Bouncing RPM with Spark Breakup at Higher RPM

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saapbaad

saapbaad

mopar or no car
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So this is the issue I'm having and can't find a thread to help get me started on a diagnosis or to actually cure the problem. I've come to a few of my own conclusions but maybe someone can chime in.

Car: 74' duster with a 340 out of a '72

Problem: spark breakup attributed to the voltage regulator

Scenario: While driving, when I load up the engine (3/4 to full throttle) and the RPMs run between 2500 and 5000 I get spark breakup. My tach is bouncing all over the place when the breakup occurs, right near the 8-10K mark (which is not the true engine RPM). Even at idle the needle sits around 800-1000 but it isn't smooth, still kind of jumps around. When I disconnect the voltage regulator, the car runs perfect and the tach is smooth and displays the correct engine RPM. The voltage is normal sitting between 12 and 14 volts. What could this be?

At first I thought it was interference with the coil wires so I unbundled and separated them from their electrical tape cocoon but still no dice. Is something not grounded? Could this be a faulty regulator? I obtained this duster like this so I don't have any prior knowledge about the engine swap.

Any help is appreciated,
Andy
 
Might check for a ground strap between
the motor, frame and body.
 
It might be the alternator, regulator, or wiring. A multi-meter would be very useful. Observe what the DC voltage measures at the battery and switched 12V at ballast resistor as DC volts, and also place in AC mode and measure voltages. An alternator has three windings in a Y configuration with diodes to convert to DC. The AC voltage measurement should show less than 0.5VAC, while the DC voltage 13.5 to 14.5V. A bad diode results in bumpy voltage with an AC component. This is often results head lights rapidly flickering, and other noise issues including ignition. Your car should have a solid state regulator, not a mechanical relay type. The voltage regulator also needs to be properly grounded. A loose ground, or field supply might also cause similar problems.
 
i have this issue on my dart... its not spark scatter... its the feed wire to the ballast wire(blue and yellow i think)... feeds the ballast and then goes to the VR...

67dart273 post the diag steps...
 
Very first question is what are you running, exactly, for an ignition system?

How long is the distributor pickup wire between dist. and ignition box, and where is ignition/ regulator mounted?
 
longarm said:
Might check for a ground strap between
the motor, frame and body.
Click to expand...
There's a nice fat ground between the block, the battery and the body.

It's definitely something to do with the voltage regulator - it runs way better with it disconnected. I noticed there's an ammeter on the dash with a +40/-40 gauge sweep. Can this cause any problems?
 
67Dart273 said:
Very first question is what are you running, exactly, for an ignition system?

How long is the distributor pickup wire between dist. and ignition box, and where is ignition/ regulator mounted?
Click to expand...

Stock ignition system, no pickup wire between an ignition box because I don't have one, regulator is mounted on the passenger side fender, ignition resistor is located on the driver side firewall.
 
Ground connections can be verified with a volt meter. While mechanical connections may look good, a voltage reading provides true proof. Simply place the meter to read DC volts, place one lead at the battery (-), the other on the regulator case, ignition box case, firewall ground point, they should all measure near zero volts with engine running. If any measure more than a few tenths of a volt there is a bad ground connection at that point.

If you do not have a meter, fairly good ones can be purchased for $20 to $50.

The alternator diode issue may happen when jump starting a vehicle with the engine running. There are 6 diodes to provide full wave rectification for the 3 phases. If any diode fails the voltage supply regulation is compromised, and becomes electrically noisy. When you disconnect the regulator, the car runs off the battery only, and all is well suggests alternator at fault. The battery helps filter the normal alternator 3 phase rectified supply, however loss of one or more phases, results in the AC component that varies rapidly. You could think of it as a 3 cylinder engine running on two or less cylinders. When the dip in voltage occurs, the regulator responds and then the good phase over-shoots the voltage. The noisy voltage is not viewed with a normal DC meter since meters are damped and the frequency is fairly high such that the meter does not respond well. The AC setting measures, just the noise component. As suggested earlier an AC measurement of more than 0.5VAC may indicate a problem. If the +-40A sweep, means the amp meter is quickly bouncing the needle then it likely indicates a shorted diode or winding in alternator.

