1998 Suburban 5.7l TBI. ECM-B fuse blows

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

  1. MrMarty51

    MrMarty51 Full Member

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    Hi Florida HorseMa, welcome to the forum, pull up a chair, here have a cold :woot:and sit a spell.
    Let us see if We can get Your fueling problems figured out. :woot:
     
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  2. chevychase

    chevychase Semi-Admin
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    Okay here is the thread on ops how it works ect..91 has same setup ..look it over www.truckforum.org/forums/chevy-truck-forum/34884-vortec-oil-pressure.html
    So here in a nutshell Ecm is good..if it was bad your truck would not run.
    In this circuit the ecm energies the fuel relay. So ecm is good.
    You are blowing the ecmb fuse after the fuse i think..might be contacts at fuse area could check at fuse.
    Could try replacing the ops do not really think it would be though...all the ops does reads pressure...orange wire power..when oil pressure is at least 6psi contacts open sending power to fuel pump gray wire. As seen in diagram i left.
    When you say fuel module are you saying fuel pump?
    Fuel pump maybe getting hot drawing to many amps blowing the fuse.
    Going to have to hunt on the orange..and gray..and the red fuel pump prime connector...for one of these wires grounding.
    Simple circuit..about all i have on this.
     
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  3. dirtrider73068

    dirtrider73068 Moderator
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    Wasn't trying to be snippy just pointed out that the font was very light in color and hard to read, yes I tried to read but was too faint, if was coming off snippy sorry my eyes aren't the best neither is my hearing for me to see it has to be bold dark colors.
     
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  4. florida horsema

    florida horsema Full Member

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    #34 florida horsema, Jan 21, 2014
    Last edited: Jan 21, 2014
    Re; 1998 K3500, 454, 178K, randomly blowing ECM B...

    Thanks for the link, Chevy. Yes. My reference to the fuel module is the pump and sending unit complete. The one in the vehicle is 3 weeks old. It should not be overheating. But with confusing power sources (relay and /or OPS) it could be running continuously and I've been told it's not supposed to run continuously. Even continuous duty shouldn't draw in excess of 20 amps IMO.

    Your quote from that OPS thread is interesting;

    "Went to my older 91 truck. Started it pulled three wire connection off at ops ..truck kept running. Left it loose turned truck off.
    Truck started . Well i thought..so i started the truck removed the fuel pump relay truck kept running.
    Then i disconnected the ops..it quit. So what oil sending unit/ops does reads oil pressure at 6 psi truck will start???
    But mine started disconnected..only thing i can think of is old oil in sending unit has contact open....????"

    I did the same test on mine today with the same results. The truck would start and run with the OPS disconnected, running on the fuel pump relay only. Turned truck off. Then, with the OPS reconnected and holding residual fuel pressure at 55 PSI I REMOVED the fuel pump relay and the truck started and ran. Even with the relay missing the fuel pump ran and maintained 55 PSI obviously powered only by the OPS side of the circuit. With the truck running on the OPS only I probed the relay socket. Pins 87, 85 and 30 were all hot, indicating the relay, if inserted, would also be hot, so the fuel pump is getting 12v from 2 seemingly independent, redundant and parallel sources. And that would defeat the fuel-pump-kill intent of the OPS circuitry. For that to work the relay needs to be downstream of the singular OPS power source after the initial pump prime function has been completed.

    But here's where things go really hinkey. Late today I decided to replace the OPS with a genuine AC Delco part because the old one occasionally pegged the gauge full scale with the key off and stayed there until the truck was restarted. The old OPS appears to be a big box parts store brand. Now, with the AC Delco OPS installed the gauge works normally (holding last indicated pressure and resetting to zero when key advanced to ON) but the truck will now die if I pull the fuel pump relay. With the old OPS plugged in it ran even after the relay was removed.

    And here's where I also start suspecting an accompanying PCM problem. The engine running without an operational fuel pump relay should've triggered a DTC (54 for OBD1, not sure about OBD2), as should the disconnected OPS (P0520 thru 0523). Not forgetting my freeze frame scan returned a "OBD System is not compliant" fault. The OBD function of the PCM appears to be inop. I can scan running systems but there are NEVER any DTC's, even after repeated obvious system failures.

