Hi All,

I just installed the MSD StreetFire CDI Box in my 74 barracuda. Ignition was stock with black Mopar ecu 5 pin and dual ballast resistor. Alternator looks stock 2 field wires, one direct to battery, I bypassed the dash ammeter last year and installed an under dash voltmeter.
After the msd install, car starts right up, which was a big improvement, but above 1500-2000 rpm, lights flicker and voltage pulses from 13.5 to 15.5 approx. Pretty sure before I installed this box the voltage bounced around 12.5 to 13.5 but never made the lights pulse and never climbed up past 15.
I did a search here and found one solution that uses the diode that came with the MSD box, that did not work, tried it both directions.
I paid careful attention to the instructions and verified all wiring.
The culprit to me seems like the small red wire, it goes to the switched 12 v which was on the positive side of the coil, but it is also connected to the regulator and the field wires of the alternator.

Any ideas?

I am not familiar with the MSD setup but the above statement sounds confusing.

The field wires are what controls the charge rate of the Alternator and should be independent. IE: direct connection from the regulator to the Alt. Tying them to anything could create all kinds of anomalies.

In a STOCK setup the wire, usually blue, feeds the 12V side of the ballast, the VR sense pin and one of the alternator field terminals.

The wire hooked to the + coil terminal would not be switched 12V, it would be less than that, about 9 or so, in run. Only in start would it be 12V or so.

Who knows how you have it wired.

OK, I do not think it is how it is wired up. The red wire I referred to is from the MSD Box, it just powers it up. I eliminated any questions by going straight to the battery using a quick disconnect and still had the same problem. All the other wiring is straight forward and done according to the MSD schematic.

So, I took some measurements;
While running and giving some throttle;
At the Battery 14.8 to 15.5
At the Alt Blu wire 12.7 to 14.4
At the Alt Grn wire 2.4 to 14.6 ...this measurement also showed OL intermittently, with a lightening bolt symbol.

The OL means input out of range, or Unsafe voltage. Voltage ≥ 30 V, or
voltage overload (OL) according to Fluke.

At the Battery 14.8 to 15.5
At the Alt Blu wire 12.7 to 14.4

Those two should match and are the issue,

As the battery is recharging (after start) the measured battery voltage should float up to nearly equal of the alternator's.

The wire going from the key switch to the ballast provides the voltage that the regulator uses to decide how much output the alternator needs. It then controls the electricity going through the field inside the alternator. The reason there are two field wires is that one is the negative. The 'two wire' regulators like you have control the negative's connection to ground.

Your MSD 5 wire's sound very similar to MSD 6
The thin red wire goes to a relay inside the MSD that turns the MSD on/off. The MSD draws its operating power from the heavy red wire.

The MSD units tend to draw more current with increasing rpm. The amount a 5 draws may be in the instructions. 6 is roughly 1 amp per 1000 rpm IIRC.

The problem is most likely in the charging circuit. Possibilities:
the VR is not getting the correct system voltage
VR its not outputing properly for given system voltage
The alternator output isnt keeping up
The alternator output isn't getting where its supposed to - power is being lost in a poor connection, partially grounding (not likely but dangerous).
Finally, yes its possible the MSD5 is drawing too much internally.

Copy Nacho's diagrams and mark up what you did.
That will help you figure out where to look for problems.

I do not believe the MSD box is directly causing your voltage fluctuation. As mentioned before, your blue wires should all be very close to the battery voltage.
Your field wire jumping to OL is your clue. More than 30 volts only occurs in two places: the wiring from the MSD to the coil and on the coil secondary including the spark plug wires.
Capacitive discharge ignitions work by putting way more than 14 volts to the primary of the coil (I am not sure about MSD but the original CDs used nearly 400V). Make sure your wires to the primary of the coil and the plug wires are dressed well away from your regular 12 volt ignition wires. Cross coupling may be inducing the over 30 volt spike to the alternator.
The second possibility would be the regulator. If it is generating a spike, the voltmeter may not be able to properly measure the pulse and just shows OL to indicate it does not know that is going on. Because the meter is expecting fairly steady state DC, sharp rise time pulses look more like AC to the meter. Flukes and other professional meters as well as older analog meters ignore these spikes or average them into the indicated reading. Regulator spikes are usually due to a poor ground but also can be a sign it is failing.
Craig

That's a very solid answer, technically.

Streetfire specs are easy to get on MSD website.:
Brand
Street Fire

C.A.R.B. EO #
D-40-43

Current Draw
.7 Amp per 1,000 RPM

Emission Code
1

Primary Voltage
435 Volts

RPM Range
15,000 RPM with 14.4 Volts

Secondary Voltage
43,000 Volts

Spark Energy
95 mJ per spark

Spark Series Duration
20 degree Crankshaft Rotation

Voltage Required
10-18 Volts, Negative Ground


Weight and Size
1.5 lbs., 6"L x 3.5"W x 1.75"H

The highest supply voltage of a CD ignition that I know about is 535 volts and the max energy is something near 150mJ.

