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Service Engine Soon Light

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ncbloodhound, May 20, 2004.

  1. ncbloodhound

    ncbloodhound 1/2 ton status

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    This might be long, but here is the deal...

    First, the service engine soon light has never turned on when turning the key to the on position. It should turn on for a second and then turn off shortly after that.

    Second, I have tried shorting pins A & B to read any error codes and of course that doesn't work. The light doesn't come on at all.

    Third, while I was on vacation I was in 4 wheel drive the whole time (we were camping on the beach at cape lookout). I noticed that the service engine soon light came on when the key was in the on position and I was in 4 wheel drive. It did this about half the time.

    Finally, when I drive on the highway for a while, the check egine light comes on periodically, I haven't noticed a pattern yet, but it usually doesn't stay on for long.

    What do you think could be the problem? A short? bad ECM?

    Should I just take it somewhere? Everything is running fine...


    Thanks for any advice. /forums/images/graemlins/thumb.gif
     
  2. Thunder

    Thunder 3/4 ton status

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    If your CEL light does not come on when key is on and engine is not running and you cant get any codes FIRST TRY THIS.
    Disconnect the pos battery cable for at least 5 minuets. Then reconnect.
    Drive your truck and check codes.
    If you still get no codes or no CEL when key is on it is prolly a bad ECM.
    If you get codes deal with the issues. Or post up with the numbers someone will help
     
  3. ncbloodhound

    ncbloodhound 1/2 ton status

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    [ QUOTE ]
    If your CEL light does not come on when key is on and engine is not running and you cant get any codes FIRST TRY THIS.
    Disconnect the pos battery cable for at least 5 minuets. Then reconnect.
    Drive your truck and check codes.
    If you still get no codes or no CEL when key is on it is prolly a bad ECM.
    If you get codes deal with the issues. Or post up with the numbers someone will help

    [/ QUOTE ]

    Tried the battery thing and it didn't work. Guess the ECM is bad. Should I take it somewhere to confirm or just replace mine? Anything special I need to consider?

    Also, what all does the ECM control? Just error codes? Everything seems to be running fine..

    Thanks!
     
  4. 87BrnRsd

    87BrnRsd 1/2 ton status

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    I dont know aobut the light not comming on when you turn the key on, but if the light comes on after driving on the highway for a while then that is definitely a sign of a bad egr valve. I would try replacing the egr before you spent a ton of $$$ on a mechanic. Also, are you sure you shorted the a&b pins and did the code test correctly? I know for sure it works because I did it on my truck and it has worked perfectly every time. You do know to not start the truck when it is like that dont you? Anyways, hope that helps.
    -Harrison
     
  5. Thunder

    Thunder 3/4 ton status

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    It is possible that there is a short or the CEL bulb is burned out. If the bulb is burned out or shorted, the only way to check your codes and ECM is with a scanner. If you dont have acess to one try Autozone or another big parts store. Many of them will hook up and do a scan for free.
    Get your truck scanned before replacing any parts.
     
  6. xdelirious45x

    xdelirious45x Registered Member

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    The CEL light will go off after a code is stored into the ECM. When you scan for the codes, it should be saved in there and it should show up.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    I second checking the bulb and connections before doing anything else. ECM's do fail, but percentage-wise, it is very rare. If the ECM fails, I would suspect it would be pretty apparent in driveability, since it will go into "limp home" mode.

    I just changed my "choke" lens out for a TBI "check engine" lens, and when I turned the key on, I got nothing. Of course instantly I think I either wired it wrong, or the ECM is fried, but with some diagnosis, I realized the bulb wasn't making good contact in the socket. Tightened/cleaned up the socket/bulb connection, and voila, check engine light when there should be.

    The printed circuit board itself could be the culprit, but that should be obvious...one wire on the connector is all that feeds the CEL, so it is very easy to trace the printed circuit to verify the copper is still intact and making good contact with the bulb socket.
     
  8. 6.2Blazer

    6.2Blazer 1/2 ton status

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    [ QUOTE ]
    I dont know aobut the light not comming on when you turn the key on, but if the light comes on after driving on the highway for a while then that is definitely a sign of a bad egr valve. I would try replacing the egr before you spent a ton of $$$ on a mechanic.

    [/ QUOTE ]

    There are a helluva' lot of other things that could also cause the CEL to activate. I definitely do not recommend just randomly changing parts like this.................

    Based on the information given in the original post, mostly that the CEL doesn't activate during the initial key-on, I would make sure the CEL bulb, connection, and associated circuit is good first.
     
  9. ncbloodhound

    ncbloodhound 1/2 ton status

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    Thanks for all of the help. /forums/images/graemlins/waytogo.gif

    The Autozone near me doesn't have an OBDI scanner, neither does Advance and Pep Boys wants to charge $75 for a total engine diagnostic. I will check all of the connections and make sure everything looks good. I also may call around and find someone that does have an OBDI scanner.

    Thanks again!
     

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