A little hep here, 84 720 Z24 won't start

[Still Breathin]

Hi folks, my 84 720 has been awesome for the past 12yrs. it's motor is a bit tired but still runs great when running. 2 weeks ago I started it as always to let it warm up & it ran for 4 to 5 minutes then shut off like I had turned the key off, I checked & there was no spark, I had installed an MSD 6A some 10yrs ago which has performed flawless so I called MSD explained what was going on, did a few tests & ended up buying a new digital 6A & as instructed installed & wired it exactly as the old unit. Fired right up began to warm up & after 4 to 5 minutes the darn thing shut off again,   called MSD did a few more tests & as with the old unit when checking for spark I only get spark off of 1 coil   so I installed a new coil, no change & of course it won't fire up. My distributor is quite old  ( actually wabbles while running ) so I bought a new replacement which I am waiting for now as I thought perhaps the ignition module could have messed up & I needed a new one anyway. I gained quite a bit of info from reading posts placed a long time ago but I thought I would ask present day. Not looking for pitty but I am a 64yr old Congestive Heart Failure patient on fixed income trying to fix my truck out in single digit & zero weather & on foot until I get it running as it is my only ride, I am not a quiter & I fricken love my old truck as it has been the best little 4x4  I have ever owned, it's old & beat up like me but like me it can still carry it's own weight if I can just get her running again, it has Weber 32/36 slight RV cam no emission control ( not required here ) ANY input would be GREATLY APPRECIATED ! Thanks ahead of time:)

[Logical1]

You shouldn't need the MSD box as all the spark firing is done with the ignition module in the distro. I have revived many of these trucks by re-wiring the distro and coils in a simpler setup. You should read this thread:

 

https://ratsun.net/topic/80270-spark-when-key-is-turned-on-but-no-spark-when-cranked/

[datzenmike]

The 720 has a high voltage electric ignition (EI) so using it to run an MSD seems redundant to me.  If you have a replacement distributor coming, wire it up to the stock coils and forget MSD. MSD are highly over rated over hyped and over priced for stock applications.  

 

There is also the chance that that old coil has been bad for a long time and you were running on only one set of plugs. To lose spark, could be the good coil heats up and stops working. So keep this in mind after the distributor change. Check for spark on both coils.

[Still Breathin]

Thank you both for your input & 2 out of 2  Mike & Logical1 do not like MSD !  I have run my trucks with them since the 70s but you folks obviously have far more knowledge than I do so if you say it will run well with the stock ignition I have no problem with it, I just hope after 10+yrs since I installed the original  MSD I can remember & go back to stock without major issues as when I see electrical it looks a bit greek to me, which reminds me logical1 suggested I take a look at a thread which I did indeed do, I understand most of it as the picture is very clear but I wasn't quite sure about the large red wire & there were a few other small wires down & to the left I didn't know what they were.  I am expecting my new distributor in the next day or so & when I pull the old one I will take a close look see at the wire's & another look at the picture because it looks to be pretty simple even for someone like me to do, it's a bit frustrating about the MSD as I only bought it last Wed. & it only ran for 5 minutes but I don't know if I could return it for a refund at this point. I wanted to also ask when you check power on a coil by continuity light should both sides of the coil have power ( + & - )  ( like I said not good on electrical )  I ask because I checked today with MSD in & the intake coil with key on had power on both sides but when I checked the exhaust coil it has no power either side & I wondered if the ignition unit triggers the exhaust coil ?  Anyway a BIG THANK YOU to Mike & logical1 for taking the time to respond I appreciate it much & if it's ok I will keep you posted when I get the new distributor & possibly ask a few more questions if that's ok ?  Thanks

[Logical1]

More than happy to help! I used to be a MSD fanatic with my 510 until I understood the true nature of the electronic ignition in newer (relatively) Datsuns. I had a MSD 6a wired up in my 510 with a matchbox and swore by it, removed it and it made no difference at all if not ran smoother.  I am sure I can walk you through a direct wiring of the coils/distro with more pictures. The large red wire comes out of the distro and goes to the negative side of the coil (counter intuitive). Even if you want to go back to stock wiring you could do a temporary wire up like mine to check that the distro & coils are working. On another note, I just bought some property up in Show Low, Az and am moving soon, I would be happy to stop by and help out!

