So I spent the day getting the MSD 6AL box all wired up. I of course would like to use the limiter built in the box and may be using a 2 step module here shortly as well.

However to do such a thing I need to somehow disable the trailing spark. I've seen a lot of people say just pull the trailing ignitor. This kills the spark, but also the tach signal the box needs to run the car, as well as the tach itself of course.

My question is if there is a way around this issue as far as how to get tach signal sourced.

 

The tach signal is a yellow/green wire that get it's signal from the trailing ignitor.

the easiest way would be to pull the trailing igniter and connect the trailing coils negtive wire to the leading coil... leave the rest alone and what will happen is the leading ignition will feed the tach signal threw to the yellow/green wire...

 

I know which wire gives the signal. I guess I didnt know the MSD box can get rpm signal from itself?

So more or less what youre saying is connect the factory tach signal as well as the MSD box signal to the - side of one of the coils that is running the leading?

 

Of course it can, how would it know to limit it if it did'nt oh and BTW the MSD Box has a dedicated TACH output, it's acceptible to connect it to the negitive of the coil or the tach output on the box...

The MSD upgrade is option,I installed it on my old 1984 gsl before I sold it, Now that I have an 85 gs I think that I'm going to use hei's just because they're easier to find should on fail, not to say that the MSD box is uneliable by any means... Oh and it's kinda bulky...

 

My experience is different from you. My 10+ years 6A has no problem. Possibly since it.is connected directly to the alternators B terminal and a fuse for protecion. Those who have brought their cars to me with blwon MSD boxes were due to improperly or poorly installed system

 

Maybe I should be more precise with my description of what is going on. I think I may have figured it out though.

So if the trailing is hooked up, the car will start, idle fine, and blast right past the limiter on the trailing plugs (of course)

If the trailing is not connected, the car will not start.

What I'm thinking is it may be down to my 12V switch. When the key is in the "On" position, the box powers up. However maybe when the key goes to "Start" the 12V switch turns off, thus not firing up the box. However the trailing ignition makes just enough umph to start the car. Thus I return the key to the "on" position and the MSD box fires right up.

I guess I'll try it out tomorrow and see where I'm at.

 

Sorry if this is a dumb qiestion. I cant figure out where I would put the grey(tach output) wire, on the trailing ignitor? Its a 6al sorry

 

If youre running the trailing ignition put it to the negative side of the trailing coil. If not connect it to the green/yellow wire than would go on the negative side of the coil from the factory.

 

What 12v did you hook it to? you should have hooked it the factory coil's positive for the ignition signal...

 

To be clear, I never had any issues with my MSD box, as far as I know the new owner of that car has not had any issues with it... My whole reason for not going with the MSD is the fact that should it fail, it would be easier to find an Hei Module than an msd box... Personally I choose to leave my trailing intact for increased redundancy... After all, being stuck on the side of the road in Alaska is way worse than most places, say I'm make a trip up to Fairbanks from Anchorage,(360 miles) I usually have cell service within 50 miles of either city but up in the moutains I don't have any cell coverage... So I also factor that into my choice... While my RX is my DD going up to fairbanks I would most likely take my Taurus SHO Anyways... But in the summer, RX all the way...

 

How does the MSD rev limiter work? Does it cut the 12volts to the coil or the signal line? If it cuts the 12 volt line, you could hook that up to one end of the coil on a normally open single pole single throw relay. You then ground the other end of the relay coil. then run the trailing ignition 12 volt line through the two contacts on the relay, doesn't matter which one, one goes to coil, other goes to 12volt line.

That way, if the MSD kills the 12 volt signal, the relay opens up and kills the trailing spark. Make sure you put the tach signal line to the MSD unit, otherwise, it's possible that when the rev limiter kicks in, it might kill the tach as well since it's now getting its signal from the leading coil which has just been shut off by the rev limiter.

If it just kills the negative end of the coil, the line that oscillates at the RPM level, I'd need a minute to figure out the way to do it. If you just put the relay on that line, the relay would switch on and off at the RPM level, that is if it could keep up.

Let me know how it works and I'll figure out a circuit for it; it would be simple. You just need to convert the oscillating line so the relay simply see on or off.

What do you need a rev limiter for anyways? I hear the over rev buzzer on most shifts! It sort of guarantees that you won't get carbon buildup or lug the engine. Of course, it won't help if the throttle gets stuck...

 

The easiest way to do it would be to put an msd box on the trailing... The way the msd box limits rev is by turning off spark after the set rev limit and turning it back on when the rpms drop. You'd be correct if you thought that it kill the negative end of the coil... Anyhow if you want a pour man's rev limiter then solder a wire to the 12v out on the buzzer on the back of the cluster and put a relay inline with the trailing coils + so the 12v batt is connected to 30 and the coil input on 87a, grround to 85 and the overrev buzzer signal to 86. Basically the way this works is if there is no power the relay the coil is powered but as soon the overrev buzzer sounds that 12v will activate that relay and kill trailing spark... Make sense?

 

Heres a quick diagram of the idea...

 

You need 2 MSD 6al boxes to make the rev-limiter work, if you only have one box for now, It must be connected in the Leading side to take advantage where its needed most.

 

I don't understand BUT, The negative signal/input to your tach can/should be hooked up on the side of the MSD box negative/tack output

 

Well the 12v switch was the issue after all. Everything is working per design now except I have no tach lol. It doesnt really bother me that much but I would like to have it back.

I'm a little confused as to why it isnt working. I took the gray (tach output) from the MSD and connected it directly to the green/yellow wire from the trailing coil - side. Also tried hooking the yellow/green wire to the - side of the msd powered coils, still nothing. Any ideas? Am I missing something?

 

That should work but you need to make sure they are in sync. In other words, the buzzer should go off when the rev limiter kicks in; in fact, I would set the buzzer a little lower than the MSD one.

 

So my gray tach output signal is only registering ~7.5 Volts. This could have something to do with why my tach doesnt work yes? Any idea why that would be low?

 

I guess thats normal since the multimeter will take and average of the waves.

So right now I have the 6al running the leading only, limiter works fine since trailing is disabled

Tach does not function via the tach output from the 6al

2-step does not work even though it is wired up completely correctly. (runs off two 12v switches and the tach signal)

I'm seriously getting irritated with all this crap.

 

Follow post #14 and #15 and you will not have any issues

 

Maybe you missed it but I said that I am running ONE box hooked to the LEADING ignition. The limiter is functioning just fine right now wherever I have it set. Trailing ignition is disconnected at the ignitor.

My issue is that when I hook the gray tach output wire from the MSD box to the green-yellow wire from the factory - side of the trailing coil, the tach doesnt work. If I hook the yellow-green wire to the - side of the leading coils nothing happens either.

 

ummmm....bad tach???

 

Nope. If the trailing is all hooked up, and i put the tach wire back on the - side of the trailing coil it functions just fine.

 

the gray wire isn't for tach output, it is a single spade connector right on the side of the MSD box itself, no external wires running from the box. read the MSD diagram again.

generic diagram but you see the tach output in the lower left, the magentic pickup is wired to the J109 ignitor:


add a spade connector to the original tach wire from the trailing ignition and pop it in the tach output on the 6A, done.

 

Thats on the older style msd boxes. The new digiatal (PN6425) has the wire in the loom itself.