Keeps blowing the ECM-b fuse when trying to start the engine

1986 GMC JIMMY
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TIGERFANCY
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I have the vehicle listed above with a 2.8 in it and 114,365 miles on it. It quit running one day and would not start afterwards. We found out it kept blowing the ECM-b fuse every time you tried to start it. We have replaced the ECM, the ignition module under the distributor, the fuel pump relay switch, and the electronic ignition module, as well as the fuel filter with all new parts but it is still blowing the fuse. The truck tries to start but is not getting any fuel. I am not even sure what the ECM-b fuse runs so I'm not sure what else to do. any help would be appreciated.

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Apr 2, 2007 at 8:03 AM
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PEPPERMRJ
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Is the fuel pump running? You should be able to hear it run when you first turn the key to on. Listen with the gas cap off to be sure. Are you getting spark?
The ECM-b fuse provides power to the ECM, fuel pump relay, fuel pump, and the oil pressure switch. Test the pressure switch and check the wiring for shorts.

Good luck and let us know. :)
Apr 2, 2007 at 11:46 PM
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TIGERFANCY
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It blows the fuse as soon as we turn on the key so I guess the answer would be no the fuel pump isn't running. We have replaced the fuel pump relay switch. How do I test the oil pressure switch? We have also replaced the ecm and i would assume it is getting spark because even with the fuse blown it tries to turn over it just won't fire.
Apr 3, 2007 at 9:03 AM
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RASMATAZ
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The oil pressure switch is the back up for the fuel pump relay when it goes. The PCM/ECM energizes the fuel pump relay. No power to ECM means nothing is going to happen.

You need to track down the short involving the ECM-B circuit.

Disconnect all connectors at the ECM, replace the ECM-B fuse, now turn key to On-did the fuse blow?
Apr 3, 2007 at 10:01 AM
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KASEKENNY
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Here is the wiring diagram for this fuse. If the fuse continues to fail while cranking then there is more then 10 amps on this circuit. It is correct that we need to disconnect the components on this circuit which would be the oil pressure switch. If this is shorted it could cause over load to the circuit. However, if this ok then we need to load test the circuit because it is most likely shorted to power. You will want to do this with the switch and fuel pump unhooked. Use each portion of the circuit to supply the 12 volts to a headlamp bulb and then just ground it to the battery. If the fuse doesn't blow then that wire is ok. Do this until you find the section of wiring that causes the fuse to fail.
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