12 votlt cranking source for msd • The Dodge Challenger Message Board

by **73challengerguy** » 12 Jun 2008 17:42

can any one tell me were i tap into for 12volt cranking source to run my msd......i have it connected to my forward harness at the firewall....but i only have juice there when the key is turned on in the first position......no power when cranking it over....msd says thats why i dont have any spark........thank you ......

by **drewcrane** » 12 Jun 2008 19:04

i think i connected mine to the starter relay box stud, it has power all the time,its by the battery,with a post ,you should have room there . im not sure if every one has theres there but it works for me good luck.

by **73challengerguy** » 12 Jun 2008 20:13

but then wont it run and not shut off when the key is turned off???....wont it be on constantly?....thanks

by **dave-r** » 12 Jun 2008 20:57

On my MSD Digital Plus there are two live connections. I have not wired it up yet but I am sure I remember the instructions state you need to connect a heavy wire to a permanent source and there is another live wire that goes to a source that is live both when cranking and also when in the "run" position.

There must be an internal relay in the MSD unit that switches on the battery source from the key.

What model MSD unit is it? They have all the instruction manuals on their web site so I could have a look for you.

by **73challengerguy** » 12 Jun 2008 21:01

yes its a 6 al........pretty basic too but i cant see to find or talk to anyone that really knows where a 12v sourse i can use....i had it hooked to my origianl stock coil + wire that runs out of my forward wiring harness but when i click the key to the first position....i have juice then when i crank i lose it so i need another source.....msd said to find a constant with key cranking in the fuse block>???????????

by **dave-r** » 12 Jun 2008 21:09

You shouldn't lose the voltage from the coil + wire when cranking. How would the engine start with the stock or any ingition system?

My guess is you are using the wire that comes from the "run" position on the ignition switch. You need to use the wire from the "start" side of the ballast resistor.

by **73challengerguy** » 12 Jun 2008 21:17

well when i bought the car ( and had a old msd that plugged into my factory plug)....the ballast was and still is alimnated.....sooooo...i guess im looking for another source....i have a diagram for a 73 cuda /challenger but do not see any othe choices for constant 12v in crank position.?......u sent me the scematic by the way....could u look at it and make heads or tails out of it?....thank you for any help

by **dave-r** » 13 Jun 2008 7:35

If you have no ballast then you need the wires from each side of the ballast to go to your MSD. One wire is live when cranking and the other when in the "run" position. This is why you are losing the battery voltage at one point.

by **73challengerguy** » 13 Jun 2008 14:20

thanks dave........i appreciate it...........so i need to connect both of those wires to the one wire thats the 12volt red wire that runs into the msd?

by **dave-r** » 13 Jun 2008 14:39

Yes. If your MSD only has one 12 volt supply needed.

by **73challengerguy** » 13 Jun 2008 14:44

well theres a large red wire that goes to my pos batt side and this small red 12volt that suppose to be on a ign switched wire.....just asking if its alright to put all 3 wires together......my luck id burn it down........did u get to see my pics , i just posted some in the challenger pic section....wadda ya think?......thanks dave

by **73challengerguy** » 13 Jun 2008 14:45

i mean the one small wire from the msd and the two wires u mentioned from my ballast.( thats been removed)

by **dave-r** » 14 Jun 2008 8:53

Yes. The two wires from your ignition switch that went to the ballast resistor now both go to the small wire from the MSD. That is your switched 12v source.

by **73challengerguy** » 14 Jun 2008 13:48

did that and still nothing....................whats weird is i checked the msd and its working....they have a test u go through...and it works......so i took a test lite and probe with key on to find a sourse of 12v...then once i found a source i would crank the key.....every thing but one wire...would go out??...so i thought i found the problem.....hook a 12vot source and the one wire that gave me cranking 12 volt.....and still nothing.....whats wierd is i could probe some wires....and the test light would pulse on and off?????.....some leads were on all the time....but i only found one that had crank juice.....why would the others either turn off whencranked....or pulse????....do i have some other electrical problem?

by **dave-r** » 14 Jun 2008 15:45

Starting to sound like you have some sort of problem.

To test if it is working of not why not connect both your red wires directly to battery and see if it starts that way?

by **73challengerguy** » 14 Jun 2008 16:05

will that in no way hurt the coil or msd?

by **dave-r** » 14 Jun 2008 16:16

It could hurt the coil if you left it on long enough with the engine not running. Keep an eye on it to make sure it is not getting too hot.

Just do it long enough to see if the car will start. That way you know for sure if it is the MSD or the wiring in your car.