jwbeck17

As I am working through some challenges on my 928, I am also using it as an opportunity to learn more about my car and how cars work in general.

The 928 S4 has two Ignition Control Modules that sit at the front of the engine bay in front of the radiator on the drivers side, underneath a plastic cover. How do these two components function?

While in short, these are intended to drive the firing of the spark plugs, but would love a little more detail on how the interact and manage the spark process in the 928.

Adamant1971

My understanding is that are basically little MOSFAT amps. They feed power to the coils.

If you are replacing don’t forget the heat sink paste. I use a silver based paste from a local computer repair shop.

jwbeck17

Interesting. So if am surmising their role correctly and am speaking in EXTREMELY basic terms...

1. Power is sent to the Ignition Amps - there are two
2. The Ignition Amps send power to the Ignition Coils (each amp is connected to one coil)
3. The Ignition Coils transfer that power as an electrical charge to the distributor caps
4. The distributor caps send the charges to the spark plugs to fire following the pattern that the distributor rotor provides

Now let's got to what happens before the Ignition Amps. Howe do they get their power? I believe they get signals from the Crank Positioning Sensor, which tells them the RPM, and in turn they send timed power to the ignition coils. Do they get 12V power all the time but the CPS tells them when to send the signal to the coils?

Are the Ignition Amps truly providing the power need to the Coils to function, to do the Coils have the power connected to them directly and the Ignition Amps act as a timing switch for engaging that power?


Is that the correct way of thinking of it?

worf928

You left out the two engine control units (computers): EZK for spark control and LH for fuel control. They are the spiders at the center of the web.

jwbeck17

Correct, and those are before the ignition amps. I looked to the wiring schematic to see if I can make sense of how those are triggered, so let me know if this makes sense.

(Since this thread is focused on how the ignition amps work, I am leaving the LH and fuel control out of the convo, though they are obviously a critical part of of the process.)

• The ignition amps get a signal directly from the EZK
• The EZK gets the CPS signal to confirm that there are RPMs

So if this is true, is a constant signal sent from the EZK to the amps, and the amps set the timing of the coils, or does the EZK set the timing of the signals and the amps just "amplify" them to the coils?

76FJ55

The ignition amp actually acts on the ground side of the ignition coil. If you look at the wiring diagram you will see that the 2 ignition coils are supplied with 12V of the 15 bus (ignition on). The ignition amp receivs the signal from the computer and grounds the negative wire off the coil.
edit:The EZF/EZK sets the timing the amp is just there to work as the on-off switch, because the computer circuitry can't handle the necessary load from the coil.

worf928

This —^

jwbeck17

Got it, and it makes a lot of sense. So the EZF/EZK sends timed pulses to turn on and off the coils using the injector modules as a switches to each coil. In the end, EZF/EZK determines the spark timing.

Thanks for making that clear...very helpful!

Alan

The ignition final stages are completely dumb - they are just a remote controlled high power switch. Think of it like a relay - except it operates much much faster.

All the smarts are in the ECU's: the EZF/EZK controls spark and the LH controls fuel injection - though since some sensors apply to both spark timing and fuel they share data as needed - hence the large number of connections between them.

Alan

GregBBRD

Very concisely worded!
I would have needed a paragraph to say the same thing.

Brett Jenkins

Bump to see if I can get some info here to help me with my troubleshooting. On my 85 with S4 engine and lh/ezk, I am getting no spark. With known good lh/ezk, new ignition amps,, new CPS, and jumpering the relevant relays, I get no spark to plugs during cranking and I have verified that the coils don't get grounded. I have tried with new, verified working relays also. I have tested and verified continuity to every sensor under the hood. I have scoped the cps signal and verified it is sending information to the ecu's. I have verified continuity between the negative side of the coils and the ignition amps. I am still going through the wiring diagrams for my 85 CE panel and the S4 lh/ezk to see what needs to be switched, so this might be a trigger issue, but I wanted to understand what is going on in the ignition amps.

So, when cranking, CPS sends signal to EZK, which sends signal to the ignition amps. At that point, what happens in the amps? What should I look/test for with each pin on the connector?

Kevin in Atlanta

Is there any possibility you've swapped the CPS and the TPS harnesses?

Alan

The ground to the coils is what is actively switched so don't be surprised you didn't see it steady state (not running) - anyway its only short pulses when working correctly (you'd need an oscilloscope).

First see that you have ignition power on pins 15 of the coils (e.g. on with ignition) and also on pins 4 of the ignition amps. Check that Pins 2 of the ignition amps are grounded (to GP III).

Alan

Brett Jenkins

I had considered that, but verified it was correct when checking continuity.

928S MN

Should checking/replacing the heat sink paste on these Ignition Control Modules be a 5 year, or 10 year, maintenance item?

Could a Barn find no start car have this simple issue as the reason it was "Parked"?

Asking because I had to replace my non functioning IGM's a couple of years back. That paste was rock hard, totally shot. Took me quite a while to figure out this was the problem.