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Old 05-31-2004, 09:13 PM   #1
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GM IGNITION MODULE - Technical Discussion: Info not covered in the Service Manual

I have a couple specific technical questions about the 7 & 8 pin ignition modules used on TPI and TBI cars.

Here is shematic photo of the module, that I found, indicating the waveforms at the module:

http://www.slantsix.org/articles/dib...odule-schm.jpg

Let's start with what I think I know about the operation of these modules:
1) The reluctor style pickup in the distributer sends a spiked AC signal across terminals P & N of the module. approx +/- 3V?
-see: http://jaguar.professional.org/ignition/

2) The module converts this AC signal to a square wave, which outputs to the ECM via the "R" Reference Terminal.

3) When the ECM is controlling ignition advance, it sends a 5V? signal to the "B" bypass terminal of the Ignition Module.

4) The ECM then controls ignition advance by varying the pulse width duty cycle of the Square wave signal that it sends to Ignition Module terminal "E" EST.


My questions are as follows:

A) In regard the the SQ wave reference signal that the module sends to the ECM: Is it the leading or trailing edge of the signal that is the trigger point?

B) Does the EST signal Directly trigger the module, or does the module just measure the duty cycle of the EST signal and use that figure to calculate the required advance from the reluctor trigger point? If the EST directly triggers the firing of the module, is it on the leading or trailing edge of the variable duty cycle square wave?
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Old 06-01-2004, 11:19 AM   #2
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FWIW - As far as I have been able to tell, the front half of a 7-pin HEI module resembles the
National Semi LM 815 chip. The 'ref output' on pin 12 in the diagram is an open-collector switch.
Search their site for the application docs. Note the 'Input Select' gate on pin 11 - that determines
whether the HEI module is in local or ECM control of the output pulse.

The back half of the HEI module is only a power transistor. All the timing/dwell calcs seem to be
done in the ECM, which controls the power transistor when the ECM is in 'ESC' mode (via pin 11).

HTH
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File Type: jpg 1815.jpg (35.1 KB, 913 views)
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Old 06-01-2004, 09:28 PM   #3
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Great Info !!! Thanks! I'll have to take some time to digest the Datasheet. Here is the data sheet link for the LM1815:

http://www.national.com/ds/LM/LM1815.pdf

Last edited by formulaman; 06-01-2004 at 09:39 PM.
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Old 06-02-2004, 11:07 AM   #4
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You're welcome. I tried to figure out the 7-pin modules for a while; they only began to make sense after I saw
that 1815 schematic. BTW, I couldn't open all of your references above - the one for "fairtrade(?)" wants a fee to
look at a vendor .pdf(!) so I skipped it.

Also note - the back end of an HEI module is more than just a power transistor of course - though you didn't ask
about it, it is actually some kind of current-limited darlington, with zener protection AFAIK. Never took one
apart to look inside, but I believe it contains something similar to the back end of the 4-pin HEI module, shown in
this Motorola (4-pin) diagram:
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Old 06-02-2004, 11:07 AM
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