[Still Breathin]

I can't tell you how much I appreciate your input & willingness to help, to be honest after reading the thread & looking at the picture it looks to me to be common sense in action even though at this point I am a bit unclear on exactly how to do it but then again I have always been one that had to do it to understand it & in most cases I don't forget so even though a bit thick headed once I understand how it's done it sinks in. Awesome to hear of your move to Show Low  that is a beautiful  area, don't know where you are coming from but if you are not use to the cold  " PACK A COAT "  trust me I am a native Arizonan 64yrs  & in that area you will enjoy a real but fairly short winter, I spent a couple of years in New England, New Hampshire Cape Cod where the winter is fricken WINTER which is why I say fairly short😷

[Still Breathin]

I spent a bit of time thinking about what both Datzenmike & Logical1 said about the MSD, so I spent yesterday pulling out the new MSD unit & talking with Holley performance who were cool about it & gave me an RMA to return the unit for a refund, when I installed the original MSD it only effected the wiring on 1 coil & the other coil stayed wired the same out of the harness. So right now I have one coil that has a black with  a white stripe on the positive from the harness &  a red on the negative from the harness, the other coil was wired to the MSD,   I have a brown wire & a  white with blue stripe wire  that was hooked to the MSD  but my shop manual doesn't indicate   which wire goes to + or -  or which is intake or exhaust so I need to find out which wire goes where. I also still don't know if when checking a coil with key on should both poles of the coil be hot as mine is with the black with white stripe & red wire from the harness ?  I got my new distributor yesterday with ignition module so I am thinking once I find out which wires go where   I am going to pull the new module & install it in my old distributor as it is in & the timing is set,  once I get the coils & their wiring figured out knowing they have juice it should fire up then I can swap the module back & install the new distributor. So input on the juice to both poles of the coil when testing & which is intake/exhaust  & which wires go where would be greatly appreciated:) I also want to understand Logical1 's way of wiring cause I don't trust my wiring harness & if need be i could wire it Logical1's way  because I am still on foot &  I like to learn new things when I can. Thanks

[datzenmike]

Ignition ON you should have 12v on the positive terminals of both coils.

 

 

Brown on the intake coil positive and Black/White on the exhaust coil positive.

 

The Brown comes directly through the ignition switch from a fusible link and is also the supply power to the ignition module inside the distributor.

The Back/White comes from a fuse in the fuse box and then through the ignition and back to the fusible link.

 

White/Blue to the intake coil negative and Red to the exhaust coil negative. Both these wire go to the ignition module inside the distributor.

[Still Breathin]

Mike thank you very much, now if you don't mind I have another question & I would still like to know if when checking a coil with a continuity light & key on should both posts have power because the 1 coil that does have power ( the 1 with  black/white stripe to +  & red to negative ) it's the new coil )  has power to both posts.  Also when you 1st answered my original post you mentioned you suspected only 1  coil was firing & only half the plugs, I took the dist. cap off to clean & pull the module & when I did of the 2 carbon contacts in the cap only the outside one looked worn and the center contact looks new, why did you suspect this ? Thanks again

[datzenmike]

Power is applied to the + terminal of the coil but if you check the other side of the coil it will read something because you are reading some of the voltage through it. The coil is just a coil of wire.

 

Because a single failed coil would not have stopped the engine because the other coil would still be working. If one side quits and the engine stoops then the other must have not been working all along. I assumed the coil but the break at the distributor cap is basically the same thing.

 

With both coils firing you will notice a BIG change in running power. Two ignition points allows a shorter burn time so the timing is run later at 5 BTDC. If one side plugs don't fire the burn time is lengthened and the correct timing would be more advanced like 10-12 degrees BTDC. As it's still at 5 and should be double that, it is effectively retarded 5 degrees and will lack power.

[Still Breathin]

WOW & I mean WOW !  I can't wait to get her going cause I have driven her that way it seems forever & to this day I have ALWAYS gotten 18mpg no matter what I did or what I was doing, on the highway in town & even when I was forced to haul a ton &  half of water & trailer  for 2yrs.  She began to smoke just a bit when cold but once warmed up clear as a bell, so I quit hauling & had it delivered besides I got tired of being pushed through intersections when it snowed cause the water & trailer were a lot heavier than me & the truck even slowing down way early once it's broke loose your just along for the ride ha ha:)   So if I can manage to get her running again you think it may well have a bit more power ?  even with the firing issues it's still the best little 4x4 I have ever had, besides I have put quite a bit in it in the last year like tires front end  alignment cv axle etc. so I don't want to have wasted the funds & besides I don't need sparkly I need functional to get from a to b & back, it's also the only ride I have so I am going to do whatever I have to.  Thank you again, I will try not to bother you folks but I have to be honest don't hold your breath cause I figure the only dumb question is the one you don't ask & I really enjoy learning from others with a vast knowledge I don't have, it's the way I have learned my whole life, ask learn  grow & share😷

[KELMO]

Wish I knew more about the 720.

I could really use a good excuse to make a trip to Williams

[Still Breathin]

Well folks " IT'S ALIVE "   can't thank you enough for all your help & input. I rewired it stock  thanks to datsunmike,  thanks again for taking the time to explain to a thickheaded old man. I also replaced the distr. cap rotor & plug wires just to be sure, she fired right up & honestly sounds better than she has in a long time. I haven't replaced the distr. yet because the old one is in & timing set already so I figured less variables at first get her fired up & then replace the distr.  although I had forgotten that the last time I replaced the oil pump I noticed the spindle shaft was bent/warped a bit so I straightened it out & the distr. didn't wabble anymore so I was a bit ify on changing it out but I think I am going to replace it just to be sure. Anyway a BIG THANK YOU to all, you guys are great !