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90 Suburban ECM Fuse Popping

Discussion in 'Electrical & Audio' started by Bright Idea, Oct 15, 2019.

  1. Bright Idea

    Bright Idea Junior Member

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    Truck Model:
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    Engine Size:
    5.7
    I have a 1990 Suburban that has random 10 amp ECM fuse failure. This has happened 4 or 5 times over the last few months. This shuts down the engine. It still cranks but will not start until the 10 amp fuse is replaced. It has always happened when moving and all but once when I have hit a noticeable bump. I am concerned that this may be the beginning of an ECM module failure ? Engine runs great and there are no codes. A head scratcher for sure. I would like to figure this out before it leaves me stranded. Any ideas ?
    Thanks in advance for you replies.
     
  2. chengny

    chengny Full Access Member

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    The 10A ECMB fuse supplies power to the fuel pump relay, the fuel pump oil pressure switch and both sections of the ECM on the 440 ORN circuit. From the fuse block the 440 ORN lead transits the firewall at the main bulkhead connector. In the engine compartment it splits into 4 branch lines that run to the components noted above. The leads to the FPR and oil pressure switch run directly from the splice. The 2 ECM power supply leads are run back into the cab in the ECM harness via the grommet on the RH side of the firewall.

    That is the entire 440 circuit. But, there is more to the story. The leads from the load sides of both the FPR and the pressure switch contacts are spliced together along with two other branches. So when the contacts are closed, it extends the circuit.

    These leads form the 120 TAN/WHT circuit. One branch from the splice goes through the grommet and supplies the ECM with a signal that there is power available to the fuel pump. The other branch heads back to the gas tank and provides the fuel pump with power.

    Complete wiring diagram:
    Pages from ST_350_90_1990_Chevrolet_R_V_G_P_Truck_Models_Wiring_Manual.JPG


    Simplified version:

    Pages from X9136_1991_GMC_Light_Duty_Truck_Fuel_and_Emissions_Including_Driveability[1].JPG


    Anyway, to find the short to ground you will have to cover a lot of ground.
     
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  3. Bright Idea

    Bright Idea Junior Member

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    Truck Model:
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    Thank You, Makes Sense finding it may be the fun part. Fuel Pump is new, relay is not. Looks as though the oil pressure switch and the fuel pump relay may be the place to start. Thanks for the info.
     
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  4. chengny

    chengny Full Access Member

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    You might want to inspect the rear lighting wiring loom too.

    It runs from the main bulkhead connector at the fire wall and heads back. In addition to the leads for the rear lights, that loom carries the wiring to the fuel pump/gas gauge sender. I am not at all familiar with 06 body style (Suburban, Tahoe, Yukon). But if the rear lighting wiring loom runs near the exhaust system, it can get damaged by heat - especially if the exhaust has been altered.

    Good luck with this. Locating a short - especially one that only occurs intermittently - can suck. Lay in a big supply of 10 amp fuses.
     
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  5. Cuba

    Cuba Likes to work alone in her woman cave

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    What you can do, is install a light bulb in place of the fuse. I know you can't jamb one in there, but you take an old socket with leads intact and put 1 lead into each slot. When it's shorting, the bulb will light. Have a buddy or child, sit and keep an eye on the bulb, while you go around rubbing or moving the harness. Trick is, NOT to move it too much. Look for "contact points". Then push the harness into those points.

    Heat, vibration, & contact points are your main focus. Hope that helps.
     
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  6. Bright Idea

    Bright Idea Junior Member

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    Thanks for the suggestions. One question in regard to the oil pressure switch. The switch was replace a couple of years ago. Is there a test procedure for the switch or is there a way to bypass the switch ? I suppose it is possible this switch is bad ? My luck with newer components has not been al that good. I know that NAPA has the Echlin line and the Economy line. Not sure which was used when replaced.
     
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  7. Bennyt

    Bennyt Full Access Member

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    Had the same problem on my '89 Jimmy(K5).

    It was a wire that ran over the transmission that had rubbed through the insulation. Every once in awhile I'd hit a bump and it would bounce down and short on top of the trans. One day I was changing a part, maybe an o2 sensor or something, and spotted it.