    The fuel system diagram in your post was OBD1. This one is OBD2 but for a '97 Blazer. There are some differences. http://www.wiringschematic.info/wp-...Chevrolet-Blazer-Fuel-Pump-Wiring-Diagram.gif

    Still pondering the imponderable... ~FH


    *Font color and size approved by AARP Committee For Vision Impaired Geezers-
    :hah: FH, Head Geezer
     
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  5. chevychase

    chevychase Semi-Admin
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    OBD System is not compliant" fault means your scanner will not read info..ect.
    ON YOUR TRUCK WILL BE PO230 ON THE FUEL RELAY...BUT I,AM PRETTY SURE VOLTAGE FROM THE RELAY DOES NOT GO THROUGH THE ECM/PCM ..PCM GROUNDS THE RELAY AT START .VOLTAGES GOES STRAIGHT TO FUEL CIRCUIT PCM GROUNDS THE SYSTEM. MAYBE WILL NOT GET A CODE FOR IT UNLESS RELAY IS BAD FOR A NO START..NOT SURE WILL SEE WHAT I CAN FIND ON THIS. DID THE OPS FIX YOUR PROBLEM ? DIAGRAMS I LEFT AND YOURS THE SAME JUST LAID OUT DIFF.

     
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  6. chevychase

    chevychase Semi-Admin
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  7. chevychase

    chevychase Semi-Admin
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    OH FUEL PUMP RUNS ALL THE TIME ..TRUCK IS RUNNING,OR TRUCK WOULD QUIT...WILL ONLY RUN FOR A FEW SECONDS WHEN KEY TURNED TO ON.
     
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  8. chevychase

    chevychase Semi-Admin
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    Noticed you are here...Next time it blows a fuse take a test light connect it to positive at battery..probe both sides of fuse if probe lights you have a ground.You changed the relay..pump.. ops...has to be a wire loose making ground.
     
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  9. florida horsema

    florida horsema Full Member

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    #39 florida horsema, Jan 25, 2014
    Last edited: Jan 25, 2014
    Here's a better diagram but this one has a mistake at the fuel pump relay. The pin marked C is 86 and C2 is 85, the coil circuit (grn/wht & blk). Pin A2 is 87, C2 is 30 and B2 is 87a. The mistake in this diagram is the relay points should be open (not energized) by connecting 30 to 87a, not 87 to 87a (fuel pump prime connector). With the key off the prime connector needs to be manually jumped to 12v and power up the fuel pump. When the key on energizes the coil (85 & 86) it should pull the points closed (away from 87a and connect 30 to 87 for 2-3 seconds on command from the ECM to prime the fuel system then return to the open position (30 to 87a) until the relay coil is activated again during engine crank. When the oil pressure exceeds 6 PSI the fuel pump side of the OPS begins to supply the 12v to the fuel pump and the relay coil should disengage.

    The AC Delco replacement OPS I got was bad right out of the box. Good oil pressure but no power to the fuel pump. That's why the engine would die when I removed the relay while running. The fuel pump side worked on the old OPS so I put it back in. But here's the rub. The fuel pump relay is NOT disengaging after the OPS kicks in. So the fuel pump is getting 12v from 2 sources simultaneously. That's why the truck ran when I had the OPS disconnected. I've been told the relay side is only supposed to function on start up and shut down. If it runs concurrently with the OPS source it defeats the safety shut down feature.

    I cannot get to all three wires (gray, tan and orange) at the OPS connector due to it's insane location between the firewall and bell housing behind the intake plenum and below the distributor. But I did manage to cut the ORN (12v) switch power lead at the OPS and run a new wire outside the harness bundle to eliminate that portion as the possible ground problem... and still managed to blow another fuse. I pretty much have the ORN wire isolated from the others and all that's left are the 3 gray fuel pump power wire legs; 1 from the relay and 1 from the OPS that become 1 wire at the pump somewhere between the firewall and the tank, and another gray leg that splits off the same OPS output and goes to the ECM signalling active oil pressure. I have an access port in the truck bed for the fuel pump so I'm going to put a 15A test fuse on the gray wire back there and see if that blows before it takes out the 20A ECM B. I think that should at least tell me on what side of the switch and relay the short is. I suppose I could probably dig the gray wire out of the Black ECM connector and fuse that one too but I want to see if the gray side is even involved first.

    I'll try that test light on the blown fuse the next time it happens, Chevy. I appreciate your help. ~FH


    [​IMG]
     
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  10. chevychase

    chevychase Semi-Admin
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    sounds like your doing some great diagnostics..like that..yes the relay is in the pic only for a few seconds at start...then the ecm takes over after it receives rpm pulse from dist.and oil pressure from the ops circuit.
    I believe you will track this down for sure..going to call you Sherlock mech man .
    Good ideal on the fuses..yes that is a tight spot gm put the ops in.
    If all else fells could try running new wires back to the pump..gray wire.From relay.. to ops.. back to the pump.
    Keep us updated for sure..i,am thinking on wires going to the pump touching some wheres we shall see.
    And your mighty welcome fh!!
     
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