Wiring an MSD6AL

Paul Bearman · 11-09-04, 02:13 PM

I recently managed to 'blow up' my MSD unit when the engine continued to run even though the ignition switch had been turned off.(Read previous post on MSD).
Before I install the new unit I would like to know if it is possible to fit a battery master cut out switch in the main feed (large red wire) to the MSD from the battery? I have conflicting information, as some say that no switch or fuse should be installed in this cable.

Paul Thompson 'Hooligan' · 11-09-04, 06:12 PM

Hi Paul

It really should not be required. Removing power (+12v) from the 'control wire' (thin red) will turn off the MSD anyway. The problem is that if the thin red is attached to additional ccts as it often would be, a reverse feed situation (eg fan cct energised) can allow enough of a voltage level back to the ign cct to keep the MSD turned on.

One thing though - the way that most main power killl switches are wired would do exactly what you are suggesting anyway. i.e. - break ALL +12V available from battery/alternator and hence stop the car.

Hope this makes sense [img]/ubbthreads/images/graemlins/confused.gif[/img] it did when I started typing but now i'm not sure it is clear..... [img]/ubbthreads/images/graemlins/confused.gif[/img]

Howard Jones · 11-09-04, 10:45 PM

Here's how my car's main battery switch is wired. The pos. lead comming from the battery goes first to the main battery switch then on to the starter solenoid where the power is branched off for the rest of the car.

If you turn off the main battery switch the battery +12v is not attached to ANYTHING. Also I used the type that had a aux.set of contacts that open the +12v line to the alt.

My big red lead to the MSD box gets its power from the switched side of the main power switch. That is switch on power on to MSD box. The little orange wire gets its +12V from the key switch, on posistion.

If you really want to make sure the MDS turns off run the orange lead to its own switch on the dashboard and then on to the key switch. Then you can turn off the MSD by itself.

Julian West · 11-10-04, 06:14 AM

Where are you guys running the feed from the Alternator?? I intend to wire the alternator direct to the battery, & then the battery feed as per Howards setup.

I used this setup for many years on my race car. the scrutes' would often check the operation of the kill switch.

If there is concern over reverse feeding the ignition unit, why not wire it via a relay??

Paul Thompson 'Hooligan' · 11-10-04, 12:37 PM

Howard -

For 6AL I think you mean thin red - not orange, the orange and black as a pair go to the coil and have 'lots' of volts on them - pulsed.. [img]/ubbthreads/images/graemlins/smile.gif[/img]

Howard Jones · 11-10-04, 07:26 PM

Paul you are correct, I was working from memory and my car isn't at home right now. I will add that I ran my MSD ground directly to the neg. post of the battery.

Iron Sheik · 11-10-04, 09:12 PM

A Diode solved this problem in my MSD 6. They are usually supplied with all the accesories in the unit.