Why does the trailing coil need to be hooked up for the ignitor to fire(and therefore give an rpm signal)?

If the coil is unplugged the tach goes dead. Why?

 

The tach signal can come from either coil. In fact for the SA it comes from the leading because
the trailing is turned on and off in stock operation to help with emissions.

The ignitor firing triggers the coil, not the reverse. Its that trigger signal thats used basically for
the tach.

 

Unfortunately the tach signal wire is buried down in the harness somewhere so you'll have to do some digging if you want to separate and hook it to the leading coil.

Are you trying to eliminate trailing for some reason? Or just curious about these interesting cars?

 

Yeah, so hard... you switch the connectors at the distributor and then switch the coil wires.

I think putting the tach on trailing is a good thing. It lets you know that your trailing is dead. When the leading dies, it's pretty obvious, but you can drive perfectly well with no trailing.

 

If you pay attention, you can hear when trailing is firing. It's subtle but noticeable with experience. SA guys can hear it kick in and out as it's switched.
Smog guys, on the other hand, insist on clamping the timing lead to the top(trailing) plug wire, then can't understand why it cuts out under early acceleration.

 

I about spit up my coffee on that one!

 

Curious...and wanting to have an ignition cut rev limit.

I have a Haltech F9 (fuel only) on a turbo'd n/a. It needs the rpm signal to fire the injectors and what not. So, I need to cut the spark, but continue to have an RPM signal.

 

the FC has the same problem, they test if the gas tank holds vacuum, and they do this by putting a vacuum fitting in place of the gas cap, and then putting a clamp on the tank vent hose right next to the charcoal canister.

there are two hoses there, a big one and a small one, and i guess every other car has the big one being the tank vent, the FC uses the small one (FB does too).

so when the smog guy goes to put the pliers on the wrong hose, they are surprised when i yell at em....

 

where are you getting the RPM signal from? if its the - side of the coil, you can't do an ignition fuel cut.

if you're using the pickup in the distributor i think you can.

however ALL the OEM's use a fuel cut, so you should think about just doing that.

 

And on the SA, the canister is inside the air cleaner lid & only has one external hose, which leaves them at a complete loss trying to find it, even though there's a detailed diagram staring them in the face inside the hood.

I've had to lead several by the hand.

I even colored my tank vent line on the firewall red so it's easy to find.

 

Cutting ignition but not fuel on a WOT rotary would be a good way to eject your exhaust system, I would think... BOOM!

 

I attached how I currently have things hooked up. The "tach signal" is where I am running the tach and getting a signal to the haltech.
The msd is running off a gutted ignitor. The leading is direct fire and the trailing is one 2nd gen trailing coil firing through the dizzy.

 

How does that 2G trailing coil know which plug to fire? Just curious.

 

it's going through the dizzy cap. It's only ONE coil not the whole 2nd gen trailing assembly.

 

you can't do ignition cut the way you have it, so just do fuel cut.

 

Not really, no. I have an ignition cut and it mostly does nothing.

What I have on my car is a Megasquirt and an MSD. The MSD reads from the leading pickup and the MSD's tach signal gives the Megasquirt its tach signal. MSD drives both ignition systems, I run with no split. (It's actually an FC trailing coil assembly, with one of the trailing coils replaced with a leading coil. Leading is direct fire, trailing goes through the distributor)

The trailing pickup and ignitor go to a little thingy I made where the original trailing coil leads attach to it and a small resistor (1k ohm?) connects the two. This gives the wire going to the tach the alternating power and ground that it needs.

You can sort of see what is going on here: