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Old 02-09-2009, 09:04 PM
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chevy HEI Basics

I'm trying to wrap my head around the ignition system on my 88 silverado.
I understand the obd2 systems much more.
What is the function of the ignition module. My hanes explaines
the spark timing is controlled by the ECM ( separate from the ignition module )
If I were to re-install the distributor after a repair off by a tooth or two on
the driven gear what would be the effect? Would I need to move the cap
around to compensate. Does moving the cap effect the ignition timing?
And if so , how? I guess I need a lesson on the fundamental functions
of the system.
Thanks Hotrodders!
Brian

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Old 02-10-2009, 10:24 AM
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Quote:
Originally Posted by briansansone
I'm trying to wrap my head around the ignition system on my 88 silverado.
I understand the obd2 systems much more.
What is the function of the ignition module. My hanes explaines
the spark timing is controlled by the ECM ( separate from the ignition module )
If I were to re-install the distributor after a repair off by a tooth or two on
the driven gear what would be the effect? Would I need to move the cap
around to compensate. Does moving the cap effect the ignition timing?
And if so , how? I guess I need a lesson on the fundamental functions
of the system.
Thanks Hotrodders!
Brian

The 88 is pretty simple compared to OBD II systems. The engine module isn't hunting for number one. If the distributor is installed off a tooth and you compensate by rotating the housing, the whole thing will run just fine.

Bogie
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Old 02-10-2009, 10:48 AM
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The ECM advances the spark by x degrees. It does not monitor timing and do the work for you, it assumes that you have properly set the initial advance. It assumes you have put it at the proper initial, so that when the computer reads 42% throttle, 3000 rpms, and 4" of vacuum from the map, it responds by spitting out "10 degrees of advance". If you have properly put it on the correct initial, it will be right.

Installing a distributor on the right tooth is not an issue with HEIs or almost any electronic ignition. It was a problem with points. But you can stab it in any way you want, then just rotate it to bring the #1 wire to the right spot.
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Old 02-10-2009, 01:23 PM
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thanks guys!

Does the ignition module function soley as a on/off
relay, while the ecm signals the module to advance according rpm/map/etc?
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Old 02-10-2009, 03:00 PM
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Yes. The ignition module functions like any other ignition module. It generates the switching for the coil. The advance is strictly controlled by the ecm
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Old 02-11-2009, 10:56 AM
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Thanks again!
last question, i promise.
How does the ECM control the advance? By controlling the ig module switching, or by physically advancing the rotor?
Brian
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Old 02-11-2009, 11:07 AM
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Quote:
Originally Posted by briansansone
Thanks again!
last question, i promise.
How does the ECM control the advance? By controlling the ig module switching, or by physically advancing the rotor?
Brian
All done electronically to control the switch point of the power transistors that operate the coil. There is a map on a PROM (Programmable Read Only Memory) chip. It takes sensor data from the throttle position and manifold absolute pressure sensors and engine RPM using this it determines the engine's power demand and selects an advance value from the map. One needs to realize the computer isn't predicting the future, this data is actually running a few nano-seconds behind what the engine is actually doing but the computer is constantly updating at a rate of several times a second.

Bogie
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