Inside an Ignition Module
02-16-2012, 05:17 AM
#1
Race Director
Thread Starter
Inside an Ignition Module
I always wondered what the inside of an ignition module looked like (physically and circuit-wise). I think this one is the original one that came from the factory in my '86 Corvette:
I used a router bit in a Dremel tool to cut away the epoxy around the lid. The epoxy seems to be very abrasive, as the router bit was square when I started. The module is filled with a clear sticky gel, which appears to be a form of silicone. The router bit tended to catch it and fling it in my face. Kinda messy:
This is what it looks like all cleaned up, with the weapons of mess destruction at the top. This is a typical high-reliability circuit from the mid eighties. The ceramic substrate is glued to the metal heat sink. The black areas are thick-film resistors that are laser trimmed by cutting tiny grooves into the resistive material to get the exact value desired. The brown parts are ceramic capacitors. The connecting wires are spot welded to pads on the substrate. There is a mystery integrated circuit in the center, probably with several op-amps inside, but knowing the way GM does these things, it could be a custom part. On the left is a very large part bonded to the heat sink. It connects to pin C, but only has that one connection (and the grounded heat sink). It has a strange looking part bonded on top with comb shaped etching on it, possibly a heat sensor of some kind. The two parts don't seem to be connected together, but that doesn't make any sense. I think the combination might be a transistor or thyristor. I suppose the next step is to attempt to figure out a schematic for this thing...
I used a router bit in a Dremel tool to cut away the epoxy around the lid. The epoxy seems to be very abrasive, as the router bit was square when I started. The module is filled with a clear sticky gel, which appears to be a form of silicone. The router bit tended to catch it and fling it in my face. Kinda messy:
This is what it looks like all cleaned up, with the weapons of mess destruction at the top. This is a typical high-reliability circuit from the mid eighties. The ceramic substrate is glued to the metal heat sink. The black areas are thick-film resistors that are laser trimmed by cutting tiny grooves into the resistive material to get the exact value desired. The brown parts are ceramic capacitors. The connecting wires are spot welded to pads on the substrate. There is a mystery integrated circuit in the center, probably with several op-amps inside, but knowing the way GM does these things, it could be a custom part. On the left is a very large part bonded to the heat sink. It connects to pin C, but only has that one connection (and the grounded heat sink). It has a strange looking part bonded on top with comb shaped etching on it, possibly a heat sensor of some kind. The two parts don't seem to be connected together, but that doesn't make any sense. I think the combination might be a transistor or thyristor. I suppose the next step is to attempt to figure out a schematic for this thing...
Last edited by Cliff Harris; 02-16-2012 at 05:36 AM.
02-16-2012, 08:21 AM
#2
Le Mans Master
Cool, that looks kind of high tech for 1986, wonder what the new one's for the 2012 Corvette look like?...WW
02-16-2012, 10:28 AM
#3
Race Director
Wow, now we know what to blame when our cars don't run right.
02-16-2012, 11:24 AM
#4
Drifting
Cliff, while I certainly respect all you do for this forum.......
You must have a lot of spare time on your hands. I've been an electrical/electronic trouble shooter since 1967 and I've never had the need to know what's inside.
I'm now retired and have some time available but I've never thought to scoop out all the potting compound and look at the internal components....even if it is interesting. Good luck with the schematic but......Why?
I know, I know, because it's there. Right.
Good luck.
You must have a lot of spare time on your hands. I've been an electrical/electronic trouble shooter since 1967 and I've never had the need to know what's inside.
I'm now retired and have some time available but I've never thought to scoop out all the potting compound and look at the internal components....even if it is interesting. Good luck with the schematic but......Why?I know, I know, because it's there. Right.
Good luck.
02-16-2012, 12:29 PM
#5
Melting Slicks
I love these 'take 'em apart and see whats in them' posts.
The component on the left looks like a power transistor, probably a bipolar from the top pattern. Back side collector, upper top connection base, and lower top connection emitter. Got a microscope handy?
The component on the left looks like a power transistor, probably a bipolar from the top pattern. Back side collector, upper top connection base, and lower top connection emitter. Got a microscope handy?
02-16-2012, 01:30 PM
#7
You know what's inside? - PFM
Pure F'n Magic.
Same thing that makes computers work.
It's broke if you let out the Magic Smoke.
FWIW I do low voltage electronics/troubleshootingfor a living too.
At least I'm not the only one who takes everything apart.
Pure F'n Magic.
Same thing that makes computers work.
