Ignition wiring (negative coil)

[tparker]

I have been using the Hemi wiring diagram since it makes the most sense  for my car. But it says a grey wire with tracer (number 20) connects to the negative side of the coil. I don't have a grey wire in terminal 20. Looking around in another diagram I see that goes to the tack on a ralley cluster, which I wish I had but don't. The regular dash doesn't have the grey wire as expected nor does the diagram for a standard challenger. Again, this wiring diagram is different from my car so I typically don't use it.

So how does the negative side of the coil get wired up. Straight to ground? Looks like it might be grounded to the distributor. Does that sound correct?

[Dakota]

That grey wire to the coil negative post is for a tach.  If you're running a normal non-Rallye gauge cluster, you won't need or have this wire.  In my case, an aftermarket tach was added in the clock position for my non-Rallye cluster so I had to add the wire too.

Here's a source for the tach wire if you end up needing one:

https://www.classicindustries.com/product/all-years/dodge/all-models/parts/me8112.html

That negative post on the coil is used as a tach signal source for add-ons like an MSD spark box or, at least in my case, a FiTech throttle body fuel injection system.     

[Jay Bee]

Mine is like the diagram. I added the wire for the tach. Otherwise the negative doesn't connect to anything.

[DeathProofCuda]

Mine is like the diagram. I added the wire for the tach. Otherwise the negative doesn't connect to anything.

How does your coil fire if the negative side isn't connected to anything?

If you are running a points style distributor, the negative side of your coil connects to the grounding lead on your distributor.

If you are running electronic ignition, the negative side of your coil connects to a wire on the harness that plugs into your ECU.

[tparker]

Do you need an ECU with an aftermarket Electronic Ignition or only with the ones provided by mopar? I had a mallory (from the 80's) but I don't recall having an ECU.

Looking at the diagram below, the negative wire goes back to the bulk head IF you have the tach. There is a second wire and it goes to the distributor. I assumed it was to ground. If not, where would that go? So many people are running with nothing on Negative terminal? If so, that doesn't seem to make sense to me electrically. Hmmmm

@Dakota awesome. Thanks for the link. I will be adding a tach and this will be perfect.

[DeathProofCuda]

Do you need an ECU with an aftermarket Electronic Ignition or only with the ones provided by mopar? I had a mallory (from the 80's) but I don't recall having an ECU.

Looking at the diagram below, the negative wire goes back to the bulk head IF you have the tach. There is a second wire and it goes to the distributor. I assumed it was to ground. If not, where would that go? So many people are running with nothing on Negative terminal? If so, that doesn't seem to make sense to me electrically. Hmmmm

@Dakota awesome. Thanks for the link. I will be adding a tach and this will be perfect.

@tparker The wiring diagram you posted must be from 1970 or 1971, since it makes mention of the Hemi engine.  Therefore, it is for a points ignition setup.  Mopar didn't introduce electronic ignition until the 1972 models.  As I mentioned previously, with a points ignition system, the negative side of the coil connects to a grounding lead on the distributor.  If you look closely at the diagram you posted, that connection from the distributor to the negative side of the coil is shown with an unlabeled black line (see attachment).

If your car is not a 70 or 71, or if your car has been converted to electronic ignition, then this wiring diagram does not apply to your application.

[DeathProofCuda]

Some electronic ignition setups do not use an ECU, for example the Petronix conversions do not use an ECU.  I'm not familiar with old Mallory set ups.

The important question is, what ignition setup are you using now?

[Jay Bee]

If you are running electronic ignition, the negative side of your coil connects to a wire on the harness that plugs into your ECU.

My bad. You are so right. I forgot about my ECU wire and couldn't see it when the breather was in the way.

[tparker]

@DeathProofCuda Thanks! I think  I'm getting it. Part of this is educational and part is to get the car running. My challenger is a '70 with a point system that was converted to a mallory electronic distributor. I picked up another electronic system but there is no instructions. So I am just trying to figure out how the wiring for both systems operate so I can hook up the electronic one correctly and account for all the wires and parts. I think I get it now. Thanks for the info

I just finished this video which gives some good info on the 1972 electronic ignition and describe what all the parts do and how they work. https://www.youtube.com/watch?v=D6TK0pfeYl0, if anyone is interested,

[DeathProofCuda]

Good luck with it.   I don't envy you trying to figure out what you have, with no instructions, but you are right about it being an educational experience and there is probably a bunch of other good resources on the web to figure out exactly what you have.

I love those old Chrysler tech videos!  I still have a bunch of the original hard copies of the bulletins that go along with the videos.  They are some of the very few things remaining from my dad's days as a Chrysler-Plymouth service rep back in the day.