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Wiring guru's: trying to figure out an MSD "limp home" mode mod
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tjet Premium Member
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PostPosted: Wed Apr 05, 2023 2:15 pm    Post subject: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

I'm working on an old baja bug with a points distributor, and I plan to install an MSD box (and I'm posting it here because you guys do a lot of vehicle rewiring).

I'm trying to figure out a limp home mode using one 4PDT switch in the event the MSD box fails. The switch has 12 lugs and 3 positions: ON-OFF-ON. The idea is to have a single switch that avoids bypassing the msd and redoing the original wiring manually.

When the switch is in "A" position, all 4 MSD switches close, and circuit "B" opens. Vise-versa when switch is in "B" position.

Take a look at the drawing below to see if I got it right.

Thanks!

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MSD wiring

http://www.914world.com/bbs2/index.php?act=Attach&type=post&id=79384
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MarkWard
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PostPosted: Wed Apr 05, 2023 3:07 pm    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode Reply with quote

Unless the quality has gone in the crapper, I wouldn’t worry about adding a point of failure. I installed an MSD on a digifant to attempt at isolating a problem with the van eating Bosch coils.

To easily revert, I used insulated male and female spade connectors. In your case, use connectors to attach your condenser wire to the white lead. If your MSD dies, unhook the white wire from the condenser wire and connect to negative side of coil. Disconnect the key on power connector to the MSD and connect to the positive terminal of the coil and you are bypassed.

One note. The MSD has constant 12 volt or 30 terminal power and ground. They are not keyed. Your diagram doesn’t show that. I’ve never had an MSD properly installed fail. I’ve also never replaced a blaster coil.

I would can the points. Just installed a Pertronix distributor in a bug from the big “A” for about the same as converting a stock Bosch distributor to electronic Pertronix. New advance diaphragm included.
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DanHoug
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PostPosted: Wed Apr 05, 2023 5:58 pm    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

just as a bit of nostalgia, the Delta Mark 10 B and C CD unit had a bypass switch built in. it was fun to engage/disengage it and hear the idle difference.

still have my old 10 C. rebuilt it with new caps and SCRs. works great. built back when manufacturers provided detailed schematics with component values. that unit introduced me to MSW, Magnetic Suppression Wires, with the spiral Monel core. high tech stuff in the late '70s.
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PostPosted: Thu Apr 06, 2023 3:23 am    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

Yeah the MSD 6T in my other car has lasted for 30 years, but this particular 6A died. I plan to send it back to MSD (Holley now) for repairs. It got me thinking about installing a switch in the cabin to bypass it, rather than messing with the wires. On my diagram, I didn't include any wires that will remain untouched, like the MSD main power & ground, and the mag pickup wires. As for using points, they last forever when paired with an MSD box, and I also like having an EMP bomb proof Baja Laughing

I do have a question on the diodes I added. I want to avoid any power or ground signals backfeeding the MSD. Are they pointing the right direction?
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PostPosted: Thu Apr 06, 2023 7:14 am    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

Can't comment on the diode direction other than you seem to know what path you want to interrupt. You will need to calculate the current to know how to properly size the diodes.

I still think, utilizing male and female insulated crimp connectors would be a more reliable way to revert back to stock. Adding diodes and a toggle switch with no relay to handle the switch load is not how I'd approach it.

As far as points, I realize the MSD takes the load off the points, but I've seen some pretty cheap ass replica points lately. I wouldn't be so concerned about them burning out as wearing out. As the rubbing block wears, your engine timing does change. Electronic ignition set it and forget it. Good luck with your project.
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PostPosted: Thu Apr 06, 2023 10:47 am    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

here's a schematic of how the Delta Mark 10 C did bypass switching. your wiring connections are nearly identical with the MSD.

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'87 Westy & '89 Westy both 2.1 4spd

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PostPosted: Thu Apr 06, 2023 2:03 pm    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

That's perfect Dan.

I'm gonna see how well this works with the aircraft 4PDT switch. If it doesn't work out, I can always revert to the wire swap method.
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PostPosted: Thu Apr 06, 2023 2:38 pm    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

a 4PDT will work, it'll just take some quiet time to get the terminals and bridge straps correct on the poles of the switch.
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60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

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www.thefixitworkshop.com
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PostPosted: Wed Apr 12, 2023 3:18 pm    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

Just wanted to post an update on this. I know it's not relevant to most people here, but I wanted to make sure it's accurate. I simplified the wiring, along with adding a "pin out" of the switch at the bottom. Note, the MSD orange and black trigger leads still go to the coil, but they are not shown.

Will let you know how it works out.

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PostPosted: Wed Apr 12, 2023 3:29 pm    Post subject: Re: Wiring guru's: trying to figure out an MSD "limp home" mode mod Reply with quote

i'm dying for you to try this!!! it's the Sudoku of wiring.
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60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com
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