ok so i hear this about timing all the time "timing is controlled by the computer once the computer takes over"
not sure to make of that so i guess my question is simple when does the ecm take over?
i was under the impression that the ecm took over the moment you turn key to run, so does the ecm already have control of timing advance durring cranking or is it only once engine is running?

also is there any advance built into the module? i belive not but again i read it both ways. if there is some , any idea how much?

just would like verification on this to be sure.. thanks

 

The ecm takes control as soon as the EST bypass wire is plugged together. I've heard people claim it adjusts according to conditions at the time, but I've always thought it is based on measured vacuum, and rpm period. There's cold start tables that modify the main spark dependent on temp measured at the CTS even upon startup.

 

The computer is always in control, unplug it and see what happens. Never heard of the advance built in theory.

 

sounds as i figured thanks

lol obviously if unplug the ecm(computer) but i'm just talking about the ignition timing part of it and specifically the cranking and startup part of the ecm controlling it from its tables.
as for the module having built in adv, there may be something to this because when est bypass is unhooked and you rev the engine it does advance as well, not sure how much as i need an adv timing light or better yet some timing tape... or i suppose i could set timing way down so its all within the tab and read it to see.

just playing and trying to truely understand the system and how it all works and handles stuff.. thanks

 

The timing is controlled by the ignition module while cranking.

The computer takes over when it thinks the engine is running. The number I've heard is 400 RPM, but I have not verified that (one of the projects on my list). The computer doesn't measure RPM directly, it goes by the time between ignition pulses and calculates the RPM from that.

If you look at electrical diagrams for the ignition system you will see a signal called BYPASS. That's the signal that the ECM sends to the ignition module to switch the timing to the computer (it bypasses the ignition module timing).

There's a diagram in my '86 FSM on page 8A-20-8 that shows the distributor and ignition module in a different way than the other diagrams. The bypass circuit is shown sorta like a relay but the switching is actually done with transistors.

 

ahh kk so mystery solved.. any idea how much the module adds durring cranking? and im assuming that when the ecm takes over that the module then adds non of its own advance and any it added is at that point null and void using only the advance the ecm is telling it?

so the advance amounts shown in the bin are the only advance after engine is running?

 

The module adds nothing during cranking. The advance during cranking is the "base advance", also known as "mechanical advance", which is the 6° that you set the timing to.

There is a parameter in the EPROM for the base advance. That parameter is set to 6° in an unmodified chip. If the base advance is set to something else then that parameter should also be set to the new advance number. The reason for this is that the ECM limits the maximum advance to 42°. If the base advance is not 6° then it's possible that the ECM could add its "normal" advance, which would be too much total advance and the engine could knock.

The ignition module DOES have an advance curve built in. If the ECM has a problem and can't control the advance then it will be done by the ignition module instead. One of the mysteries of this setup is at what RPM the ignition module advance kicks in and what that curve looks like. Normally the ignition module advance is bypassed, so it isn't a factor.

The advance amounts in the .BIN are added to the base advance to get the total advance. The ECM adds several advance factors, from tables, etc. depending on temperature and other things. Then the calculated advance is compared to 42°. The ECM uses whichever one is LOWER, which means that the maximum total advance you can get is 42°. The advance calculation is very complex and the ECM takes into account 5 or 6 factors/conditions that each add (or don't) to the advance. I vaguely recall seeing an article by Bob Rauscher on this but I have not been able to find it recently when I looked.