Does a mag (-) wire give a signal?

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sc2dave
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Does a mag (-) wire give a signal?

Post by sc2dave » Sun Oct 08, 2017 11:46 pm

Trying to see if my Neg mag. pickup wire is giving a signal to the MSD box for it to signal. I have tried grounding the white wire and the box makes the coil spark, but I want to find out if the mag is indeed sending some signal. Can I check this with a multimeter?

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Re: Does a mag (-) wire give a signal?

Post by JohnP » Mon Oct 09, 2017 2:11 am

The Green magnetic pickup wire is grounded inside the MSD. The signal is on the violet wire.

To test, you would have to bring the violet wire below ground, then above ground to make it spark.

You could do it with two AA cells and a ground wire. Have a friend help hold them for you.

Stack them in series, head to tail, with the ground wire in between them.

One end will be +1.5v, the other end will be -1.5v.

Touch the violet wire to the -1.5v end, then to the +1.5v end. The box should make the spark when you go from the minus end to the plus end.
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Re: Does a mag (-) wire give a signal?

Post by Tuner » Mon Oct 09, 2017 2:46 am

Set a multimeter on AC volts lowest scale (2 AC Volt?) and connect the probes to the purple and green wires, polarity does not matter. At normal cranking speed it should generate .4 to 1.5 or so AC volts.

If the distributor is not in the engine you can spin it by hand. I just checked one by twirling the shaft with my fingers and it was easy to make 1 to 1.5 AC Volts.

The amount of voltage is not relevant, as the distributor speed increases the voltage increases, the falling voltage of the AC signal triggers the box, like described above. If it has AC voltage output, it will trigger the box.

If you have a soldering gun, a way to check the system is hold the hot tip of the gun near the pickup coil and trigger it. The oscillation of the magnetic field will generate a signal in the pickup coil and fire the system. 60 CPS = 900 RPM

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Re: Does a mag (-) wire give a signal?

Post by peejay » Mon Oct 09, 2017 3:49 am

JohnP wrote:
Mon Oct 09, 2017 2:11 am
The Green magnetic pickup wire is grounded inside the MSD. The signal is on the violet wire.

To test, you would have to bring the violet wire below ground, then above ground to make it spark.

You could do it with two AA cells and a ground wire. Have a friend help hold them for you.

Stack them in series, head to tail, with the ground wire in between them.

One end will be +1.5v, the other end will be -1.5v.

Touch the violet wire to the -1.5v end, then to the +1.5v end. The box should make the spark when you go from the minus end to the plus end.
You can also just make continuity between the two wires with a paper clip. This is the method that MSD actually calls out in their manual for testing the ignition system.

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Re: Does a mag (-) wire give a signal?

Post by Carnut1 » Mon Oct 09, 2017 10:27 am

Anytime a conductor has current flow a magnetic field will be produced around the conductor. If the current is dc the field will be as steady as the current flow. Ac current flow produces a growing and collapsing magnetic field. Fyi.Thanks, Charlie

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Re: Does a mag (-) wire give a signal?

Post by JohnP » Mon Oct 09, 2017 2:12 pm

Peejay:

Thanks for the tip. I assumed the signal had to go below ground.

I googled their tip page on it:

https://www.msdperformance.com/support/ ... echniques/
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Re: Does a mag (-) wire give a signal?

Post by sc2dave » Mon Oct 09, 2017 11:03 pm

Damn, I should have said it's for my motorcycle. Same principal, stock ignition is transistorized. ignition, 4 cyl 4 stroke. Will I still get a signal even if I connect all wires and tap into the mag (neg) wire ,to get a signal with the voltmeter, while it's cranking?

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Re: Does a mag (-) wire give a signal?

Post by Krooser » Tue Oct 10, 2017 6:36 am

Does all of this info also apply to an analog 6T box?
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Re: Does a mag (-) wire give a signal?

Post by JohnP » Tue Oct 10, 2017 12:28 pm

The negative wire of the magnetic pickup is usually grounded in the ignition module. Nothing to see.

The signal will be on the positive wire.

Disconnect the wires, connect an ohm meter to the pickup coil, and you should read a few hundred ohms.
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Re: Does a mag (-) wire give a signal?

Post by sc2dave » Thu Oct 12, 2017 7:49 pm

So the neg coil wire will be the trigger wire? The instructions are here; https://www.mpsracing.com/instructions/MSD/MC2.pdf . I'm pretty sure that I have it wired right. Page 5 wiring for a stock dual coil system.

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Re: Does a mag (-) wire give a signal?

Post by JohnP » Fri Oct 13, 2017 5:19 am

Figure 5 shows we are talking about two different things.

I thought you were asking about the pickup coil in a two wire mag pickup, as used in a distributor or crank trigger.

You are talking about the signals on an ignition coil.


Think of a points ignition:

Coil plus is connected to 12v from the key, through a ballast resistor.

Coil minus goes to the points.

The points are just a switch. One side is connected to ground, the other side goes to coil minus.

When the points are open, coil minus reads 12v

When the points are closed, coil minus reads 0v

When the engine is running, you will see a 12v square wave on the coil minus. At the rising edge of the square wave, the signal actually goes to 300v to 400v for a few microseconds, due to the "inductive kickback" from the coil. This is when the plug fires.

If the points are replaced by a transistorized ignition, you will see the same waveform on the coil minus.

Now, install an MSD, and the wire that used to go to the coil minus goes into the MSD instead.

When the engine is running, you should see a 12v square wave on the wire that used to go to C-, except there is no 300v to 400v kickback signal. Just a 12v square wave.
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Re: Does a mag (-) wire give a signal?

Post by sc2dave » Sun Oct 15, 2017 10:10 pm

I accidentally found out what to do to get it running . I had disconnected one of the neg wires to check to see if it indeed was neg and not pos. when I hooked up my voltmeter to crank and check for a signal, I heard the engine kinda rumble a little. So what I did was keep those wires disconnected, cranked the engine, and it fired right up. According to the instructions, both of these wires should be hooked up, along with the other set for the other coil. Any comments?

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Re: Does a mag (-) wire give a signal?

Post by JohnP » Mon Oct 16, 2017 4:29 am

That's weird. Can you post a drawing of how you have it wired?
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Re: Does a mag (-) wire give a signal?

Post by sc2dave » Fri Oct 20, 2017 12:34 am

20171019_212514.jpg
The only wire that was disconnected is the Green wire to the original (-) neg wire. Today, I tried starting it and now it woudn't start. I tried intermittently grounding the Green wire and it wouldn't make the coils spark. I occasionally would hear a very slight tick noise from the box every time the Green wire touched ground. After a while of tapping the Green wire to ground, it would make a good spark, but it wasn't consistent. Can someone rotate the picture for me?
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Re: Does a mag (-) wire give a signal?

Post by JohnP » Fri Oct 20, 2017 1:02 am

The drawing goes right side up when I click on it.

Yes, I've seen that drawing. I was hoping you would post a hand drawn one showing how YOU have it wired, so we can check your work. :)

In most CDI installations, the wire to the negative terminal of the coil is grounded inside the box.

Key off
Disconnect the Coil+ wire from each coil.
Note that the output signal from the MSD that goes to the Coil+ of each coil is a 500v NEGATIVE pulse.

Use your DVM to verify 0 ohms from chassis ground to the Coil- of each coil.
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