Here is a closeup of the rotor. The scratches on the plastic probably came from my initial attempt to clean off possible corrosion - I very lightly scratched at it with some sandpaper but stopped after a couple swipes.

The coating is gone from a significant portion of it. I don't see any tiny wire which you alluded to.

http://forums.pelicanparts.com/uploa...1427055625.jpg
http://forums.pelicanparts.com/uploa...1427055637.jpg

It was the half circle logo stamped into the copper tip that looked like a wire.

:D

See:

http://forums.pelicanparts.com/porsche-911-technical-forum/857201-another-bouncing-tach-but-problem-finally-solved.html

Nowhere have I seen a voltage meter check at the battery terminals. The cap, rotor and plug wires are maintenance items. Not like you will lose out replacing them

Test the alternator and read the voltage when the tach is bouncing around. If you get voltage spikes exceeding 14.5v. It's the alternator/VR or both.

I wonder....

If the battery is being overcharged does the battery terminal/post voltage rise very much before he electrolyte has a least been partially boiled away?

Batteries have a lot of bulk and can absorb a huge voltage spike without being damaged.


If the alternator output voltage is to high can you measure that except nearest the alternator?

Where would you recommend I attach my voltmeter? So far I have checked voltage at the battery terminals.

The tach still bounced with the alternator/VR disconnected (I removed the alternator light which disables that system).

I should have new cap+rotor tomorrow, so we'll see what effect that has.

The best place to check for voltage excursions is electrically nearest the source, the alternator. The voltage at the battery will have been filtered of any spikes.

The source side of the fuse panel in the engine compartment is directly connected to the alternator output.

With a poor connection, ground or +, from the alternator to the battery the alternator output might rise fairly high in the engine compartment.

Most commonly at the starter solenoid, or engine case to chassis.

I had the rotor turned wrong. A couple minutes in good light with the dust cover off and it all became clear. At first the tach no longer bounced. But after driving around the block a couple times, it is back to it's happily bounding ways. Maybe something is arcing? My spark plug wires are all shielded, and I looked carefully but didn't see any arcing. I can check again when it is dark outside.

Ok, I have now measured back here. Is a consistent 14.44, +/1 0.02 volts.

The tick/arc is definitely matching the tach bounce, although it becomes more pronounced after the RPM drops from the cold-start RPM and before it reaches normal operating temp.

Is it possible there is a little arc forming inside one of the wire bundles in the engine bay? I'm thinking about removing the plastic wrap/shield thing and checking within. A spark noise audible over the engine has got to be visible too, right?

Check your engine compartment in total darkness with the engine running. Be prepared for a light show. Your description fits arcing ignition wires. I had the same bouncing tach which cleared with new spark plug wires. I had the shielded ones, OEM Beru. I now have Magnecors from Pelican and no tach bounce and no light show in the engine compartment.

Zap Zap! Nice little blue lights!
It was arcing between the center wire (runs from the coil to the distributor) and the shield for one of the other wires. After I nudged those apart with a wooden stick, it started arcing to the nut next to the coil.
I guess this means new wires, right? All? Or just the center wire? I'm going to pick over my "big binder" to see when they were last replaced. What is the usual maintenance interval for such things?

http://forums.pelicanparts.com/uploa...1427338946.jpg

Checked my records - Leland bought "Ignition Wire Set" #911-609-010-ST from PP on 4/29/02. Nice braided stainless steel Turbo ones too. That's about 25K miles ago.
I love good record keepers!

The center ignition wire isn't usually shielded however. Perhaps it has failed and the rest are fine? I'll see if I can get just an ignition wire at the local parts store tomorrow.

The spark WILL NOT jump at those points absent a HV open circuit downstream.

Most likely the rotor is not closely aligned with the cap conductor when the CDI fires, that likely also why you're seeing spark carbon path traces aligned with the rotor tip rotation.

Those shield braids are more a detriment than a help. They detrimentally slow the risetime of the HV spark and Porsche was wrong about the causative factor for airbox backfire/explosions.

Trying to see the relative rotational/timing positions, yours vs mine, inconclusive.


http://forums.pelicanparts.com/uploa...1427343370.jpg
http://forums.pelicanparts.com/uploa...1427343430.jpg

Hmmm, so the wire is not the main issue? I'll do some CDI reading to determine what I should pursue next.

Electricity follows the path of least resistance. First it followed the grounded shield plug wire. The next choice was the negative terminal of the CDI coil wires. So the question is, why is the resistance in the path of the coil to center cap to rotor to plug wire so high that it preferred the other paths?

Replace the rotor, cap and coil wire. You're on the right path there. When the pulley is on the Z1 mark, what is the position of the rotor? It is possible you have too large of an air gap between the rotor and the individual plug contacts inside the cap. Maybe your distributor position is off by a tooth? When was the last time you checked your timing?

The magnetic ignition sensor is under that dust cap/cover below the rotor.

You should see a 6 point reluctor ring one point exactly aligned with rotor tip/contact.

There should be no "looseness" rotationally between the rotor and the reluctor ring.

Something is causing the CDI to fire at rotational positions other than with the rotor tip closely aligned with one of the 6 the distributor "contact" points.

http://forums.pelicanparts.com/uploa...1427347408.jpg

Pulled the dust cover off and did a little engine turning by hand before work this morning.
There is no play between the rotor position and the 6-point reluctor ring. They solidly move together. I can rotate them a little bit (together), apparently against a spring of some kind. When released they move right back to their original rotation.

As you can sort of tell in the first picture below picture, the rotor appears to be about 1 spline off exact alignment with the reluctor ring. I last had the distributor removed about 3 years ago when I replaced the chain tensioner oil lines.

I haven't checked the timing since acquiring the vehicle 3 years ago. I do have a timing light.

I turned the engine to TDC (that's the first notch in the pair, right?) The 2nd nice fuzzy picture below is the rotor/reluctor position when the engine was in that position. The reluctor points seemed to be quite close to alignment with the 6 "posts" on the distributor body.

The arcing noise does not happen at perfect intervals. At times it is intermittent, but then has short bursts of activity. The bursts correspond to the tach bouncing. Meanwhile the engine seems to be running smoothly. Is it possible that the coil is firing at random-ish times? Maybe some sort of signal wire is failing?

Thanks again everyone for all your help.

Reluctor - Rotor Alignment
http://forums.pelicanparts.com/uploa...1427374411.jpg


Rotor position when at TDC.
http://forums.pelicanparts.com/uploa...1427374854.jpg

Z1 (TDC) is the second mark. Edit: Z1 is the 2nd mark for '81-'83. Not 100% sure about your '79.

You see the very faint mark on the top right of the outer ring (top picture)? The rotor should be lined up with that mark when the pulley is at TDC.

Let's make sure you are correctly lined up at Z1. Once you are, then if the rotor is still not completely aligned you will need to move the distributor CW by at least one tooth. Remove the adjustment nut and pull the distributor up until you feel it disengage. Carefully rotate the distributor CW until you feel the next tooth. The splines are curved so as you push the distributor back down it will move further CCW. Then reinstall the adjustment nut. Finally, you'll need to re-adjust your timing.

If the distributor is properly aligned then the signs seem to point to high resistance in your distributor to coil wire or in the plug wires themselves or both.