If the 12VDC measured was with engine, off or regulator unplugged, and 14VDC running connected, the regulator is likely good. My guess would be the alternator or connections. The alternator could be taken in and tested by most car part stores.
 
KitCarlson said:
Ground connections can be verified with a volt meter. While mechanical connections may look good, a voltage reading provides true proof. Simply place the meter to read DC volts, place one lead at the battery (-), the other on the regulator case, ignition box case, firewall ground point, they should all measure near zero volts with engine running. If any measure more than a few tenths of a volt there is a bad ground connection at that point.

If you do not have a meter, fairly good ones can be purchased for $20 to $50.

The alternator diode issue happens when jump starting a vehicle with your engine running. Diodes can be replaced with skill. There are 6 diodes to provide full wave rectification for the 3 phases. If any diode is failed the voltage supply regulation is compromised, and very noisy. When you disconnect the regulator, the car runs off the battery only. The battery helps filter the normal alternator 3 phase rectified supply, however loss of one or more phases results in the AC that varies rapidly. This voltage is not often viewed with a normal DC meter since meters are damped and the frequency is fairly high such that the meter does not respond. The AC setting measures, the noise component.
Click to expand...
Ran your tests and everything checked out.
 
805moparkid said:
i have this issue on my dart... its not spark scatter... its the feed wire to the ballast wire(blue and yellow i think)... feeds the ballast and then goes to the VR...

67dart273 post the diag steps...
Click to expand...
I'd like to hear more about this. I can see there's a yellow wire/black stripe that's soldered to a yellow wire, yellow wire runs to a plug on the firewall which obviously goes into the cabin.
 
saapbaad said:
Stock ignition system, no pickup wire between an ignition box because I don't have one, regulator is mounted on the passenger side fender, ignition resistor is located on the driver side firewall.
Click to expand...

What means "Stock?".

You are running what, EXACTLY?

Mopar single points? Dual points? "Ready to run" integrated of some kind?

You ARE running an isolated field alternator with the 70/ later regulator?

For the moment, I agree with Kit, that this might be one or more bad diode. These cause the alternator output to be HORRIDLY "spikey" and noisy. If you have an AM radio, it will sound AWFUL.

You can check diodes if you can get to someone with EITHER an automotive tune-up scope or just a bench oscilloscope, or you can check them by loading them individually, but to do that, you'll have to unsolder the diodes from the stator

If this turns out not to be the case, try running the tach wire very close to the "ground plane" of the car, or even replace the tach wire with shielded cable.

Here's a normal alternator waveform, showing what is known as "ripple" from the rectified AC. The "little bumps" are pulses of current coming from each diode

misc_1-1.png


Here's a couple of scope waveforms showing bad diodes:

You can see they are horrid and spikey looking. A perfect DC waveform would simply be a straight line

_2344_20BADdiode.gif


_2345_20BADdiode1.gif
 
67Dart273 said:
What means "Stock?".

You are running what, EXACTLY?

Mopar single points? Dual points? "Ready to run" integrated of some kind?

You ARE running an isolated field alternator with the 70/ later regulator?

For the moment, I agree with Kit, that this might be one or more bad diode. These cause the alternator output to be HORRIDLY "spikey" and noisy. If you have an AM radio, it will sound AWFUL.

You can check diodes if you can get to someone with EITHER an automotive tune-up scope or just a bench oscilloscope, or you can check them by loading them individually, but to do that, you'll have to unsolder the diodes from the stator

If this turns out not to be the case, try running the tach wire very close to the "ground plane" of the car, or even replace the tach wire with shielded cable.

Here's a normal alternator waveform, showing what is known as "ripple" from the rectified AC. The "little bumps" are pulses of current coming from each diode

misc_1-1.png


Here's a couple of scope waveforms showing bad diodes:

You can see they are horrid and spikey looking. A perfect DC waveform would simply be a straight line

_2344_20BADdiode.gif


_2345_20BADdiode1.gif
Click to expand...
Forgive me for my lack of technical includes as Mopars are all new to me (so is V8/carb/distributor type ignitions).