    Buy a resettable fuse ($3-4)until you figure it out.
     
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  8. Bright Idea

    Bright Idea Junior Member

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    spent some time today on the problem replaced the oil pressure switch, got good visual inspection on the relay and switch wiring and cleaned up some other wiring but found nothing obvious going to ground. wiring down the frame rail to the fuel pump looks good. I drove it a few miles and as I drove up my driveway hit a bump and the fuse blew. Any chance there is a short in the ECM module itself ?
     
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  9. Poppy 87

    Poppy 87 Supporting Member Supporting Member

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    Read post#5, GM refers to this as a "wiggle" test, manipulate the harness while monitoring DVOM . Time consuming but cheaper than part replacement. It's possible but unlikely the ECM due to the fault occurs when hitting bumps.

    Sent from my XT1254 using Tapatalk
     
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  10. Bright Idea

    Bright Idea Junior Member

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    After replacing the Oil Pressure Switch the problem remained. I have now disconnected the switch and the problem has been resolved. I have not visibly found a shorted wire. The Oil Pressure guage now pegs when the key is turned on. Previously it had a very low reading. Any chance it could be the guage itself ? I am fine without it or I could install a mechanical guage but I have not had a fuse blow since the disconnect of the switch/sending unit.
     
  11. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    You mentioned hitting a bump and everything went to hell so try this. I was having the same issue on my Caprice this summer. It took me three weeks to nail it, but my ESC wires were too close to the manifold, and the insulation was flaking off. Everytime the car flexed around or the air hit the wires right, either they’d touch ground or touch each other. Then pop! Look for stuff like that. Do you have any rodents getting in there? They can really throw a monkey wrench into the whole thing.

    You’re gonna have to get your DVM, plug it into the troubled fuse’s spot, get a base resistance reading after you’ve put a fuse in there and it didn’t pop, and manipulate the whole engine control harness in as many specific areas as possible while having someone watch the DVM to see if the numbers drastically change when you move the wires a certain way.
     
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  12. Snoots

    Snoots Supporting Member Supporting Member

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    Not relating to a squarebody but in the GM V6 that they put in Fiero's the oil pressure sensor is a PITA. Many have been the owners that went through several before they got one that worked correctly.

    One major problem was that there is a vent hole in the sensor inside the connector. The fix was to put a small dab of RTV over the hole. End of problem. Most problems were with 'off-brand' sensors. Fewer if any with the GM sensor.

    I think 1987 GMC Jimmy has hit the nail on the head tho. Unless something is bouncing around inside the ECM, it's a wire shorting out. Without a doubt the hardest problem to find.

    Careful examination and patience will get you through. Keep us posted.
     
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  13. Bright Idea

    Bright Idea Junior Member

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    I may continue to explore this but with the sensor disconnected I have lost my pressure guage in the dash (such as it is ) and the continued supply of power to the fuel pump in the event of fuel pump relay failure if I understand the circuit correctly. I am not concerned about relay failure as I bave a backup mounted and ready to plug the harness in if I have a failure. Right now things are good as far as the fuse not blowing.
     
  14. chengny

    chengny Full Access Member

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    On the 5.7 motor, a dual function oil pressure switch/sender is used. It has three leads connected to it.

    A single TAN lead is connected to ground via a variable resistance - and that just drives the dash gauge.

    The ORN 440 supplies power to both the fuel pump relay and the fuel pump pressure switch. It also provides power to the ECM - both banks.

    The fuel pump is supplied on the TAN/WHT 120.


    The oil pressure switch is provided to allow the truck to start in the event of an FPR relay failure.

    This shows just the TAN and ORN circuit:

    Pages from ST_350_90_1990_Chevrolet_R_V_G_P_Truck_Models_Wiring_Manual.JPG

    This shows the dual function switch/sender:

    Pages from ST_350_90_1990_Chevrolet_R_V_G_P_Truck_Models_Wiring.JPG
     
    Last edited: Nov 11, 2019 at 6:22 PM
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