94 Blazer Code 42 diagnositics

Trying to figure out problem with 4.3 TBI that has no off idle power and flashes code 42. I've checked the IC circuit and it reads 285 ohms and shows no voltage through the by-pass circuit. I've checked the continuity for the wire from the module connection to the ECM connection and there don't appear to be any shorts. What do I check next? Thanks for helping

Trying to figure out problem with 4.3 TBI that has no off idle power and flashes code 42. I've checked the IC circuit and it reads 285 ohms and shows no voltage through the by-pass circuit.

When the system is running strictly on the ignition module (no voltage on the bypass wire) the ignition module grounds the EST circuit to the ECM. The ECM expects to see no voltage on the EST signal line during this condition. If it sees any voltage, the ECM sets a 42 and will not go into the EST mode.

You can try this:

1. Turn Ignition on, engine off;
2. Disconnect both ECM connectors (this will not damage the ECM, but will erase stored data and codes);
3. Measure resistance on the white wire between the 4 pole connector at the distributor and the ECM, reference the ohmmeter to ground. The resistance be less than 1K ohms.

I've checked the continuity for the wire from the module connection to the ECM connection and there don't appear to be any shorts. What do I check next? Thanks for helping

Here is how it should operate:

When engine RPM adequate to enable the EST is reached (usually about 400 RPM) and bypass circuit voltage is applied, the EST circuit should no longer be grounded by the ignition module. As a result, the EST circuit voltage from the ECM to the ESC should be varying. This variation in voltage will have a direct relationship to spark advance by the ignition module.

If the EST bypass line is disconnected (open) or shorted (grounded), the ignition module will not switch to EST mode so the EST voltage will be low and Code 42 will be set.

If the EST line is grounded between the ESC and ECM, the ignition module will attempt to switch to EST mode, but because the line is grounded there will be no EST signal, thus no advance. A code 42 will be set.

If the circuit from the detonation sensor is open or grounded, will be no advance regardless of EST signal, and a code 42 will be set.

If there is no EST signal voltage on the bypass circuit, the circuit may be grounded by a short or failed distributor HEI module.

Thanks for the input and advice but still no definitive answer as to what is causing the code 42. The rig still runs fine at an idle but does't like any mid-range RPM with a load. Each time the computer is reset the Service Engine Soon light comes on within 10 seconds of starting the engine. As suggested I checked out the detonation circuit and found that it is showing a direct ground when I checked the continuity from the ECM connector to the engine. There is about 3.5 volts from the ECM to the knock sensor though. I've had the module checked twice at a local auto parts and they say it checks out OK. I'm starting to think the problem is in the ECM itself. I've had to change it out twice and replace the PROM a little over a year ago. I'm about to give up myself and take it to a shop (something I rarely do). Any last thoughts on what else I can try? Thanks again for your time.

Disconnect the knock sensor and install a 3.9K resistor in its place (lead to ground) and see if the timing stabilizes.