replaced..
it might be worth rechecking the dwell and resetting the gaps

no guarantee but they might clean up the signal and help some more after the wear in.


I think the signal will be better after the 123 install
might be worth buying that early in the new project and getting the benefit of having it on the old motor before the new motor.

[FlacaProductions]

Why was the Tach-adapt put in by the PO? Curious of the need if it's a stock 1.8.

Glad you're back to running order!

[Superhawk996]

Burned points are typically related to:

Bad condenser

OR

Coil not running proper ballast resistor. Bosch coil is about 3-4 ohms primary resistance with a built-in ballast resistor. Aftermarket low ohm “hot” coils will burn points.

The tach adapter should not be there unless the tach is for a 6 cylinder (doesn’t look like it based on animated GIF) or they are trying to use it as a band-aid for an otherwise failing tach.

I’d have a look at the back of the tach and see if it has been opened up at the bezel ring. See if anyone has been inside mucking around potentially swapping to something that would require the tach adapter.

As a general statement it sounds like a classic “there I fixed it” cobble job of prior owner adding in stuff that shouldn’t be there and potentially messing up something that is causing fuse to blow when you don’t use it. (IMG:style_emoticons/default/headbang.gif)