Got a code 42 + 43 on a 91 Z28 with L98. Anybody know where electronic spark module is on this car? Not the distributor control, the module for the knock sensors. I checked firewall, its not there.

On a 90-92 speed density f-body, the spark module is contained on the MEMCAL on the chip inside the computer. A code 42 will set when you disconnect the timing sensor to set the initial timing. A code 43 will set if there is a problem with the knock sensor. Are these codes coming on all the time?
A board for Code 42 or Code 43 will reveal more info.

The code 42 is probably just from setting timing. But the 43 is kind of intermittent, but seems to happen when car is hot. I took the knock sensor out, and put in foam, and then grounded it to the battery. When cruising, seems to be ok, but when trying to accel, timing retards up to 20 deg., and car stumbles. I didn't look underneath yet, but schematic from Hayne's book shows two knock sensors. Not sure if thats true or not. It doesn't seem to always happen during a certain condition, so its hard to pinpoint what sets it off. Any help would be appreciated. I guess I can try ohm testing the wire, but I dont have a good schematic to go by. I work at a dealership, but we don't have All-Data, just GM, and that only goes back to 97. Thanks

I checked the voltage at the blue wire for the knock sensor, and to battery ground and had 2.4 - 2.5 volts. I think I read somewhere it should be 5 volts. Maybe someone has more info. Thanks

That's 2.5, backprobing the connector.

There needs to be 3.9K resistance going to ground if you want to fool the ECM that there is no knock. (for a 350 motor)
Leaving the wire disconnected or Grounding the wire will not work correctly.

You may have a melted wire to the KS that is intermittantly touching somewhere, check the wire carefully to see.
HTH

Thanks for the reply. The knock sensor is still connected, its just packed in foam, and insulation. I then took a ground strap and attached it the sensor and ran the ground to the battery. Doesn't the sensor offer the resistance itself? I will try and trace the wire, that could be the issue to, or run a new wire to the pcm. Know where i can get a schematic of pcm connectors? Thanks

Not trying to be off topic but where is the timing sensor at??? I am trying to set the timing on my 91 z and im having a little trouble.

If you look on the passenger side of the engine compartment near the strut tower there should be a brownish orange wire with a connector. You have to unplug the connector to set timing. Once your done, reconnect it, and disconnect battery for 30 sec. Then you should be good.

Did you ever solve this problem? It sounds very similar to what I am experiencing with my 87 LB9.

Thanks,

I am also interested in the outcome of this. I posted a very similar problem in the TPI section and really didn't get much help. I believe the problem is with the knock sensor but everything seems to have checked out but I have not changed the module in the distributor but I have changed the knock sensor without any change. I am considering using a resistor in place of the knock sensor to see if there is any improvement but haven't done so yet. Regardless the car still has periods where the timing changes and runs like it timing is very retarded. This is even verified with a timing light. I am running a speed density system.