Injuneer

There is a table in the PCM that provides ignition advance as a function of RPM and engine load (represented by MAP). There appear to bo some tables that add or subtract small increments of advance, maybe something like IAT. The PCM gets the cam position from the optical cam position sensor in the Optispark distributor, calculates when to fire the cylinder based on the tables, and sends a pulse to the ICM that tells it to fire the plug at the correct time.

Everything is digital. No spinning weights, no vacuum modulator, nothing.

When the knock sensor (a piezo microphone) picks up knock, it sends the noise frequency signature to the knock module. The knock module is a filter, to prevent extraneous and unwanted signals from passing through. The PCM reacts to the knock signal by subtracting timing, based on the severity of the knock. The timing is pulled rapidly (attack), and then slowly returned (decay) to "no knock" levels when the threat disappears.

The PCM can be programmed to set the rate of attack and decay, and the maximum of timing retard allowed (15 degrees in a stock program). A special knock module is available, the "LT4" knock module, which was installed in the 1996 Corvette LT4 variant of the LT1 engine. It specifically filtered out the noise from the LT4’s Crane Gold full roller rocker arms. Without this filter, the PCM would react to "false knock", retarding timing. It was a popular replacement part for people who installed full roller rockers in their LT1’s.

What are you calling "the sensor ground"? The block is the knock sensor ground. The PCM to block ground is the black/white wire near the starter?

Have you looked at Shoebox's clickable 1995 harness photograph:

1995 Z28 A4 Engine Harness

Lots of 95 wiring diagrams:

4th Gen LT1 F-Body Tech Aids

Do you have the knock sensor wire running in parallel with a plug wire? That can induce voltage in the circuit.

Jank315

The ground I was calling “sensor ground” is the PCM ground in the starter harness when I started trouble shooting this it was not hooked up but I still had good ground to the PCM case and all 2 wire sensors. The KS lead is isolated right now I have it pulled out and running straight to a 3.9k ohm resistor. I was trying to eliminate any problems there might be in the lead or with the new sensor even though it tested ok. It does the same thing no matter how I have it ran. Straight to the sensor, to the resistor, or open and not hooked up at all like I found it. I understand the PCM sets the code at either low voltage or high voltage situations. I have 2.41 volts going right into the pcm when it sets the code. The voltage never changes and is dead steady. Thanks for the explanation above that’s what I was looking for. I have looked at all Shoebox’s diagrams they have been very helpful. I’ve also searched about 200 threads with DTC 43 issues and it seems everybody gets them figured out with the basic tests. That’s why this has been so frustrating everything I test is coming back in spec but still get the code. That’s the reason I tried a new PCM as well. Again thanks for all the help guys I will keep going on it and see if I can figure it out.

Injuneer

I guess I'm not following your explanations. In post #30 you said you found the knock sensor wire was cut, along with the sensor ground Was that new info, or were you simply recapping ancient history?

Jank315

I’m sorry. Yes I originally found the KS lead and pcm ground in the starter harness cut. That was not new information that was the first thing I found wrong. Didn’t mean to confuse you.

GaryDoug

While DTC43 is set by either a too high or too low voltage, the existence of the MALF43d flag indicates the reading is high. MALF43c would indicate the reading to be low.

Jank315

I really don’t where to look anymore. I think there has to be something else that is causing the code at this point but everything I’ve read says that’s not possible. I don’t see how I can isolate the circuit right to the pcm give it the correct resistance and still get the code. Again I’m getting the Malf43c as soon as I clear it it comes back within 10 seconds.

GaryDoug

If you like, you can send me either pcm with knock module and I can check it on my benchtop pcm setup. At least then I can compare the results with my known-good pcm if it fails. If it doesn't fail then the fault must be external.

Jank315

Thanks I sent you a PM.

GaryDoug

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