How does a DIS work ?

[Malcolm Robb, LC 0112G]

Hi,
Could someone please explain the connections between a GMP4 and a Delco DIS.

I have a 6 cylinder engine with a 3 coil DIS. The DIS gets an input from a
crank sensor, which detects a notched wheel bolted to the front of the
crank. The wheel has 6 notches at 60 degrees to each other, plus a seventh
notch which indicates TDC for cylinder #1. The seventh notch is at 10
degrees to the notch for cylinder #1, which I believe (guess!) sets the
default 10 degrees of ignition advance.

But how does the ECU cause the DIS to retime the ignition based on engine
load, knock retard, etc, etc ?

There are 4 wires going between the ECU and DIS. According to the workshop
manual (WSM), these 4 are called BYPASS, EST (Electronic Spark Timing?), REF
line 'high' and REF line 'low'. The WSM gives some timing diagrams which
seem to indicate that the two REF lines are a differential pair with an
engine speed type waveform on it. However, I don't know if this signal is
going DIS->ECU or ECU->DIS.

My guess is that the REF signals go from the ECU->DIS, and that when the EST
line is active, the DIS uses the REF signal to time ignition instead of  the
crank sensor input. This would allow the ECU to 'tweak' timing to it's own
liking. But this doesn't explain the use BYPASS line.

If anyone knows how the DIS works, rather than just guessing like me, could
you please put me out of my mysery.

Cheers,
Malcolm Robb


----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org