With the use of 1984 corvette manual and having received code 42, says to disconnect the HEI 4 way connector and perform some simple tasks. Well for anyone having ever done this probably knows that this connector is down behind the distributor is extremly difficult to access specially the art of re-connection, as one needs a hand down behind dist and between engine and firewall which is not going to be an easy task. To me, it was just as easy to buy a new HEI module till i found out it was $76 and would be an expensive proposition if found it wasn't the problem.

The car runs rough at start up and after warm up seems to operate much better and engine starts to accelerates smother. I just wanted to throw it out there to you guys if there was other things i could check that would prove the HEI was the problem or other checks i could do to fix this code.

Best regards, Dave...

 

Typically a HEI module either works or it don't work and that means that it either runs or it doesn't. However you have a EST circuit problem which can be the module or a connection at that 4 way connector or it could even be the ECM. Mainly because the EST circuit includes the ECM as well. I have seen those older cars with a 4 way connector that is rotted causing a failure that you describe. As bad as it maybe to get to it back there, it is in your best interest to check it out as per the diagnostic tree.

 

I just went thru the diagnostic thing involving that 4 way connector- NO WAY for me. I ended up pulling this dist. unfortunately when I reinstalled the dist. I clamped it down on the connection-crimped a wire really bad, thru a code 42, saw what I had done-a new 4 way connector cost $95. Sooooo, becareful, I know no one else would do something as stupid as I did-but

 

I finally ended up, after disconnecting that 4 way connector behind the distributor, re-routing the chassis end under and behind the large loom which gave me extra inches to complete the tests. Manual says connect "B" and "D" together and check voltage at "A" chassis end. There was no way it would start without "C" being connected also so am suggesting an error in the instructions. Recorded 2 volts AC and proceeded to connect "A" as instructed and manual mentions looking for 0.7 volts AC and recorded nothing (Zero) so instructs then to replace the HEI module. At least I'm more confident now on dishing out the $76. Thanks for your replies, its great to have this forum to discuss our findings. Dave...

 

Dave I have a more recent GM HEI diagnostic manual to test the old EFI ignition systems.

Step 1:
Clear codes, start engine, let it idle for 1 minute. Does the Check engine lamp return and does it set code 42? If not, then the code is intermittent if it does then:

Step 2:
Ignition off then, disconnect the ECM connectors, then with the key in the run position, with a ohm meter set to read to at least 2000 ohms. Probe circuit 423 which is the white wire (EST Circuit) and the other test lead to a known good ground, it should read less than 1000 ohms.

If the reading is more than 2000 ohms then its a faulty connection or a bad ignition moduel.

If it reads less than 1000 ohms then:

With a test light connected to battery positive (12v) probe circuit 424 which is the tan/blk (bypass circuit) wire. The test light should be off. If off proceed to step 3.

If the test light is on, then disconnect the 4 way connector at the distributor, if it remains on then circuit 424 is shorted to ground somewhere between the ECM and that 4 way connector, if the light goes off then the ignition module is shorted.

Step 3:
If the test light was off, then with the ohmmeter still connected to circuit 423 and ground, again probe circuit 424 with the test light connected to battery positive, observe the ohm meter as you probe circuit 424, when it is probed the ohm meter should go to above 2000 ohms with the test light connected to circuit 424 (tan/blk wire).

If it does, then reconnect the ECM start the engine, let it idle for 1 minute or until code 42 sets, if the code sets then its a faulty ECM, if it does not then the code is intermittent.

If the ohm meter does not switch to over 2000 ohms proceed to step 4:

Step 4:
Disconnect the 4 way connector, with the ohm meter still connected to circuit 423, observe it as you disconnect the 4 way connector, the meter should read OL or high indicating a open circuit. If it doesn then you have a faulty ignition module, if it does not, then circuit 423 is shorted to ground.

This test procedure was valid for all TBI engines in cars and trucks up to at least 1993.

 

Thanks for the post tjwong, helped by getting me into that 4 way socket.
Removed the ecm's plugs and ended up checking continuity between the ecm and the actual hei module. Found 424 (bypass) open circuit and is a Tan/Black color wire. Checked the harness the best I can for breaks and drawing shows a connector or something in that circuit only and not the others. By total accident, saw a connector just inboard and beside the hydraulic brake cylinder with two tan/black wires. Found over the twenty years has developed a fault connection and after checking continuity and re-connecting solved the problem. Operating the car at approx. 1800 rpm and shorting the A & B, the rpm now increases as per the manual and now no engine check light. Will not hurt for those that read this to make not of this for future reference.