It's broke if you let out the Magic Smoke.
FWIW I do low voltage electronics/troubleshootingfor a living too.
At least I'm not the only one who takes everything apart.
02-16-2012, 02:14 PM
#8
Melting Slicks
later...
I seem to recall some fab lots getting trashed because of lithium grease contamination at one of the companies I worked for.
Last edited by 3D87C4; 02-16-2012 at 02:36 PM.
glad I could clear that issue up
02-16-2012, 09:20 PM
#12
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The item on the left is the Darlington switching transistor that controls the ignition coil current. The integrated circuit in the middle does all the signal processing and logic operation. It takes the waveform from the pickup coil, squares it up, and sends it out the R terminal to the ECM for timing and RPM information. The IC also controls the coil dwell time if the computer fails, and IIRC has a built in "centrifigal advance" feature to improve drivability in the event of an ECM failure.
The 466 on the cover tells you that it's a genuine Delco part, designed in Kokomo, Indiana.
The 466 on the cover tells you that it's a genuine Delco part, designed in Kokomo, Indiana.
02-17-2012, 03:49 AM
#15
Race Director
Thread Starter
First, to answer a question/remark earlier in the thread: I'm an unemployed electronic engineer, so I've got lots of time (no money, though). I'm VERY curious about how things work (which might have something to do with why I became an engineer in the first place). When I had the fire in my engine last year I actually got out of bed at 2:00 AM and went down to the garage and tore the plenum off because I HAD to know what was happening underneath.
The power transistor appears to be a plain NPN bipolar part. I didn't pay that much attention to the junction voltages, but in retrospect I don't think it is a Darlington. I was just trying to determine what it was by probing it with my DVM in the diode mode.
This is the schematic for the ignition module inside the distributor. It has several sections that have different functions. I added the external connections to show how it fits into the overall system. The reference designators on the parts are arbitrary and just what I assigned based on their locations. The pin numbers on the IC are also arbitrary, as there are no markings on it. I showed the ground connections on the right side with a different ground symbol because the two grounds are not connected together on the ceramic substrate. They are apparently connected inside the IC, as there is only about 2 ohms difference between them.
The parts in the upper right corner handle the signal from the pickup coil. I saw the waveform somewhere. It's a very low level signal and looks like a ramp (/|/|) with VERY rounded corners. The signal is amplified by an op-amp and then a comparator decides where the trigger point is.
The signal from the pickup signal shaping circuit is used to generate a short pulse that is sent to the ECM through the REF connection. There is also a REF- signal which is connected to ground inside the distributor. The diagrams in the shop manual incorrectly show the REF- signal as connected to ground inside the ECM. It actually goes to the negative input of an op-amp. The REF signal goes to the positive input in a differential configuration. The REF signal is used by the ECM to generate the EST timing signal for the distributor with the appropriate ignition advance for the load and RPM of the engine. This signal is also used to synchronize the fuel injector pulses to the rotation of the engine.
The BYPASS signal comes from the ECM and controls the routing of the timing pulses inside the module. This signal normally causes the EST signal to be used for the ignition advance. If the ECM goes into LIMP mode (and possibly other conditions), it uses the BYPASS signal to disable the EST signal. This is the same signal that is disabled by opening the EST connector (shown in the schematic) to set the base (mechanical) timing. When the BYPASS signal is asserted or the EST connector is open the timing signal comes from the REF signal and the ignition advance from the ECM is not used.
The IGN input is 12 volts and comes from the ignition switch. It is used to power the module and also to generate the high voltage pulses that fire the spark plugs.
The TACH output goes to the tachometer on the instrument cluster which shows the RPM of the engine. It also is connected to the ignition coil, which is connected as an autotransformer, which is a transformer where the primary and secondary windings are connected together. There is a very high turns ratio between the primary and secondary windings and the 12 volt input creates approximately 50,000 volts on the HIGH_VOLTAGE output. This voltage goes to the center button in the distributor cap and through the rotor to the various spark plug wires and then to the spark plugs. The voltage has to jump two gaps on this trip, one between the rotor and the distributor cap terminals and the other gap on the spark plug. The voltage actually available at the spark plug gap is mostly determined by the pressure in the cylinder and builds up to approximately 10,000 volts (despite claims by aftermarket ignition system manufacturers) until the spark plug fires. When the gas between spark plug electrodes ionizes, the resistance (and voltage) drops to a very low value. The current is then limited by the resistance in the spark plug wires and the amount of energy in the collapsing magnetic field in the coil.
