OBDI Code 42 on my 1991 Suburban

Discussion in 'GM Powertrain' started by Crawdaddy, Dec 27, 2011.

  1. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    Over the past couple of days I've pulled the Suburban out and been driving it a lot. It has been running fine. Now, when I go for a drive, the first 30 miles or so of driving is good, but once I get off the interstate or slow down a lot, the check engine light comes on. The engine doesn't seem to be running rough or complaining, so I just let it be.

    I checked the code, and it's OBDI code 42 which my Haynes book says is the Electronic Spark Timing issue. Trick is, the manual keeps referring to the HEI ignition setup, which I don't have. I have the normal electronic ignition system. So, I'm not quite sure where to start. A code 43 references an Electronic Spark Control module, which I think I do have, but it's not throwing this code. Any idea where to start looking? I'm thinking about checking out the ignition module under the rotor in the distributor, but until this weekend I'm not in a position to check it out. Any ideas?

    99'HEARTBEAT MODERATOR Staff Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor

    Christoper....here's is some information, that may be able to help??, regarding Trouble Codes 42 and 43.......

    CODE 42
    Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system. During cranking, the timing is controlled by the ignition module and the ECM grounds the EST line. It expects to see no activity on this line at this time. When the ECM enters EST mode, it applies +5 volts to the BYPASS line and expects to see voltage variations on the EST line.
    Reasons for the ECM to set Code 42:
    ? System in BYPASS mode (i.e. EST line supposedly grounded) but activity sensed on EST line
    ? System in EST mode (i.e. BYPASS line driven with +5 volts) but no activity seen on the EST line

    Possible causes include:
    1. BYPASS line is open or grounded
    2. EST line is open or grounded
    3. Poor connections between ignition module and ECM
    4. Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise
    5. Faulty or incorrect ignition module
    6. Faulty ECM

    CODE 43
    Trouble Code 43 indicates that there may be a malfunction in the Electronic Spark Control (ESC) circuit. ESC is used to sense spark knock (pinging) and retard the timing to eliminate it. The knock sensor is located at the rear of the engine block. The ECM will retard the timing by as much as 20 degrees in 1 degree increments. A loss of knock sensor signal or loss of ground at the ESC module will cause the signal at the ECM to remain high. The ECM will act as if no knock is present, and
    may possibly result in engine damage, due to detonation.
    Code 43 is set when:
    ? Voltage at Knock Sensor is above 4.8 volts or below .64 volts.
    ? Either condition is met for about 10 seconds.

    Possible causes:
    1. Open or shorted knock sensor
    2. Loose knock sensor
    3. Excessive mechanical noise within engine
    4. Improper or incorrectly installed MEM-CAL in the ECM or defective ECM
    5. Intermittent open in the EST line to the ignition module
  3. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    I know that throwing parts at a vehicle blindly without troubleshooting is generally a bad idea, but considering how often these modules go out, do you think a reasonable first step is to replace the ignition control module? As far as I know, it's still original to the truck. I'll also inspect the wiring while I'm in there. I don't think there's any wiring issues, but you never know with 20+ year old wiring.

    While I'm in there, I'd also do cap, rotor, plugs, and wires since I have to remove some of those to get to the module. I think also while I'm at it, I'll upgrade the coil, rotor, cap, wires, and plugs to a hotter model in preparation for bumping the timing in an attempt to get better power and MPG. But anyways.
  4. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    This is so confusing. I haven't replaced the module yet. I don't understand why I won't get the check engine light for over 30 miles, then I get it. It's obviously intermittent, but I wonder if it's the module, wiring, or otherwise. And to make things worse, I don't think I can reproduce this issue sitting in my driveway unless I run the truck for hours. Any ideas as to how to troubleshoot it?
  5. LoneWolf'burban

    LoneWolf'burban Rockstar 100 Posts

    I'm having the same problem with my 92 GMC W code 4.3L. One day being tired of it intermittently and then permenetly retarding the timing I crawled under the hood and found a wire hanging on the driver side beneith the master cylider and it was the knock sensor wire that broke off. I replaced the rat tail and sensor. It ran much better but still have code 43. Cleared the memory but the code came back.
  6. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    I'm getting a code 42, not 43. Not sure if they both relate to the same module, but 42 is an electronic spark timing issue, and 43 is an electronic spark control issue. I'm not 100% sure the EST module is the one under the distributor cap or if it's somewhere else because the book only talks about EST with the HEI ignition system that I don't have.
  7. Josh

    Josh Member 100 Posts

    i too have the same problem in my 94. have code 42 and 43. replaced knock sensor today with no change. talked to a few mechanics in my area said they get heat soaked and malfunction. mine is after 1993 so its in my PCM. yours is on the black bracket on the left side of your tbi. my code reader says ESC Failure and a ES Timing error. likely due to the failure of the internal module. ill let u know what i find out on mine maybe itl help u out.
  8. LoneWolf'burban

    LoneWolf'burban Rockstar 100 Posts

    My bad. I was really tired when I posted that. On my truck a code 42 pops up when I disconnect the tan and black wire under the dash on the pass side right where the carpet ends. It's the one that you disconect to time the engine. I'm having a bit of trouble with that now.
  9. Josh

    Josh Member 100 Posts

    i went and got a replacement computer today switched em out and not a thing changed. put a new ign. module in and cleaned the cap terminals and runs much smoother. where is this said wire is it a one wire connector or a couple wires. maybe mine came undone or someone screwed around in there before, or maybe theres gremlins at work. the cap terminals were rusted lol. clr and sandpaper made it like new. runs like a raped ape too. i just let it idle and idled as smooth as a new ls engine. im at 230,000 runs nice but will know if the problem persists tomorrow. i did try something. i unplugged my IAC before going to parts store. i noticed my rps in park at 1450rpm and drive was 975rpm it only dropped rpm once. its just real wierd that it wasnt acting up all but once. lemme know what u find out mine is like i said code 42 and 43.
  10. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    I'm not 100% sure, but I'm fairly sure that the timing advance disable connector is next to your brake master cylinder. It should be a 1-wire connector and the wire color is brown. I'll replace my ignition module when I get a chance, hopefully this weekend. It runs fine without the module so I haven't been worrying too much about it. Right now the more pressing issue is why am I losing ATF from around my transmission cooler.

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