Bouncing Tach when Cold, and Zap Noises

[tirwin]

On the oscilloscope... you could do that on the green wire. You would need a reference for "normal" to compare to. And that's the kind of thing I would do -- but then on the other hand by the time you did all that you could just replace the green wire and have knocked one more preventative maintenance item off the list.

Personally I would replace the green wire and then try to find a loaner CDI/coil to test with. If the problem persists at that point then you're down to dizzy rebuild.

[OsoMoore]

Originally Posted by tirwin

On the oscilloscope... you could do that on the green wire. You would need a reference for "normal" to compare to. And that's the kind of thing I would do -- but then on the other hand by the time you did all that you could just replace the green wire and have knocked one more preventative maintenance item off the list.

Personally I would replace the green wire and then try to find a loaner CDI/coil to test with. If the problem persists at that point then you're down to dizzy rebuild.

I was thinking I should see a regular frequency on the 'scope. I'll ask around in the area about a spare CDI or coil. My best option unfortunately just had his SC go into the shop.

I'll see about getting that green wire on order. I believe I'll also need the special connector to splice it on to the current direct line from the CDI, right?

[tirwin]

Yes. Get in touch with timmy2 and he can tell you where to get the connector. He just rebuilt my engine harness and he went ahead and did it. Now changing the green wire is plug and play!

[OsoMoore]

Related question - the new and non-brand spark plug wires have a loose connector on one spark plug, and measure 2K ohms resistance. The spec is 3K. I am thinking maybe I should just return them and get a new set when I order my green wire. Should I go with Red or Black?

[wwest]

Originally Posted by tirwin

OsoMoore,

You're getting closer. So we know it is inconsistent. Let's talk through other possible causes.

Failing CDI: usually gets worse with heat.

I think Op said spark "smooths" out as the car warms up.

try getting a can of freeze spray and see if the condition improves when it cools down.

In this case does cooling cause the symptom to return? Fine spray/mist from a garden hose.

That could be why it seems inconsistent. Do you notice that the coil fires properly at cold start and symptoms worsen as the engine warms up?

Pretty sure Op said the opposite.

As we discussed the other day this could be the common element to the bouncing tach wire too.

The tach signal originates with the dizzy pickup, amplified within the CDI.

Green wire: The green wire is the trigger for the CDI to fire the coil. If it is damaged it could be causing the coil to fire before or after it should.

Coil: I don't remember if you had tried a new coil yet. You might want to think about getting one. Worst case you have a spare handy.

Coil is not likely to generate spontaneous/random CDI triggering.

Distributor: Internals could be in need of refresh. I would try to rule out a couple more things that are simple first. Dizzy rebuilds are not cheap.

Any chance a local Pelican could lend you a known good CDI/coil to try?

Do the sparks smooth out with rising RPM?

For some time my thinking has been that a strong magnetic field, say shorting inside the alternator, might "couple" extra firing signals into the dizzy's MAGNETIC pickup.

But we likely removed that possibility by disabling the alternator's initial rotor field excitation current.

[wwest]

Originally Posted by tirwin

OsoMoore,

You're getting closer. So we know it is inconsistent. Let's talk through other possible causes.

Failing CDI: usually gets worse with heat.

I think Op said spark "smooths" out as the car warms up.

try getting a can of freeze spray and see if the condition improves when it cools down.

In this case does cooling cause the symptom to return? Fine spray/mist from a garden hose.

That could be why it seems inconsistent. Do you notice that the coil fires properly at cold start and symptoms worsen as the engine warms up?

Pretty sure Op said the opposite.

As we discussed the other day this could be the common element to the bouncing tach wire too.

The tach signal originates with the dizzy pickup, amplified within the CDI.

Green wire: The green wire is the trigger for the CDI to fire the coil. If it is damaged it could be causing the coil to fire before or after it should.

Coil: I don't remember if you had tried a new coil yet. You might want to think about getting one. Worst case you have a spare handy.

Coil is not likely to generate spontaneous/random CDI triggering.

Distributor: Internals could be in need of refresh. I would try to rule out a couple more things that are simple first. Dizzy rebuilds are not cheap.

Any chance a local Pelican could lend you a known good CDI/coil to try?

Do the sparks smooth out with rising RPM?

For some time my thinking has been that a strong magnetic field, say shorting inside the alternator, might "couple" extra firing signals into the dizzy's MAGNETIC pickup.

But we likely removed that possibility by disabling the alternator's initial rotor field excitation current.

Likely, maybe..

On the off chance that the alternator rotor had enough left-over magnetism to self-start maybe we should try removing the alternator drive belt temporarily.

A lot of trouble but.....

By chance is there a nearby emi/rfi source nearby??

Years ago at Point Barrow I had a 6 Megawatt Dewline radar about 2 miles away interfere with the read head on a 9-track tape drive. After HOURS of troubleshooting a seemingly intermittent tape drive my O-scope happened to capture the radar transmit pattern as it swept by our position.

[wwest]

And let's not forget the scratching on the side of the dizzy cap seemingly aligned with the rotor tip.

