Ignition question involving use of ballast resistor

1987 Dodge Dakota 3.9L carbed

I recently posted on rec.autos.tech about the problems I was having with
an MSD 5900 Blaster ignition and an MSD 8202 Baster 2 coil. It seems I
could use one or the other with no problem, either the MSD ignition with
the stock coil or the MSD coil with the stock ignition, but if I had
both installed I would get a stumble on accelerating from a dead stop.

From this behavior I deduced that, for some reason, the problem was
being caused by too much ignition voltage. To test out that theory I
installed a 1 ohm ballast resistor in line with the +12 volts going to
the coil. Voila, the problem vanished and I was back to strong, smooth
acceleration from a dead stop. So then I tried a 1/2 ohm ballast
resistor and the stumble returned, but not as bad, so I went back to the
1 ohm resistor.

The stock coil has about a 1.5 ohm primary and the MSD coil has a .7 ohm
or so primary. I don't know what impact this has on the situation but
it's info that I have and I figured I might as well throw it in if I am
going to all the trouble of posting this and you are going to all the
trouble of reading it.

Now we get to meat of the my puzzlement. The ignition wires that I have
on the engine measure about 3000 ohms per foot. For about $45 I can buy
some MSD Street Fire wires that are only 500 ohms per foot. I know a bit
about electricity, but not enough to know whether or not these wires
would enable me to run the MSD ignition and coil without the ballast
resistor. After all, the point of an MSD ignition upgrade is to get
longer stronger spark to the plug.

I don't have a clue as to why the engine stumbles with both the MSD
ignition and coil installed. From the symptoms it seems to me that it
must be because of too much voltage at the coil tower, but why that
might be causing the problem is beyond me.

Could it be because the high voltage is leaking out to ground somewhere
on the way to the plugs? I checked the engine at idle in a dark area at
night and saw no evidence of arcing. Could the high resistance wires be
the cause? If so, what is the rationale? I can't think of one.

Well that's it. At present, I am more than happy with the ballast
resistors. The truck runs great. Just curious as hell as to what might
be going on.

Jack

 

ROTFLMAO !!!

 

Jack,
The purpose of a ballast resister is to allow a lower voltage ignition coil
to be used. The reason for this is that when the starter motor is engaged,
battery voltage will drop to say 8 V. You will note that when wired
correctly the ballast resister is bypassed at that time allowing the coil to
create adequate spark during lower battery voltage conditions . When the
starter is then disengaged the resister bypass is also dropped and the
resister is now in play to reduce excessive heating of the coil. Electronic
ignitions are very sensitive to coil configurations, as they provide coil
drive and can be permanently damaged if the DC resistance of the coil is too
low. Follow the manufacturer's recommendations.
Steve

 

Normally when all is working as it should, what limits voltage is the spark
plug gap and the engine compression (actually the absolute pressure inside
the cylinder). A wider gap or more pressure will cause the voltage to climb
higher before it will cross the gap. acceleration from a stop often will
produce the greatest pressure inside the cylinder.

Yes it could be that - If the voltage climbs too high too fast it finds
another path instead of the spark plug gap. Ultimately, your ballast
resister doesn't limit the voltage at the spark plug but it does slow down
the rise in voltage which means that some other points in the secondary
won't get quite as high. If stray arcing in the secondary is what is
happening it would be likely that it would get progressively worse over time
as the stray arcs develops a carbon trail that the current can follow more
easily.

The cause could also be in the primary there may be something that
saturates in the primary or even something intended to limit voltage/current
to protect against damage.

-jim

 

This was especially true when the Kettering system was used. When
capacitive
discharged systems came along, it pretty much stopped being true. Their
rise times are so short and transformer action so definite that they usually
put
up much more constant ignition performance.

I dont know what the multiple spark discharge systems do. Are they CD, or
something else?

Some early studies showed that, all things being equal, a small single spark
ignited the fuel as well as anything else. Some university studies in
England
showed that high performance ignition systems which gave significant gains
were possible, but they were not necessarily simple and cheap either.

It is my opinion, and nothing more, that there is little to be gained from
systems
like the MSD for normal driving. For racing, maybe....

 

The ballast resistor does more than just change the voltage, it can change
the shape of the pulse through the coil as well.

