On the factory tach, where does it get its signal from?

The leading coil? Or the Alternator? Or CAS?


The reason I ask is we just got a 13B carb SP 89 engine, they have used both Trailing coils to run the distributor, the tach does not work any more.


Also would it be better to use the leading coil, VS the trailing coils?

Thanks...

 

Trailing coil is the stock location ive seen people use the leading coil though. They seem to both work but then again ive seen cars where they were 300rpms off..dunno if the aftermarket tach was accurate or the leading coil is off.

 

Do you guys know if the trailing and leading coil put out the same amount of voltage?

 

The summed signal from trailing coil is more accurate, that is why mazda uses it for anything needing an engine speed signal, such as the tach in the cluster, the Power steering, the factory diagnosic connector, ETC.

 

So, would you recommend to use the Trailing Coil Pack when you dyno tune as opposed to the Leading?

 

Yep, thats a big 10-4 good buddy.

The ECU will advance and retard the leading coil as needed for idle and throttle position.

But the trailing coild fire based on rotor (well technically crank) position, and are used for increased emissions control rather than power.

 

Okay, I copy. But what would be expected with the Leading? Misread results due to constant advancing/retarding by the ECU?

 

if you connect to the leading coil the RPM reading will be 2x higher.

 

Absolutely right. When dynoing, you set It at:

Leading - 8 Cylinder
Trailing - 4 Cylinder

I'm just concerned with Icemark saying how the Trailing Is more accurate.

 

Whats the difference other that the leading and trailing?

 

The difference Is Ignition timing b'n the 2 Coil Packs.
This Is beautifully Illustrated showing them In collaboration. http://www.rotaryengineillustrated.com/

 

Here's a picture of all four signals going to/from the trailing coil pack:



Note that the green tacho pulse outputs every 180 and is the same 'width' as the purple IGf-T signal, the confirmation pulse.

The yellow IGs-T is the 'select' signal which tells the ignitor which of the two coils to fire ( they alternate back and forth ).

The blue IGt-T signal is the dwell and timing information which is sent every 180 degrees. The select signal sends it to the right coil (T1 or T2) to produce a spark every 360 on the front or rear rotor directly at or after the leading fires.

My S5 T2 stock computer fires 15 deg of split under boost and at idle but goes to zero split during most cruise conditions.

You might not get the tach to work properly if it's connected directly to IGt-L ( leading ignitor input, not pictured but EXACTLY the same shape as IGt-T, just offset by 'rotary-split' ) or IGt-T ( trailing ignitor input, blue trace above ) as these are 0 to 5v logic level signals. The tach may very well 'pull' these signals up or down so that the coil no longer fires. It would be a fatal problem for the coil if the tacho pulls it up, this = infinite dwell = melted coils & smoked ignitor.

As you can see the green tacho signal is a 0 to battery voltage pulse.

The 'accuracy' of any signal going to/from the coils is the same and not related to any programming functions.

If you break the IGf-T line going to the ECU it will shut down the trailing system as well as the tacho. You also get the check engine light and code #1

If you break the IGt-T line the ignitor will go into 'failure' mode and stop sending the IGf-T signal. This results in no tach, a check engine light, and code #1

If this is for a carb'ed car, there is no ECU to run the leading or trailing so I imagine you've gone to some kind of aftermarket igntion system or a gen 1 distributor with two coils.

In this case you need to find an adapter that can pick up the pulse from the negative side of the coil and deliver a battery voltage magnitude square-wave that is at least 1.6mSec long twice per rev.

 

Hi; is that image of the signals still somewhere floating? Thanks Ruben