L98 injector pulse trigger question
Thread Starter
Race Director
Joined: Dec 2006
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L98 injector pulse trigger question
I don't own an L98. This is only for my own curiosity and edification.
What triggers the injector pulse on an L98?
I know that the ECM alternately grounds the harness for the batches. And when there is a ground the injector opens and the squirting begins until the ground is removed and the squirting stops.
But how does the ECM know that it is time to fire the injectors?
There is the EST reference wire from the distributor. It will let the ECM know what RPMs the engine is turning. So it seems reasonable to suspect that the ECM uses that reference signal to know when to fire the injectors. Right? Mystery solved?
But hold on a minute...
The proceedure for setting base ignition timing is to disconnect that EST wire, start the engine, turn the distributor until the desired degrees BTDC are found with a timing light and then shut down the engine, reconnect the EST wire and disconnect the battery briefly to clear the code set by the EST signal being missing.
So...
If the EST reference is the trigger for the ECM to fire the injectors then how does the ECM fire them while people set ignition timing?
And here's one I really don't understand: Why is the fix for some of the cranks but won't start threads to repair/reconnect a damaged/disconnected EST reference wire?
What triggers the injector pulse on an L98?
I know that the ECM alternately grounds the harness for the batches. And when there is a ground the injector opens and the squirting begins until the ground is removed and the squirting stops.
But how does the ECM know that it is time to fire the injectors?
There is the EST reference wire from the distributor. It will let the ECM know what RPMs the engine is turning. So it seems reasonable to suspect that the ECM uses that reference signal to know when to fire the injectors. Right? Mystery solved?
But hold on a minute...
The proceedure for setting base ignition timing is to disconnect that EST wire, start the engine, turn the distributor until the desired degrees BTDC are found with a timing light and then shut down the engine, reconnect the EST wire and disconnect the battery briefly to clear the code set by the EST signal being missing.
So...
If the EST reference is the trigger for the ECM to fire the injectors then how does the ECM fire them while people set ignition timing?
And here's one I really don't understand: Why is the fix for some of the cranks but won't start threads to repair/reconnect a damaged/disconnected EST reference wire?
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
But how does the ECM know that it is time to fire the injectors?
There is the EST reference wire from the distributor. It will let the ECM know what RPMs the engine is turning. If the EST reference is the trigger for the ECM to fire the injectors then how does the ECM fire them while people set ignition timing?
There is the EST reference wire from the distributor. It will let the ECM know what RPMs the engine is turning. If the EST reference is the trigger for the ECM to fire the injectors then how does the ECM fire them while people set ignition timing?
http://www.corvettebuyers.com/c4vettes/l98.htm
Thread Starter
Race Director
Joined: Dec 2006
Posts: 10,383
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Thanks Rodj!
You pointed me in the direction of looking up that four pin connector's pinouts.
It looks like the tan/black wire that I've seen people call the "EST" wire or the "reference wire" is actually called the bypass wire.
It seems that pin A (white) is the EST wire and that is for the ECM to tell the distributor when to fire. Pin B (purple/white) is the reference wire and that is for the distributor to tell the ECM where the cam is. Pin C is the tan/brown bypass wire and that is for the ECM to tell the distributor to accept the timing from the EST wire. Pin D (black/red) is simply ground.
So...
You solved my mystery. The ECM knows when to fire the injectors because the reference is never disconnected and the wire I thought was being called the reference is really the bypass.
Thanks again!
You pointed me in the direction of looking up that four pin connector's pinouts.
It looks like the tan/black wire that I've seen people call the "EST" wire or the "reference wire" is actually called the bypass wire.
It seems that pin A (white) is the EST wire and that is for the ECM to tell the distributor when to fire. Pin B (purple/white) is the reference wire and that is for the distributor to tell the ECM where the cam is. Pin C is the tan/brown bypass wire and that is for the ECM to tell the distributor to accept the timing from the EST wire. Pin D (black/red) is simply ground.
So...
You solved my mystery. The ECM knows when to fire the injectors because the reference is never disconnected and the wire I thought was being called the reference is really the bypass.
Thanks again!
