1998 Suburban 5.7l TBI. ECM-B fuse blows

Discussion in 'GMC Truck Forum' started by PatriotYS, Aug 22, 2012.

  1. florida horsema

    florida horsema Full Member

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    #41 florida horsema, Jan 28, 2014
    Last edited: Jan 28, 2014
    Yep.... but where? So many places to look..... And you always find what you're looking for in the last place you look.... mostly because after you find what you were looking for you stop looking so it always HAS to be the last place you looked. (I sense viable PhD dissertation material here.)

    This is the last place I looked....
    [​IMG]

    The main engine harness bundle had been resting directly on top of the EGR exhaust riser tube. Over time the harness had rubbed through the heat retardant material wrapped around the riser allowing it to melt through the plastic coil wrap, the electrical tape binding the harness and eventually into the first wire it made contact with. The orange ECM B always hot wire to the OPS branch was in the perfect spot on the bottom of the bundle to make a direct short to the metal riser tube. The intermittent nature of the fuse blowing was apparently in conjunction with a slight rocking motion of the engine responding to torque loading... or whichever way I happened to be holding my mouth. The orange wire insulation was burned away and there was only about 2 strands of wire still making a connection. If you look very closely at the riser tube you can see two holes where the incessant arcing actually burned through the metal. Here's another photo highlighting the burnt and separated (by me) wire;

    [​IMG]

    Some of the surrounding wires were a little "crispy" but none burnt all the way through. I wasn't about to dig really hard and create more problems than I already had. To be clear, I had "checked" that harness for rub-throughs before but I was looking straight down from above and did not see the worn and frayed EGR insulation nor did I feel the small burn hole on the bottom of the plastic coil. The harness appeared to be fully intact and uncompromised. It wasn't until I saw the holes and bare metal in the riser tube that I stripped away the harness covers and found the burnt wire.

    So, the case of the blowing ECM B fuse appears to be resolved. I still, however, cannot account for the continuous operation of the fuel pump relay circuit GRN/WHT wire keeping the relay coil energized after the truck has started and established oil pressure. But that's a problem for another day and another thread... after my head stops hurting from this one. ~FH



     
  2. chevychase

    chevychase Semi-Admin
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    I NEW YOU WOULD FIND IT YOUR A GITTER DONE GUY..CAN TELL. I HAD A VAN ALL OF A SUDDEN GOT ABOUT 12 CODES..THEN A FEW HOURS LATER WOULD NOT START. FOUND A BIG HARNESS AT BACK OF THE ENGINE LAYING ON THE EXHAUST MANIFOLD. THE WHOLE HARNESS WAS BURNED BARE..WELL IT TOOK THE PCM OUT..WHAT A MESS TO GET STRAIGHTENED OUT..I,AM GOING TO STICKY THIS ..THANKS SO MUCH FOR POSTING THIS FH..SHERLOCK MECH.
     
  3. chevychase

    chevychase Semi-Admin
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    OH I HAVE NEVER FIGURED THE WHY OPS ...I WILL TRY TO FIND INFO ON THE RELAY ENERGIZED AS I CAN FH ALSO
    BE A GOOD HUNT FOR SHERLOCK AND WATSON!!! I LOVE GOING ON HUNTS FOR PROBLEMS SURE LEARN A LOT BY DOING THIS..IF ONE LIKES READING..LEARNING. IF I DID NOT I WOULD NOT HAVE MANY POSTS ..OR MUCH TO ADD/SAY. LEARN SOMETHING NEW ON EVERY SEARCH..!!
     
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  4. florida horsema

    florida horsema Full Member

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    UPDATE- on the continuously hot fuel pump relay coil (with engine running) conundrum.

    The guy who produced this Fuel Pump Relay Problem - YouTube and other quasi-professional diagnostic videos states unequivocally the fuel pump side of the Oil Pressure Switch is NOT a safety shut down feature as most people think but instead is a back-up power supply for the fuel pump should the relay fail. If that is true, then both the OPS and the fuel pump relay are supposed to be continuously hot with 12v to the fuel pump while the engine is running and there is no fault occuring with the GRN/WHT relay power wire at PIN 85.

    Time to follow my own best advice I think. 'If it ain't broke, don't fix it.'‚Äč But it sure helps if you understand how it's supposed to work in the first place.
    :idea:~FH
     
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  5. chevychase

    chevychase Semi-Admin
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    what i thought on the ops not a safety..just for pcm to reference from ..seeing oil pressure pcm will run fuel pump via the ops circuit..the pcm will ground the circuit via relay not seeing the forest for the trees in the diagram deal..ahh i will have to watch the vid later...to see what you are saying.
    you need to stick around ,and help out here!! Sure would be welcome FH.
     
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  6. chevychase

    chevychase Semi-Admin
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    That was a very interesting video!!! What i thought all along on the ops. I use to tell others that it was not a safety for low oil..everyone argued it now we have proof FH. Everyone needs to watch what good diagnostic will result in!!
     
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  7. 96silverado

    96silverado Full Member

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    One could always retort
    "Hi-Jacked now"
     
  8. MrMarty51

    MrMarty51 Full Member

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    OK then.
    I have read back through this "OLD" thread and am now curious.
    If the OPS is for a safety, in case the relay fails then, how do We know that the relay has gone bad ? DTC ?
     
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  9. chevychase

    chevychase Semi-Admin
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    #49 chevychase, Sep 22, 2016
    Last edited: Sep 22, 2016
    Thanks for asking Marty! The vehicle will not start back. OPS keeps fuel pump running as long as vehicle is running/400rpm,s @MrMarty51
     
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