Edit: Based on remarks by 69427, I have changed the transistor to a Darlington type.
The power transistor appears to be a plain NPN bipolar part. I didn't pay that much attention to the junction voltages, but in retrospect I don't think it is a Darlington. I was just trying to determine what it was by probing it with my DVM in the diode mode.
This is the schematic for the ignition module inside the distributor. It has several sections that have different functions. I added the external connections to show how it fits into the overall system. The reference designators on the parts are arbitrary and just what I assigned based on their locations. The pin numbers on the IC are also arbitrary, as there are no markings on it. I showed the ground connections on the right side with a different ground symbol because the two grounds are not connected together on the ceramic substrate. They are apparently connected inside the IC, as there is only about 2 ohms difference between them.
The parts in the upper right corner handle the signal from the pickup coil. I saw the waveform somewhere. It's a very low level signal and looks like a ramp (/|/|) with VERY rounded corners. The signal is amplified by an op-amp and then a comparator decides where the trigger point is.
The signal from the pickup signal shaping circuit is used to generate a short pulse that is sent to the ECM through the REF connection. There is also a REF- signal which is connected to ground inside the distributor. The diagrams in the shop manual incorrectly show the REF- signal as connected to ground inside the ECM. It actually goes to the negative input of an op-amp. The REF signal goes to the positive input in a differential configuration. The REF signal is used by the ECM to generate the EST timing signal for the distributor with the appropriate ignition advance for the load and RPM of the engine. This signal is also used to synchronize the fuel injector pulses to the rotation of the engine.
The BYPASS signal comes from the ECM and controls the routing of the timing pulses inside the module. This signal normally causes the EST signal to be used for the ignition advance. If the ECM goes into LIMP mode (and possibly other conditions), it uses the BYPASS signal to disable the EST signal. This is the same signal that is disabled by opening the EST connector (shown in the schematic) to set the base (mechanical) timing. When the BYPASS signal is asserted or the EST connector is open the timing signal comes from the REF signal and the ignition advance from the ECM is not used.
The IGN input is 12 volts and comes from the ignition switch. It is used to power the module and also to generate the high voltage pulses that fire the spark plugs.
The TACH output goes to the tachometer on the instrument cluster which shows the RPM of the engine. It also is connected to the ignition coil, which is connected as an autotransformer, which is a transformer where the primary and secondary windings are connected together. There is a very high turns ratio between the primary and secondary windings and the 12 volt input creates approximately 50,000 volts on the HIGH_VOLTAGE output. This voltage goes to the center button in the distributor cap and through the rotor to the various spark plug wires and then to the spark plugs. The voltage has to jump two gaps on this trip, one between the rotor and the distributor cap terminals and the other gap on the spark plug. The voltage actually available at the spark plug gap is mostly determined by the pressure in the cylinder and builds up to approximately 10,000 volts (despite claims by aftermarket ignition system manufacturers) until the spark plug fires. When the gas between spark plug electrodes ionizes, the resistance (and voltage) drops to a very low value. The current is then limited by the resistance in the spark plug wires and the amount of energy in the collapsing magnetic field in the coil.
Edit: Based on remarks by 69427, I have changed the transistor to a Darlington type.
Last edited by Cliff Harris; 02-19-2012 at 01:53 AM. Reason: Updated schematic per 69427.
02-17-2012, 10:20 AM
#16
Did you know that you can glue one of those to a inner fender, run wires from and to it, and run 'any' module based pointless ignition system and a few with points. Good to know if you're out in the middle of nowhere and cant get parts. Ford guys use them a bunch, at least I did, cheaper and more reliable than the ford system. Has to go to a switched 12volts though, the ford resistor wire wont let it get the full 12v. Hope this helps
Last edited by oldalaskaman; 02-17-2012 at 10:23 AM.
02-17-2012, 02:14 PM
#17
Race Director
Thread Starter
Anybody know what the yellow dot is for?
I heard that the pickup coil and module need to be compatible. Is that the meaning of the dot color?
I heard that the pickup coil and module need to be compatible. Is that the meaning of the dot color?
02-17-2012, 02:59 PM
#18
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I have a couple technical disagreements with your previous post.
02-17-2012, 03:02 PM
#19
they dont need to be compatable, if you want to get real tech bout chevy modules, go to the ford sites and even some of the toyota sites. with your electrical back ground , that shouldnt even be a question, 12volts is 12 volts, these things will work in volvos too.