Single point or dual point dist - how can I tell? A quick google has yielded nothing to help me along here.

Yes the alternator is an isolated field type with a 70/later regulator.

My bad on the ignition it looks to be electronic Mopar as there is a unit/plug near the ignition resistor that I have just found to be such. Which wire off the dist is the pickup? There's only two - then I can measure the length. (EDIT: looks to be the red from dist to brown/white on the ignition harness side)

If it looks to be the alternator I have no problem grabbing another after the long weekend. The time it will take to diagnose this one is worth more than the alternator itself.

Thanks for the help thus far guys.
 
By length of the pickup wires, I meant just estimate. I was thinking the /6 cars had a VERY long lead, but I think they also have a removable "extension."

The point I was trying to show about the waveforms is that bad diodes make interference, known by many names, "electromagnetic interference", noise, spikes, etc.

If you have some friends, the easiest way just might be to try another alternator.

I would also gently twist the distributor pickup leads, and remove the connectors from the distributor, the ballast, and the ignition box (ECU) and "work" them in/ out several times to scrub the terminals clean. Also "feel" for how tight they are, and visually inspect for corrosion, etc.

Also try unhooking the tach and see if the miss goes away.

One other member claims (I'm not disputing this) that his ECU "multifired" in a similar way, and it is POSSIBLE that a bad coil could cause a misfire, but I'm not sure that it could cause this tach problem.
 
67Dart273 said:
By length of the pickup wires, I meant just estimate. I was thinking the /6 cars had a VERY long lead, but I think they also have a removable "extension."

The point I was trying to show about the waveforms is that bad diodes make interference, known by many names, "electromagnetic interference", noise, spikes, etc.

If you have some friends, the easiest way just might be to try another alternator.

I would also gently twist the distributor pickup leads, and remove the connectors from the distributor, the ballast, and the ignition box (ECU) and "work" them in/ out several times to scrub the terminals clean. Also "feel" for how tight they are, and visually inspect for corrosion, etc.

Also try unhooking the tach and see if the miss goes away.

One other member claims (I'm not disputing this) that his ECU "multifired" in a similar way, and it is POSSIBLE that a bad coil could cause a misfire, but I'm not sure that it could cause this tach problem.
Click to expand...
Yea that's what I got from those pictures, thanks for clarifying though.

Anyways I disconnected the tach and took her for a spin and the issue is still there. I then came back and split the entire harness along the back of the firewall apart and isolated the 2 distributor wires to the ECU, twisted them up and ran them completely away from everything else. The condition improved but it's still present. I'll just pickup a new alternator next week and post back the results and we can take it from there.

Thanks for the help,
Andy
 
Hell, I darn near forgot you have a multimeter, right?

Clip your multimeter onto the ignition feed with everything hooked up, engine running, and confirm that it is running 14V

THEN unhook the meter and change it to read AC volts. That's RIGHT, AC, and not DC volts. Hook it back up and see what it reads. This should show the amount of ripple on the 12V buss, and I'm betting it will be fairly high.
 
Good news guys, it was the alternator. Had some time this morning before we left camping so I picked up a new one and installed it, fired her up and no more problems. Looks like it's charging better too.

Thanks for the help, hope this thread helps someone else in the future.

Andy
 
Very Good.

As a side note, this is a parallel example of why I TRY to get guys NOT to yank battery cables off of a running engine "to test the charging system."

This is an excellent example of what some voltage spikes can do to a system, and MIGHT eventually damage something!!
 
67Dart273 said:
Hell, I darn near forgot you have a multimeter, right?

Clip your multimeter onto the ignition feed with everything hooked up, engine running, and confirm that it is running 14V

THEN unhook the meter and change it to read AC volts. That's RIGHT, AC, and not DC volts. Hook it back up and see what it reads. This should show the amount of ripple on the 12V buss, and I'm betting it will be fairly high.
Click to expand...
Great tip this helped verify it was the alternator.
 
How much did you have, curious? I'm betting a couple of volts?
 
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