1998 Suburban 5.7l TBI. ECM-B fuse blows

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

  1. chevychase

    chevychase Semi-Admin
    Staff Member

    Joined:
    Oct 15, 2010
    Messages:
    22,216
    Likes Received:
    12,874
    Trophy Points:
    113
    Occupation:
    SELF EMPLOYED
    Location:
    truck forum .org
    I figured it would blow ...a bad oil sending unit will do this so check it. and wiring to it .
    Probably have to take distributor cap off for easier access ..not it has to be a shorted wire.
     
  2. PatriotYS

    PatriotYS Member

    Joined:
    Aug 22, 2012
    Messages:
    10
    Likes Received:
    4
    Trophy Points:
    0
    Occupation:
    Boat Repair/restoration business owner
    Location:
    Florida Panhandle
    I've actually disconnected just about everything in the car. Only thing untouched in the sensors on the engine. That oil sender/fuel pump sw in the schematic looks like a prime culprit, but with the ACM unplugged....no power should be getting there.

    Stay tuned.....more to follow tomorrow.
     
  3. PatriotYS

    PatriotYS Member

    Joined:
    Aug 22, 2012
    Messages:
    10
    Likes Received:
    4
    Trophy Points:
    0
    Occupation:
    Boat Repair/restoration business owner
    Location:
    Florida Panhandle
    Day 3....

    Ok. Today started looking at harness again.

    Started at tranmission area underneath since vehicle already lifted.

    1. Checked fuel pump harness running along frame to see if it got pinched by transmission cross support. No problems there.
    2. Checked main harness along top of bellhouse....again. No probelm there.

    Next went topside engine comparment.

    1 Checked main harness that routes on top of tranny up to top of engine. No problems there.
    2. Checked oil sender/fuel pump switch........PROBLEM FOUND!!!!'

    The small two wire harness for the sender has gotten pinched between bellhousing and engine. Very hard to see. I actually had to feel around behind the distributer and use a flashlight and mirror. Tell tale was there was no slack in the harness and wires took an immediate 90 degree turn down to the bell housing right out of the plug.

    In process of loosening bellhouse bolts enough to get harness out.

    Tomorrow morning I will repair the harness and report back if this fixed it.
     
    1 person likes this.
  4. chevychase

    chevychase Semi-Admin
    Staff Member

    Joined:
    Oct 15, 2010
    Messages:
    22,216
    Likes Received:
    12,874
    Trophy Points:
    113
    Occupation:
    SELF EMPLOYED
    Location:
    truck forum .org
    What i was thinking patriotys thanks for posting back..Let us know for sure about the fix.
    Tell friends about this great site
    Thanks cc
     
    1 person likes this.
  5. dirtrider73068

    dirtrider73068 Senior Moderator
    Staff Member

    Joined:
    Jul 14, 2011
    Messages:
    6,536
    Likes Received:
    3,913
    Trophy Points:
    113
    Gender:
    Male
    Occupation:
    dump trucker driver
    Location:
    Central Oklahoma
    Good deal thought it might be a pinched wire but just finding it.
     
    1 person likes this.
  6. PatriotYS

    PatriotYS Member

    Joined:
    Aug 22, 2012
    Messages:
    10
    Likes Received:
    4
    Trophy Points:
    0
    Occupation:
    Boat Repair/restoration business owner
    Location:
    Florida Panhandle
    Suburban back up and running!!!

    Repaired the pinched wire harness and put everything back together. Changed the fuel filter, just to take any strain off the fuel pump and ran the motor before lifting the tank back in place......just in case the fuel pump was going south. Didn't have a fuel pressure gage that would fit GM so I don't know if the fuel pressure is above 60psi, but she is running smooth as silk so I thnk the pressure is good.

    Thank guys for the tips. Apppppreeeeciate it big time!
     
    1 person likes this.
  7. chevychase

    chevychase Semi-Admin
    Staff Member

    Joined:
    Oct 15, 2010
    Messages:
    22,216
    Likes Received:
    12,874
    Trophy Points:
    113
    Occupation:
    SELF EMPLOYED
    Location:
    truck forum .org
    Well alright and outta sight ..Gitter Done PatriotYS.
    Your mighty welcome.
     