Cause of the dizzy cap vibration you observed?

Wait... Might the bevel gear teeth of the dizzy drive move the dizzy shaft vertically?

But hold it steady for a vertical pull test?

[OsoMoore]

I haven't check if the sparks drop off smooth with rising RPM. I'll have my wife step on the gas while I check it out with my ear and timing light.

The magnetic pickup wire was far away from the actual wires, which were shielded. Perhaps arcing inside the distributor might affect the pickup in the distributor, thus leading to a sort of feedback loop.

[OsoMoore]

Originally Posted by wwest

And let's not forget the scratching on the side of the dizzy cap seemingly aligned with the rotor tip.

Cause of the dizzy cap vibration you observed?

Wait... Might the bevel gear teeth of the dizzy drive move the dizzy shaft vertically?

But hold it steady for a vertical pull test?

The new dizzy and rotor have not exhibited the scratching I saw on the old one.

[wwest]

As a general rule magnetic pickup signals are sharper as the speed of mechanical induction source rises.

The "wire" from the dizzy is actually a coaxial cable, a single wire inside a metal braid/shield, the braid being a protection against outside EMI/RFI signal source coupling into the shielded signal. As such the braid/shield is only grounded at one end. In this case within the CDI.

Electrical diagram indicates an extra 2-pin connector between the dizzy and the CDI connector. Find the connector, unplug, inspect, reconnect?

[OsoMoore]

Originally Posted by wwest

As a general rule magnetic pickup signals are sharper as the speed of mechanical induction source rises.

The "wire" from the dizzy is actually a coaxial cable, a single wire inside a metal braid/shield, the braid being a protection against outside EMI/RFI signal source coupling into the shielded signal. As such the braid/shield is only grounded at one end. In this case within the CDI.

Electrical diagram indicates an extra 2-pin connector between the dizzy and the CDI connector. Find the connector, unplug, inspect, reconnect?

I don't yet have this connector - mine is wires straight through to the CDI plug. This will change soon when I replace the green wire, and then I'll have an intermediate connector.

[tirwin]

Willard, technically we can't rule out aliens either. I'm just saying start with the highest probability and lowest cost failure sources.

A coil is comparatively cheap. Not the likeliest candidate but I saw an issue recently where a failing coil that was struggling to fire was firing late. I didn't hesitate to recommend this because trying a new coil is not a dollar lost. Like I said, worst case is OsoMoore has a spare that saves him from being stranded one day.

Replacing the green wire is good preventative maintenance.

So we're left with CDI and dizzy. Both are relatively expensive - at least $500 as I recall. Being able to consistently reproduce the issue by cooling the CDI is would be great progress.

If the CDI could be conclusively ruled out, then we're left with tearing down the dizzy.

If the problem turns out to be the CDI then OsoMoore has to consider whether to go aftermarket or go back with OEM Bosch.

[tirwin]

Related question - the new and non-brand spark plug wires have a loose connector on one spark plug, and measure 2K ohms resistance. The spec is 3K. I am thinking maybe I should just return them and get a new set when I order my green wire. Should I go with Red or Black?

Color choice is up to you.

I am running Clewett wires on my car with a resistor-less rotor, MSD Streetfire, MSD Blaster II High Vibration coil and NGK plugs.

I won't try to sell you on one ignition or the other. I'm not married to MSD but the Streetfire is like $120 and it was a noticeable improvement over the old PermaTune at 1/4 or 1/5 the cost of a reman Bosch as I recall. There are other equally good choices out there.

The only reason I'm saying this is if you do end up finding it's a bad CDI then you need to consider the coil, CDI and plugs wires as a system and purchase accordingly. For example, MSD clearly states that you can't use solid core wires like the OEM Berus so if you buy Berus now and it turns out to be the CDI then you will have to swap wires again.

[CCM911]

Dumb question guys. Could Oso have gotten a bad distributor cap when he ordered the new one?

[wwest]

If Op buys a new coil is should be a CDI type, not available in the timeframe of our cars.

Accel 140024, Accel GM HEI Remote Mount Super Coil | Accel

[wwest]

Originally Posted by CCM911

Dumb question guys. Could Oso have gotten a bad distributor cap when he ordered the new one?

Good point, he should compare the 2 closely, including the rotors.

The problem started, existed, with the old cap/rotor.

The spark is jumping "before" reaching the dizzy input, typically indicating an "OPEN" circuit downstream of the dizzy center post input.

In this case carbon trails/paths/traces left on the inside of the old dizzy cap indicates that the CDI is firing the coil at other than the desired point, rotor closely aligned with the dizzy contact.

Unplug the HV wire from the coil, attempt an engine start, the spark with either jump inside the coil or through the coil HV "tower".

[OsoMoore]

Is the pickup a hall effect or magnetic on the stock distributor?

[wwest]

Originally Posted by OsoMoore

Is the pickup a hall effect or magnetic on the stock distributor?

Magnetic.

[wwest]

Originally Posted by vmax

Maybe this will help.

Not mine...

[wwest]

Fixed? Progress?