No, using a wire with a lower resistance will _slightly_ increase the
voltage to the plug. But honestly, the difference is very small because
the voltage is so high and current so low that it takes a lot of resistance
to cause much drop.

What you want is an ignition system analyzer that will let you look at the
signal coming out of the coil on a CRT. This will tell you what is really
happening when you put the ballast resistor in. Ask older shops around town
if they still have an analyzer kicking around.

The scope will tell you, and without the scope you have no real chance of
seeing what is happening.
--scott

 

What stopped being true? The statement you are responding to has never stopped
being true. It may have become unimportant to know that it is true, but that is
hardly the same as being not true..

How does this information relate to solving the OP's problem? The rate at which
voltage is developed in the coil is going to be a function of resistance and
inductance no matter what. Which is why changing resistance produces different
results.

His intent is related to neither of those - it is to improve emissions.

-jim

 

I'm going to say MSD designed the output section of their ignition
system wrong, and maybe when the resistance is low, it goes into
current-limiting mode to protect itself. Another possibility is the
high voltage protection for the output section (typically some zener
diodes and capacitor) is interacting badly with the coil primary.

Low resistance actually doesn't help in most cases, probably because
it slows the rise time (turn-on) of the current, and I've heard of
many people running into trouble when they switched to low-resistance
plug wires.

High voltage leakage shouldn't be a problem with modern (1980s and
later) distributor caps, rotors, and wires. On the other hand after I
installed a CD ignition in my 1975 Dodge Dart, 2 caps cracked in a few
months. A slight modification to limit the primary voltage to under
400V stopped this.

 

They're usually ordinary flyback systems, just like points systems. I
never saw a CD multistrike system, but that was long ago, when every
CD system was triggered by an SCR, a device that can be turned on but
not turned off. But since then, high voltage, high current devices
that can be turned off have become more common, so multistrike CD
should be easier to implement. I had a double-strike CD system that
consisted of 2 separate CD units because I didn't know how to make
anything better.

True, thanks to the EPA and its stringent emissions standards that
include cold weather, cold engine conditions.

 

I would suspect that too much primary current just supersaturates the
coil and it doesn't discharge properly, inhibiting the next charge/
discharge cycle.

Dan

 

I suspect the pure resistance is less of a factor than the distributed
impedance, wouldnt you
imagine?

 

I am not really sure... I think you can look at the cable as a lumped-sum
anyway. It's pretty much just the resistance of the cable that will affect
how much the coil will ring on the tail end of the pulse, and I think that
is the big deal. The capacitance between the cable conductors and ground
COULD make a difference, but they are going to be different in every
installation depending on how the cables are laid out. The series inductance
of the cable should be minimal.
--scott

 

Steve, thanks for the heads up on the wiring. I didn't run the B+ wire
to the coil from the starter relay because I felt that the MSD ignition
and MSD coil together would provide enough spark going through the
ballast resistor even with the engine cranking. It proved to be true.
The engine starts up immediately with the 12+ volts from the ignition
switch going to the coil through the ballast resistor. I was mainly
intent at the time to test the idea that the stumble problem was being
caused by trying to push too high a voltage to the plug gap. The ballast
resistor was my way of reducing the voltage at the plug gap while using
both the MSD coil and ignition together.

The ballast resistor is rated at 1 ohm and measures 1 ohm. The Blaster
coil primary is rated at .7 ohms and measures .6 ohms. The resistance of
the coil primary of the MSD Blaster coil and the added ballast resistor
measures 1.6 ohms, all measured on a digital meter. The recommended
primary resistance for the coil on this vehicle is 1.34 to 1.55 ohms.

I don't know what stress is being placed on the spark control computer
when using the MSD Blaster coil with the stock ignition and no ballast
resistor, but the engine runs great under this configuration, with very
smooth acceleration a dead stop. The same is true when using the MSD
ignition box with the stock coil. The problem of a stumble on
accelerating from a dead stop happens only when both are used together.

With the addition of the ballast resistor the computer now sees what it
was designed to see at the primary winding, so there should be no
problem of the kind you are warning me about, having too low resistance
at the primary.

As to what is causing the stumble on accelerating from a dead stop using
the MSD ignition and coil and no ballast, I am still in the dark. After
all, the engines performs great with just the coil and no ballast.

