86 iroc, 305 tpi full rebuild on motor and been working through some problems. Motor is finally running but there is a code 42 and the snap on shows no spark advance at all. New cap and rotor, coil but didn't replace the module. I know it was working but I went to set timing and plugged timing wire back in before I shut the motor off and that seemed to be where cel light came on. Any pointers would be greatly appreciated.
Thanks

 

Have you verified spark advance with the est connecter plugged back in ?
If you have iirc icm control wire is white at the icm this maybe the issue.
42 is triggered when the est is disconnected.

 

Yes, after realizing I ran the procedure wrong I attempted to reset and do over. With the wire reconnected it still won't advance when revving and makes no change whether plugged in or unplugged. The tach also shows way higher rpm than it is...when the cars at 800 rpm the tach reads 2200, I'm wondering if the ignition module could be the culprit. Also the scanner shows the MAT sensor reading at 392...but there's no code listing for that ? Sorry for all the queries, this thing has just been fired after 8 years in someone's driveway...after these hurdles I'm onto bodywork.
Thanks

 

I would ohm out the white wire from the icm to pin d4 of the ecm.
This should be for advance the Blm wire ecm b7 if for retard.
If the white wire is ok I'd say bad icm.
I there is a mat issue could be from a unplugged mat the lower the resistance the higher the reading. Mat code 23 is for a high voltage, no code for too high temperature. The mat should be 25k ohms or less.

 

Thanks a ton for the info, I'll check those wires. Would the tach reading run through the icm as well ? I'm just wondering if the advance issue could be tied together ? Most of what I've read says when the icm croaks it won't run at all and I've replaced everything else in that dist. I've had that plenum off several times and the plug for that sensor isn't in the best shape but it does plug in and stay on fine...I'll really hate pulling that thing off again.

 

You can test the mat at the pigtail connecter. 20c is 68f just to get you a idea it should be around 3.5k resistance.
Essentially the tach gets a signal from the icm but the phisical connection is at the coil.

 

Well I think I found the culprit, the white wire checked out ok but upon inspection I realized I had a spark plug wire half melted into the exhaust manifold and arcing out...I'm sure that cooked the icm and that is probably my no advance and tach issue. I'll reroute/replace plug wires and replace the icm and see what she does. I think I'm going to rework the iat sensor location to the air box up front so I don't have to pull the plenum again..it just seems to be a better setup anyway.

 

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