  8. florida horsema

    florida horsema Full Member

    Joined:
    Sep 11, 2008
    Messages:
    12
    Likes Received:
    11
    Trophy Points:
    0
    Location:
    Lakeland, FL
    #28 florida horsema, Jan 20, 2014
    Last edited: Jan 21, 2014
    OK...I understand I'm resurrecting a 18 month old thread here. I have the same problem; a 20A ECM B fuse that intermittently blows in a 1998 K3500 454. There seems to be no specific circumstance that causes it. Sometimes you can drive for weeks or hundreds of miles before it starts happening. The fuse will blow under any and all road conditions; level ground, pulling a hill, hitting a bump in the road... or even on a long smooth flat stretch. Just no rhyme or reason to it. Replace the fuse and sometimes it'll go 30 feet, sometimes 30 miles, or sometimes blow three or four fuses in a row as soon as the key bumps into the CRANK position. But eventually it will accept a new fuse, start and run. When the truck does start and run, if left parked and idling it will run until it's out of gas without blowing the fuse. That kind of indicates a chaffed or open wire shorting to ground with movement somewhere. But, with the truck running, I've pulled, tugged, jumped up and down in the bed and shaken every bit of the wire loom between the tank and the ECM, pounded on the ECM with my fist and jiggled every ECM connector without being able to duplicate blowing the ECM B fuse.

    I have the wiring diagram and the only items on the ECM B circuit are the fuel pump module, fuel pump relay, oil pressure sensor and ECM. I also understand the Oil Pressure sending unit has a safety circuit that is supposed to kill the fuel pump if there's no oil pressure. That sensor is under the distributor and a BEAR to get at. But in the process of elimination thinking the OPS may be shorting internally and blowing the fuse, I managed to unplug it to see if there was oil on the end of the connector. Big surprise! The engine will start and run with the OPS unplugged and completely out of the circuit! Took it for a test drive and it STILL blows the fuse with the OPS not even connected.

    To be clear.... the fuse does NOT blow when the key is turned on and the fuel pump pressures up. It will either blow when the key is advanced to CRANK or blow at highway speed under just about any random condition.

    In a nutshell; I've replaced the fuel pump module, fuel pump relay and fuel filter. The pump pressures up to 55-60 PSI when the key is ON. The OPS is unplugged (no oil pressure info to the ECM) but it still starts and runs and still intermittently blows ECM B. The SES light works but my OBD2 scanner software shows no DTC's tripped or pending. But the scanner also indicates in the system-by-system freeze frame scan "OBD System is not compliant". Using the same software on my OBD2 '99 Firebird there is no such warning so it does not appear to be a problem with the software.

    Between the "OBD system not compliant", no SES or DTC's after major system failures and still running with no ECM observable oil pressure I'm inclined to believe my ECM has gone FUBAR. I'd like some other opinions about my diagnostic conclusions. I haul horses with this truck and can't afford to get stuck on the road 300 miles from home again. ~FH

    (Edit/update - I just drove the truck off-road in a field and down a dirt road (OPS still unplugged) with potholes the size of bomb craters and drove through every one I could find, shaking the truck every direction from Sunday and could not blow the ECM B.) ​



     
  9. dirtrider73068

    dirtrider73068 Senior Moderator
    Staff Member

    Joined:
    Jul 14, 2011
    Messages:
    6,536
    Likes Received:
    3,913
    Trophy Points:
    113
    Gender:
    Male
    Occupation:
    dump trucker driver
    Location:
    Central Oklahoma
    I am not even gonna try make heads or tails of this till the color of the font goes back to black. Its WAY to hard to read.
     
    1 person likes this.
  10. florida horsema

    florida horsema Full Member

    Joined:
    Sep 11, 2008
    Messages:
    12
    Likes Received:
    11
    Trophy Points:
    0
    Location:
    Lakeland, FL
    ... and the default font size like yours is way too SMALL to read. Sheesh.... Pretty snippy to be a moderator don't ya think?

    Apparently my text was slightly gray because I copied it from another source that did not match the default formatting here. So just for you I fixed it. Happy now? ~FH
     

Share This Page