Jack

 

I think the whole idea behind the MSW (magneto-suppressive, spiral wound
solid wire) wires was to reduce the d.c. resistance (relative to
resistive core wires - a thing of the past) for increased spark current,
but increased inductance to keep the EMI/RFI (radio interferenece)
within reasonable limits - kind of the best of both worlds ("hotter"
spark, low radio interference).

 

Thanks, jim, for the feedback. I ran the engine at idle in the dark and
could see no evidence of arcing. Of course this is different from
accelerating the engine under a load from idle.

At any rate, if the problem is caused by current leak, do you think that
changing the plug wires from 3000 ohms per foot to 500 ohms per foot
would help? It would only cost me about $50 or less to find out, but I
hate throwing money at a problem to see what sticks.

Jack

 

HLS, the unit that I have is the MSD 5900 Blaster. It is not multiple
spark, but one strong spark through 20° degrees of rotation and it is
induction discharge, not capacitive. But most, if not all, of MSD's
other ignition units, with the exception of the MSD 5, are capacitive
discharge.

I installed the MSD components in the hope that they would help pass
California smog requirements. I had to swap out the original feedback
carb for a non-feedback version because the mixture control solenoid in
the feedback carb is no longer being provided by anyone as a replacement
part and owners of vehicles with this carbs are left to fend for
themselves. I had the non-feedback carb installed the last time I had
the truck smogged and it passed, but only just. I was 2 ppm below the
max for HCs.

 

What role does the resistor play when voltage is cut off to the primary?
This is what causes the secondary to produce its voltage, right, the
sudden collapse of the primary field?

Does the inductance produced by the expanding field of the primary limit
the rate of expansionof the field? Is this why there is not a high
voltage produced in the secondary when voltage is applied to the primary?

Could it be that the lower resistance of the MSD coil primary over the
stock coil, .6 to .7 ohms, compared to 1.3 to 1.5 ohms, coupled with the
greatly increased voltage to the primary from the MSD ignition could
cause the field in the primary to expand too quickly and cause a weak
spark at the plug at the wrong time?

Scott, this was what I suspected. It seemed to me that the higher the
voltage, the less impact high resistance wires would have. My thought
was that perhaps the higher resistance wires was causing current to leak
out somewhere besides the plug gap.

Thanks, scott. I really appreciate your input.

Jack

 

MSD recommends that these two units be used together on an ignition
system such as the one in the 87 Dodge Dakota 3.9L. I am not saying that
this means squat, just that this is their recommendation. The MSD
ignition alone was causing the stock coil to get so hot that I could not
keep my fingers on for more than a second after a good run, which is why
I got the coil that MSD recommends.

Ah, this interesting. How did you accomplish this, if I might ask? MSD
specs the 5900 as producing 420-450 volts to the primary. The stock 12+
wire to coil remains and MSD adds another wire from the ignition unit to
the + coil terminal and replaces the signal wire from the computer with
another wire from the unit and the stock signal wire is connected to a
third wire from the unit. A fourth wire goes to ground. Here is a link
to a PDF file of the wiring instructions for the unit:

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/5900_instructions.pdf

Figure 2 is the one that applies.

How might I lower the voltage as you have, rather than installing a
ballast resistor in the +12 volt wire to the coil.

Oh, and thanks for sending the schematics for the different ignition
systems. Now that I have my system stabilized and running well, I plan
to check them out in detail.

Jack

 

My guess is the cause is in the primary not the secondary. But if it was due
to a leak in the secondary you would probably have to be looking at it in
the dark while it was happening.
Changing the spark plug wire resistance shouldn't have any effect. Current
doesn't really flow thru the wires until the spark plug starts to arc. The
tail end of the wave form will be affected but not the initial spike.
Getting rid of the resistance won't change the peak voltage in a measurable
way.

It is not clear how this is going to help with your emissions. If the
mixture ia a bit rich all the spark in the world is going to get the extra
fuel to burn. If it is a tad lean it might help, but it doesn't sound like
that is the case.

-jim

 

The inductance and the capacitance form the resonant circuit. The
resistance
is related to the Q, or quality factor.

The resonant circuit is responsible for the ringing.

I believe, anyway>)