MSD Ignition - not strictly regional, but I am in the US!

[neejah]

Hello,

After attempting to rectify some dubious wiring by a previous owner I've created my own issues....

I inadvertantly connected the "tach" side of the tachometer to the +ve coil terminal. Now it won't start. I read on the MST site subsequently about requiring a filter before connecting to the negative side of the coil.......

It's got an MSD distributor, coil & Digital 6 Plus (6520) ignition module. It's using the distributor pick up instead of the crank pick up.

Anyway, I've had a look at the troubleshooting section - and have tried a spark test (shorting between the green & violet and looking for a spark at the HT lead off the coil) and have got nothing.

I've checked both the primary & secondary resistance on the coil & they look within spec based upon what's on the MST site.

The LED on the Digital 6 is flickering when the car is cranked, (I assume this is linked to the mag pick up in the dizzy) but again, it's not firing.

I checked to see if there was 12V on the +ve side of the coil with the ignition on - there isn't (I assume this is fed from the MSD) I also tried to connect a 12V supply directly from the battery to the coil and it still wouldn't start.

Obviously I pulled a spark plug & looked for a spark at first (there was none).

Hope someone can lend some assistance! Short of purchasing a new MSD Digi 6 I don't know where else to go......

Oh, it's in a '72 Gran Torino...........which is shortly going to be burnt if it keeps acting up!

Thanks

Neil

[GreenV8S]

Is the tacho still connected? If so, since you've probably blown it I suggest you disconnect it and try again. In blowing it and then reconnecting it you may now have blown the ignition amp so you might be stuffed now, but it's worth as try.

[neejah]

GreenV8S said:

Is the tacho still connected? If so, since you've probably blown it I suggest you disconnect it and try again. In blowing it and then reconnecting it you may now have blown the ignition amp so you might be stuffed now, but it's worth as try.

No, I disconnected it as soon as it wouldn't start.

Do you think the MSD box is blown? I've been trying to research a little more into the workings of them but there isn't much available.

Any chance you could explain the workings of the MSD?

I was under the impression that if you connected a 12V supply to the +ve side of the coil it should fire - but the engine wouldn't switch off....it still wouldn't when I tried this (after disconnecting the +ve supply to the coil from the MSD & checking a ground was on the other terminal)?

Wish I could've brought the Chim out here........would've been a damn site quicker than the Torino & more economical......

Thanks

Neil

[GreenV8S]

I've got no experience with MSD. The systems I have got experience on all have +12V supplied to one side of the coil, the other side is initially isolated (so it floats up to 12V too). When the ignition controller demands a spark it pulls the -ve side of the coil down to earth for a fraction of a second and then isolates it. At the point it is isolated, the HT side of the coil sparks.

I'd start on the assumption that the MSD setup is similar until you find otherwise.

On that basis, you should have +12v at the coil +ve while the ignition is on, and when you crank the engine you should see the -ve terminal switching between 0V and 12V each time the coil is triggered.

You can test the coil in isolation by connecting the -ve side to earth and briefly powering the +ve side. Before you do that check what voltage the coil is rated for. Modern coils are rated at 12V (full battery voltage) but older ones were rated at 6V and needed to have a ballast resister in the circuit to drop the supply voltage down. If you connect a ballasted coil across a 12V battery you'll damage it.

[neejah]

GreenV8S said:

Good stuff - much appreciated

Thanks for the reply Pete,

It's an MSD blaster 2 coil do it's rated for 12v, and having looked into it (I got the manual online) I think it does indeed work as you said - varying the -ve side of the coil.

If I tested it in isolation, would you expect that I'd see a spark if I supply +12V & Ground & then release the ground (obviously just on the HT lead rather than the dizzy)?

Excuse my ignorance, but does the coil not just sit with +ve & -ve 12V on it and then the distributor arranges where to send the spark (simplified) or does the coil need to "fire" in a similar manner to a capacitor?

Thanks again for the input - most appreciated!........I suppose you only learn by getting involved with it - I haven't had much call to deal with aftermarket ignition systems.......have the rear end out of an Alfa 75 in 30mins for you tho!......no use in this situation!

Neil

[GreenV8S]

neejah said:

If I tested it in isolation, would you expect that I'd see a spark if I supply +12V & Ground & then release the ground (obviously just on the HT lead rather than the dizzy)?

Yes. Remember that you only need to put 12V across the coil very briefly (a fraction of a second), if you leave it connect it will overheat very quickly.

neejah said:

Excuse my ignorance, but does the coil not just sit with +ve & -ve 12V on it and then the distributor arranges where to send the spark (simplified) or does the coil need to "fire" in a similar manner to a capacitor?

It doesn't work like that. Think of it like an electrical version of a slide hammer: the ignition module puts 12V across the coil (by pulling the -ve terminal down to earth) which gradually builds up the current through the low tension winding of the coil. It takes a few milliseconds for current to build up. Then it stops the current abruptly by isolating the -ve terminal. This produces a big EMF pulse inside the coil which causes an electrical spike on the HT side of the coil. The distributor connects the HT output to the appropriate plug lead so the electrical spike is routed to the plug and arcs across it.

Edited by GreenV8S on Sunday 9th August 03:35

[neejah]

Thanks again for the advice spoke to MSD tech help - who weren't very technical.

Purchased an new digi 6 - and it fired straight up, need to send the old one back for repair....probably sell the new one on to try to recoup some costs!

Best Regards

Neil