Factory Five Racing Forum banner
1 - 8 of 8 Posts

·
Registered
Joined
·
848 Posts
Discussion Starter · #1 ·
OK - a relatively naive question regarding wiring to the coil ...

Mk4 complete kit, Ron Francis (RF) wiring harness, 5 liter engine, T-5 trans and essentially a carb'd engine (actually the new MSD "Atomic" EFI system so it's pretty much a carburated engine from a wiring perspective) and NO MSD timing box.

There are two (2) wires that I have running into the engine bay ... an orange wire labelled "efi or coil" and a light blue wire labelled "efi crank power". According to the RF supplied wiring diagram, the orange wire goes back to the fuse box and the light blue comes from the ignition switch.

My short term question: which wire should attach to the "+" terminal of the MSD coil?

My sense is the orange wire should be used for connection to the coil

And the longer term "help-me-to-learn" question is: how are they different?

Many thanks for the help ---- and helping with my continuing education!

Lynn
 

·
Official OLD GUY
Joined
·
3,877 Posts
question

Lynn,

Is the Orange wire only hot when the ignition switch is turned to on?

On older cars, there was two wires to the coil for starting purposes . . . one had a full 12 volts during cranking and the other had approximately 9 during run conditions. Even now, EFI cars have a resistor in the run circuit and a by-passed wire for starting, albeit, hidden in the wire loom to the coil.

I can't answer your color question, I'm not familiar with the harness that is supplied with the MKIV ( I have a MKII), but I would suspect that the blue wire is for starting only and the orange is for run conditions . . . best guess, sorry.

Doc :beerchug:
 

·
Registered
Joined
·
848 Posts
Discussion Starter · #3 ·
Lynn,

Is the Orange wire only hot when the ignition switch is turned to on?
Still wiring - so no battery in the car yet to check on "hotness" of either wire, nor to determine how many volts.

And no battery yet helps keep any "smoke" inside the wires in case I make any mis-que while wiring. :)

Thanks

Lynn
 

·
Registered
Joined
·
925 Posts
Use the orange wire for the coil. The blue "EFI/Crank" wire is only energized when the key is in the "Start" position.

Tim
 

·
Not a waxer
Joined
·
12,098 Posts
Tim is correct and to elaborate; for your purposes Lynn the "EFI crank" wire is unused so simply terminate and insulate it. It would come into play if you were running a Ford EEC-IV system.

Jeff
 

·
Registered
Joined
·
848 Posts
Discussion Starter · #6 ·
Thanks Tim & Jeff.

Orange wire it shall be.

Lynn
 

·
Blue Oval Scribe
Joined
·
9,175 Posts
Modern EFI systems do not limit the voltage to the coil like the old points systems. It's 12-volts during cranking. The blue EFI Start wire is used by the computer to tell it you are starting the car, which applies a different strategy (timing retard, etc.), then once the engine fires it relies on the Orange wire to power the coil and EFI computer. In the case of a carb setup you simply use the orange wire directly to the positive side of the coil.

The important thing is to be sure your coil is matched to your ignition system/distributor.

HTH...
Mark
 

·
Registered
Joined
·
848 Posts
Discussion Starter · #8 ·
Modern EFI systems do not limit the voltage to the coil like the old points systems. It's 12-volts during cranking. The blue EFI Start wire is used by the computer to tell it you are starting the car, which applies a different strategy (timing retard, etc.), then once the engine fires it relies on the Orange wire to power the coil and EFI computer. In the case of a carb setup you simply use the orange wire directly to the positive side of the coil.

Mark
Excellent! Some of the "why" along with the "which" - part of my continuing education.
Many thanks

Lynn
 
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top