It's not the MSD.

R.

My question is do you have more then one wire hooked to the + side of your coil ?? As with MSD you should only have the wire from the MSD box to the coil + side. You should not have any other wire on the coil + terminal. When you say the small red wire on your coil also goes to the regulator and field wires that sounds wrong. The normal MSD is multi-spark at lower rpm and the coil on all MSD setups I ever hooked up only gets the two primary wires from the MSD unit to it as one on the + and one on the - coil terminals. You dont want the coil + primary terminal having any other wire on it because that gets fed a spiking voltage since the MSD is multi-spark at lower rpms. That would surely effect the charging system if the regulator or field wires were getting the spiking voltage from the MSD unit as I believe it can spike up to 300 volts . Ron

Ron - I don't see where Cudabitten wrote that he has two wires on the + side of the coil.

The only thing he wrote about wiring that comes across as wrong is that one of the alternator's field wires may be going to the battery. I suspect that's actually not what he did...


At the end he wrote


This is correct. The smaller red wire goes is joined to the Run circuit - which was attached to coil positive.
Run wire from the On position of the switch goes to the ballast. On the other side of the ballast resistor it continues to the coil. The MSD on/off red wire can be attached anyplace along there as long as it gets a little power when the ignition switch is On.
https://board.moparts.org/ubbthreads/ubbt...tml#Post2334590
At least on the earlier cars, the VR wire is spliced into the Run wire at the ballast. So the VR should be seeing the voltage that is close to the same as the main splice.
Diagram

Mattax you're exactly right, that junction takes place downstream of the initial connection. Yes only one wire on the Coil, as I said I followed the directions exactly. I drew schematics before and after so I could understand what was being removed and replaced.
Sorry I have been working long days this week and haven't responded.
A few things I did on Sunday were;
I unplugged the alt wires and reg wires and ohmed them out they were less than half an ohm.I also ohmed from the MSD box to gnd, the reg to gnd and the alt to gnd all were less than .5 ohm. I also performed a "full field: test of the alt and got a steady 16.9 volts Then I bypassed the Reg and gave some throttle and watched the reading go from 14 to 15.5 at a steady rate. So it appears the Alt is fine, I then purchased a new Regulator and still have the same result. I'm a little disappointed, this is a pretty simple circuit. One thing an earlier poster mentioned was the proximity of the coil wire to the MSD harness. This is something that is possible since all that wiring is really close up against the firewall,,,my coil is mounted at the rear of this 360 on the last intake bolt. The only other thing I can think of is the connections on the Ballast and the connection at the bulkhead where the ignition comes through.
I may re-install the mopar ecu, stinks because I spent three days fabbing a bracket and re-doing harnesses.

[quote=Mattax]Ron - I don't see where Cudabitten wrote that he has two wires on the + side of the coil.

The only thing he wrote about wiring that comes across as wrong is that one of the alternator's field wires may be going to the battery. I suspect that's actually not what he did...
[quote] Alternator looks stock 2 field wires, one direct to battery,




Sorry about that as I was not sure but I thought I would ask but you seem to understand that it should only be the coil primary wires on the coil primary terminals. I just wanted to be sure since as we know we dont always know how much each of us know talking on the net.
I appoligize if I offended anyone as that was not my intention at all. You guys seem more then smart enough to understand most about the MSD. Was hoping I could help but Cudabitten looks to have that covered. Good luck with fixing it as I hate goofy problems like that. Ron

Yo 383man, All the responses I received have value. None of us have all the answers...please keep your advice coming now and in the future.
Peace out.

Ron,
You sure didn't offend me. I thought what you wrote was civil and to the point and I hope I came across the same way.
- Matt

my car did that, I wired up a relay on my ign circuit & that stabilized the 12 volt signal to the voltage regulator,, problem solved.

Thank you and thank you also Mattax as I appreciate the kind replies. Ron

Well I finally got this solved. I found on another site, I think the A Bodies only, that someone had this same issue and they sent one wire from the voltage regulator directly to the battery and the voltage swings stopped. The theory is that the voltage regulator was seeing resistance in the ignition circuit and was letting the alternator charge up to a certain voltage, 16 volts, and then dropping back down.
I sent the blue wire over to the starter relay stud and now I have a very steady charge in my circuit.
Thanks for trying to help me with this. Also ended up removing the ballast resistor.
I just saw your post CSK, now I know what you were saying.