720 Ignition module testing

[StewyN]

Hey guys im back with another problem, hoping someone knows how to test the ignition control module on an '85 pickup. The reason i ask is because Ive got a bit of a misfire on my intake side. I hook a timing light to plug one and there is a noticeable skip in the blinks, hook it to the intake coil wire, its a bit harder to notice but the lag is there. Ive tried swapping the coil wires to see if the problem moved to the exhaust side but its only on the intake side. fuse #1 is good, has new wires, fairly new cap. so i believe it may be my icm. thanks for any input  

[datzenmike]

The timing light only shows when there is an electrical discharge through the wire. If for some reason the plug does not fire from excessive resistance, carbon or oil fouled, spark finds another path to ground through cap, rotor, coil wire, coil to negative terminal, carbon tracking on any of these parts, the timing light probably won't flash.

 

Swap #1 wire with another wire, see if problem moves.

 

The Z series have 8 plug wires jammed very close together on a small cap.

Make sure the area around and between cap terminals is clean of oil dirt moisture.

Make sure inside the cap is clean of dirt or moisture.

Make sure the rotor is clean of dirt or moisture. It has two coil outputs to direct to the correct plugs in there, very crowded.

 

 

 

The distributor (and module) has 4 wires. On the distributor wire harness three are together and there is a separate white wire, 4 in all. The 4th white wire when grounded tells the module to turn off the exhaust side plugs. Of the other three, one is power from the ignition switch to power the module. (also shares the power to the intake coil positive terminal, the other coil is powered from the #1 fuse) The two remaining wires are connected to the negative sides of the two coils and are switched between ground and off by the module. There is a misconception that the two coils fire separately with the exhaust later to 'burn off emissions' on the exhaust stroke. This is not true, both coils fire together unless the exhaust are shut off by the grounding of the white wire during full throttle acceleration.