I am not sure what is going on...I said I think it is possible. I personally(and financially)am not about to run out and buy an alternator or tachometer without thinking it through. Lets work together yes?

idle bounce
 

Read some old threads on idle bounce and it appears it could be the mixture setting(I know I'm running rich). You may just have to lean her out a little...

Do you have an MSD ignition system? Do a search on my name and MSD. Mine failed this year and I had some unusual tach issues with it.

scoe911, I think you're talking about "idle bounce", which is an unrelated issue. jwakil is driving his car at a constant rpm and the tach goes bonkers, or twitches, etc.

Gogar you lost me...:confused:

Sorry. what I mean is:

In your post #22, you talk about idle bounce, which indeed can be a mixture/vacuum leak/ etc. problem, where the actual engine idle moves around, and the tach reflects that problem.

There's -also- another problem, where the engine is at a constant rpm (could be high or low), and even though the engine is running fine, the tach itself bounces around and twitches and does weird stuff. THIS problem is an electrical one, which is what (i think) jwakil is experiencing.

Hope that helps you with what I think i meant. SmileWavy

Gogar while I cannot speak for jwakil this scenario sounds like my problem.

Gogar and scoe911, looks like both of you didn't read my post 20 very carefully. I'm not talking about actual idle bounce, only needle bounce.

-Latest update, I drove it today and no needle bounce. I haven't done anything between yesterday and today, so definitely a sporadic thing which I'm sure will come back.

That voltmeter only tells you the AVERAGE circuit voltage. The alternator rotor current is being switched on and off by the VR thousands of times each minute. The duration of the "on" time vs the "off" time is the result of battery charge level AND overall 12 volt system circuit loading.

If you were to put an O-Scope directly on the alternator output you might well see voltage spiking of 3-6 volts. From 9 to 12-13 volts for a low battery charge vs 12-13 to 14-15 volts if the battery is fully charged and very little 12 volt circuit loading otherwise.

The battery acts as a HUGE voltage spike SINK, FILTER, insofar as those voltage spikes are concerned, the closer you move the O-Scope to the battery the lower those voltage spikes will be.

The CDI is back there in the engine compartment so it "sees" those voltage spikes and it does NOT like voltage spikes above ~14 volts.

One of the tests you can do is to add circuit loads to the 12 volt system, turn on the headlights, blower, etc, and see if the tach jumping reduces

Obviously any level of inordinate circuit/wiring resistance between the alternator and the battery posts would exacerbate the problem. clean and burnish the battery posts and connections. Don't forget that the ground side of the circuit is just as important as the high, positive, side. Make sure the negative battery cable is well connected to the car body. The main positive feed from the alternator goes first to the starter's HOT post so check and tighten those connections also.

Oh, in the end what we did with our '78 to get it to travel the last 100+ miles to "home" was run it with the VR disconnected until the Brand NEW battery's charge was too low, reconnected the VR until the battery become overcharged, disconnected the VR, etc, etc, etc.

So if your problem is the alternator then disconnecting the VR, provided the battery is charged, should result in a smooth tach output.

Apparently none of us are reading any of each others' posts. Best of luck with your tach, there.

Tach bounce, or not, related to the charging ciruit might well be a function of the current battery charge level and/or 12 volt "buss" loading. It might even be a function of the local climate as lead-acid batteries take/need a higher charging voltage level the colder it gets. Electronic VRs have a thermistor in the circuit to account for this while the old mechanical Vrs used a bi-metalic strip.

Reading
 

Up until this point I thought my reading and comprehension skills were above average.....and I still think they are! Now back to the bouncing tach that we both still have. I would check the mixture to see if it is running rich. What could it hurt to try all possibilities?

Gogar, I had read your post carefully and knew you understood the difference between idle bounce and needle bounce, but I noticed you had written "(I think) jwakil is experiencing" which let me know you had not read my #20, since that clearly explains I have needle bounce. In fact I stole the terms needle bounce and idle bounce from your posting.

wwwest, some good but discouraging info. If the battery is really filtering the spikes then I'm kind of screwed in being able to diagnose it. I thought most of the other posts said they were able to pick up the obvious spiking with a conventional meter like mine in the cigarette lighter and that the spiking corresponded to when the needle would bounce. So I still think my charging system and VR are OK if I'm not able to see the same thing.

scoe911...I have spent months playing around with mixtures and know when I get idle hunting and bouncing. Trust me, this problem has nothing to do with mixture or vacuum leak. The idle is constant. This is needle bounce, not actual idle bounce. After looking at the needle more carefully, I think it bounced even at the higher RPMS, but its not a noticeable since you can't hold the accellerator pedal that steady. Anyway, it is not doing it today so I'm not going to worry about it until it comes back.

Trade out the CDI or send it out for testing......

Ingo can help on that. 3.6@cox.net

Jwakil I sure hope you isolate this gremlin(cant think of anything better to call it). To be honest I have just learned to live with it because as you know it doesn't happen all the time. Three words come to mind.....I am subscribed:D

"..Obvious spiking.."

A digital voltmeter readout cannot respond to voltage spiking at the rate the VR causes the alternator output to switch up and down. However, it could, will, indicate a higher than normal AVERAGE battery voltage which might help you diagnose.

"high" battery voltage = tach bounces....

Normal battery voltage = no tach bounce.

Not to be picky, but the MM measurement obtained would be dependent upon MM instrumentation used. In my case (my M/meters-Nice RMS Flukes) ) , it would be an actual R.M.S. reading, as to AC ({even} pulsed) waveforms and NOT an "average" (.636, keeping it mathematical).

A scope would set you free, as to peak voltages...if I'm into "it" heavily, I assure you I break out both scope and multimeter (RMS preferred,..could care less about "average"),..then make the "call".

Then, again, I must submit that this ISN'T a complex circuit,..and "average" may "set you free, as well"....Me? I'm just picky about this,..no challenges delivered (BTW), for sure.

Your points are well-taken.

As all have indicated, one MUST know that the DC rail variances aren't mal-impacting that tach....I'm just saying that I've heard MORE resolutions relating to this symptom that ultimately were found to be alt/VR related,..NOT saying this is your case. In my case, I use the radar detector to provide a DC voltage reading AT the cig/lighter receptacle. I've taken the "anal" opp to check the cig lighter measurement point versus the actual battery terminals to ensure no drop differentials (or, to account for...) There unusually close....great copper and good grounding, I'd say. Not to ramble....just my info/experience.

One day my radar detector hit the overvoltage setting (audible alert) that's default on my detector....I think it's 17 VDC? (maybe 16,...I'd have to look it up...). In any case,...it alerted me to an overvoltage condition ( failing alternator ) ....not being comfortable with my DME rx'g high DC voltages, I quickly replaced the alternator and regulator,.. ..old parts discovered to be original from 1989.......so goes my story.

Now,..back to yours'.............(which may not be OV-related, at all).

Best,

Doyle

Well, the problem came back but I think I finally solved it!!. After checking the many suspect ground wires near the engine and coming up empty, I finally messed around with the ground cable that connects to transmission. Although it seemed the screws holding both ends were pretty tight, there were some very severe bends in some places and the mesh cable was coated in a pretty thick layer of grease and dirt. I cleaned it a little and wiggled a little, now the needle is very steady. Been like that for two days, so knock on wood I believe that was the problem. scoe911, try this out.

Thanks Jwakil I will try this it sounds logical and inexpensive.

Thanks guys. I'm still waiting for my parts to arrive to switch out the old alt/vr with a reblt from our host. I'll also check the ground wires after doing the install and see if I can't narrow the possibilities on the fix as well. great thread on an annoying problem.