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Missing the "Service Engine Soon" light.

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by thebigdaddyof2, Nov 3, 2002.

  1. thebigdaddyof2

    thebigdaddyof2 1/2 ton status

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    Missing the \"Service Engine Soon\" light.

    Anybody have this happen to them and what was the fix?

    I first noticed that I did not have the Service Engine Soon light when I went to check for trouble codes. I was checking for codes because my 1989 K5 350TBI kept stumbling and cutting out- anytime I gave it more than 1/4 throttle. Damn thing idles fine and has power, just wont take more than a partial throttle.

    I found a set of GMC shop manuals for model year 1987. They appear to be close enough to work off of.
    My Haynes manual is about worthless.

    I have found that the bulb is good, the power to the bulb is there and the wiring to the ECM is OK. This points to a defective ECM. I will be swapping in another ECM to see if this is the fix. I will be shocked to know that the ECM can still run- but in a degredated state. I'll post back after the swap. If I do need a new one, fortunately Auto Zone and Advanced have the ECM in stock for $71...but with a $95 core charge.
     
  2. YtseJam

    YtseJam 1/2 ton status

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    Re: Missing the \"Service Engine Soon\" light.

    Make sure the bulb socket is ok. If you remove the instrument cluster and install a test light in it's place and it illuminates than the bulb socket or connection there is faulty. If the printed ckt & the bulb & socket are ok than I'd check the ECM ground. If that is ok I'd replace the ECM.
     
  3. HarryH3

    HarryH3 1 ton status Author

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    Re: Missing the \"Service Engine Soon\" light.

    There should be power to the bulb any time the igintion is on. The ECM will apply a ground to the other side to turn the bulb on. Try unplugging the harness at the ECM end. Then, with the ignition switched on, ground the wire at the ECM harness connector that goes to the SES light. If the light comes on, then the transistor that turns on the SES light is a goner. The only fix (unless you're an ECM repair guru) is to replace the ECM. /forums/images/graemlins/mad.gif If the light doesn't come on, then start tracking down what is wrong with the connection between the ECM and the bulb. /forums/images/graemlins/cool.gif
     
  4. 88K5Jimmy

    88K5Jimmy 1/2 ton status

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    Re: Missing the \"Service Engine Soon\" light.

    When I first got my 88 K5, my "service engine soon" light didn't come on at all. It ended up being the computer behind the glove box.
     
  5. thebigdaddyof2

    thebigdaddyof2 1/2 ton status

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    Re: Missing the \"Service Engine Soon\" light.

    Found a used ECM for $20. Plugged it in and now have a SES light again....ECM was defective!
    I'm hoping the stumbling problem is fixed as well. I'll drive it tomorrow.
    I guess I'll carry a spare ECM now....along with all the other crap I carry!
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Missing the \"Service Engine Soon\" light.

    They really don't fail that often, I guess if you can get a spare for cheap, go for it, but the failure rate is less than that of alternators, I can tell you that. It still ran anyways didn't it? Not like having an axle break on you or something, when the ECM fails, you should still be able to drive it, and sounds like you were.
     

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