Injector Firing Pattern Stuff

**RobertISaar** · 06-25-2014

i know this has been dove into in a few different threads, but i think bringing as much of it as possible into one thread would be helpful.

this topic has always been somewhat confusing to me since the Ludis diagrams sometimes have cryptic descriptions for what various circuits are responsible for. the dual FMD equipped ECMs have been of particular interest, and those that are more or less derivatives of/to them. the 7749/7730 would be a good example.

after spending a lot of time looking at and theorizing what is known via diagrams and collected info, i've come to some conclusions that may explain some operating characteristics. using the 7730 diagrams as an example:

sheet 6, we see both FMDs with a lot of the ignition control stuff. interesting stuff to note on U13(FMD1):
inputs: REF, CYL, IGN
outputs: INJREF, IGNREF

REF is the reference signal coming from the ICM for cylinder position information.
CYL is the hardware cylinder select voltage formed via the dividers on the NETRES/LHM stuff
IGN is the dwell/fire signal that comes from U2(P4 Timer)
INJREF looks to give a divided REF signal based on the voltage that CYL sets, goes to the P4 Timer(and limp-home controller)
IGNREF looks to be more or less the REF signal that gets passed to the P4 Timer(and limp-home controller)

now take a look at U15. most of signals aren't used in the same manner, but some are still important:

REF can now measure and be used to report a signal that isn't the crank sensor(cam would be one example)
CYL is permanantly connected to 5V, which as per: http://www.gearhead-efi.com/Fuel-Inj...ll=1#post37369, should allow every pulse through
there is a INJREF with FMD2, labeled as CAMREF here.

moving over to sheet 7 for mostly injection stuff:

U13 and U15 again, but now the limp-home controller can come in to play. it receives U13's INJREF and IGNREF signals, so it gets the correctly divided INJREF signal from U13 and gets an engine speed signal from IGNREF. it also gets TPS, MAP and CTS signals, makes an injector pulsewidth from those along with a bunch of signals from the limp-home chip on the MEMCAL. enough about that except for INJLIMP: that is the injector pulsewidth the limp-home controller would be commanding if it were active. runs to both U13 and U15, not a seperate circuit for them, so limp-home operation with 2 injector drivers would appear to result in them firing at the same time.

U13:
INJS - injector pulsewidth signal, synchronous
INJA - injector pulsewidth signal, asynchronous

self-explanatory for those two.

U15: INJA is setup the same(and it would appear the injectors would fire an async pulse at the same time), but INJS is not.
instead, it goes through some convoluted process of logic gates, a "PWM" signal from the P4 timer(which appears to be controlled with the 3FD2 register). it also looks like OUT3 and OUT4(which aren't actually outputs) of U15 must also be set correctly. i'll have to dig around for those bits, i know i've found them before.

oddly, INJS is read by U10, which is also the VSS divisor along with some other general I/O. not really important, just something of potential interest. there are also INJLOOP that appear to be read by U13 and U15..... it's really confusing why these are needed, perhaps for DTCs that never got implimented?

anyways, that's all for the moment. some of that is theory, other parts are semi-theory. i really need a scope.