Ok here's what i've done so far, i checked for continuity at the red and green wires at the coil and also at the DSI connector, the meter read just over 1 ohm on both tests, i don't have an exact reading as my meter is analog not digital, when testing the blue wire it showed 12v with ignition in start position and zero volts with ignition in run position.

The module i have on there now works but it cuts out on me intermittingly then after it cools i can restart, so i know it's approaching the end of its service life, i went back to Napa to try a second module (6-wire), they too were just as puzzled as i as to why it wouldn't work, we went out to the parking lot hooked it up and just like before "no crank" nothing, i then tried cranking over the engine with no module connected at all, again nothing, if i'm not mistaken shouldn't the engine at least turn over with or without a module? i realize it won't fire but i'd think it would turn over, then we plug in my old module and she fires right up, i know the module is grounded through the distributer with the black wire but just out of habit i always make sure it's grounded to the inner fender as well though i'm not sure this grounds anything as all the inner workings are encased in epoxy, but i just wanted to rule that out as a possible cause.

So after Napa i go to my local salvage yard and find and old Motorcraft module (7-wire) in a '75 Ford, it looked to be original and was pretty grungy looking but for 5 bucks i bought it, long story short i bring it home, hook it up, truck fires right up, so as of now i'll just run that module and keep my old one under the seat as a semi-reliable spare, even though i'm up and running i'm still curious as to what role the blue wire plays and why was it deleted, something just seems amiss here.

 

Exactly right, it should crank without the module being present, or connected. It just wont start and run. The circut to the starter solenoid, from the ignition switch is faulty somewhere.

Something IS amiss here. The wiring on the truck is faulty, and is using the blue wire on the original module as a work around.

The blue wire was deleted, because the circut it served was placed, "INSIDE" the module. The circut that goes out the blue wire, goes through a ballast resistor, and hooks to the Red/Green coil wire on the truck is now inside the module on the new one.

The blue wire is the start wire and the Red/Green wire is the run wire to the coil on the old module. These are connected internally on the new module so the Red/Green wire is both start and run wire on the new module.

So in reality the blue wires circut wasn't deleted, it was placed inside the module where you can't see it.

 

I'm not sure how else to proceed or maybe just leave well enough alone, electrical issues are not exactly my strong suit, i'm competent enough but by no means an expert in all matters electrical, the original owner assured me all original wiring was intact and it looks to be so (though i realize things do go bad over time) he did swap in a '69 390 about eight years ago when the original 360 gave up the ghost but to my knowledge he retained all the original wiring during the swap and used the exsisting '75 duraspark ignition, do you know the reason behind "deleting" the external blue wire and placing it "inside" the module if it still performs the same function?

 

It's less wire to be running around the engine bay, for one thing. Personally I would go over your wiring with a diagram, because if this problem came up, there is an underlying issue somewhere. Especially if the PO has been into the wiring...you never know what was done.

 

What if you took the red wire and from the DSI wiring and moved it to it just up stream of the of the 1.3 Ohms resistor lead?

Cause the DSII wiring takes the red wire and puts it up stream of the 1.3 Ohm resistor and eliminates the blue wire that on the DSI that is hooked up down stream of the 1.3 Ohms resistor.

I could be totally wrong but that is just what I see.

 

My trucks been exceptionally reliable over the last four years that i've owned it so as long as i have access to a 7-wire module i think i will leave well enough alone and just let sleeping dogs lie.

 

I'd have to guess but, Design improvement is the most likely as FMC400 described.

Transistors, microchips and things that make larger circuts that were needed in the past, small enough to fit inside the module.

 

This has been a very interesting read.

I can only offer what I've read here recently - a member was having similar problems with his ignition. Turns out cleaning the ignition switch from 30 years of gunk solved the problem.

Since this is a simple & free fix, maybe you could look into this?

 

Ignition switch was replaced about a year ago for that very reason, after 33yrs it was pretty sloppy.

When fmc400 stated "I am told the addition of a Zener diode eliminated the need for the 7th wire" that coincides with the information i found as well "Blue lead connects to the "BAT" terminal of the coil and serves to bleed off any pulses of excessive voltage which may occur in the module, in 1976 the blue overload shunt lead is no longer required and is eliminated because of the internal improvements in the ignition module. The ignition module now has six leads".

The 1975 (green grommet) module is a one year only module, in searching online i came across this Motorcraft module with a description that caught my interest, is it implying that late year 75's DO REQUIRE the blue wire???

<TABLE border=0 cellSpacing=1 cellPadding=5 width="100%" align=center><TBODY><TR class=row1><TD class=tabletitle width="15%" noWrap align=middle>DY-166-B




</TD><TD class=tabletitle align=middle>MOTORCRAFT</TD><TD class=tabletitle align=middle>Ignition Control Module</TD><TD class=tabletitle width="7%" noWrap align=middle>$84.44</TD></TR><TR id=trid4 class=row1 align=left><TD vAlign=center colSpan=5><TABLE border=0 cellSpacing=0 cellPadding=0 height=16><TBODY><TR><TD vAlign=top width=25></TD><TD vAlign=center align=left>Ignition Module/Control Unit;Blue Wire NOT REQUIRED on Early 75 RPL MOD ; W/DIST, #D5te-12127ea,la,pa,ra,za; Years: 1975-1975;Qty Per Vehicle:1;</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>

 

Makes sense to me. Typically a Zener diode is biased in the reverse-breakdown region. That means that it regulates a known voltage, and any current that tries to drive the voltage higher is simply shorted to ground.

 

Just for clarification, The 1976, is a duraspark II, so don't get confused with it, and the 1975, and Late 1974 DSI.

Just because the 1976 DSII module has six wires, and the newer 1975 DSI module has six wires, they are different internally, and are not compatible with each other. Even though the connectors are the same. Just to ask, You didn't try and hook up a Blue Grommet DSII module did you?

As for the zener diode, this makes sence. This makes the large resistor 1.30-1.40 ohms that connects the red/green to the blue wire redundant. No way to get that large resistor inside the module. Not enough room.

The blue wire is hot in start as well on the original DSI, as tested by you. This also makes sence because there is a need of higher voltage in start mode than there is in run.

The circuts and wiring harnesses are rewired and simplified for the DSII in 1976.

 

D4AZ12A199C .. Ignition Module (Motorcraft DY157B) / Fits: 1974 only / Obsolete
-----------------------------------------------------------------------------------
D5AZ12A199C .. Ignition Module (Motorcraft DY166B) / Fits: 1975 only / Obsolete
-----------------------------------------------------------------------------------
1U2Z12A199AA .. Ignition Module (Motorcraft DY893) / 1U2Z-12A199-AA replaced D9VZ-12A199-A

Fits: 1976/79 except some CA vehicles / Available from Ford.

This module has a BLUE plastic square.
--------------------------------------
D7AZ12A199A .. Ignition Module (Motorcraft DY204) / Obsolete

Fits some 1976/79 CA vehicles ONLY.

This module has a RED plastic square.
--------------------------------------
D4PZ12A112A .. Stator, aka Pickup Coil (Motorcraft DU1A) Fits: 1974/79 ALL / Available from Ford.

 

Out of curiosity i attempted to, when at the parts store i had them pull a 76 module off the shelf (DSII blue grommet 6-wire) but unlike the 75 module the 76 module connectors are revised to reflect the reduction from seven to six leads and are not interchangeable.

 

See post #27.

 

Yeah, the modules definately are not interchangeable.

What I was worried about might have happened there.