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Author Topic: VCM PCM (ECM B) Fuse 20amp keeps blowing out...  (Read 8025 times)
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WestNewton
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« on: 05/25/10 02:52 PM »

Truck was driving great, until I left it outside running for a minute.

Came back out and the engine had stopped and the key was still in the "on" position...

Turned the key off and tried to restart, but had no success.  ALSO, fuel gauge and temp gauge were both inoperative.

Found a blown fuse in the engine fuse block, which was a 20 amp VCM/BCM Fuse.  The owner's manual says it's the ECM B fuse.

When I replace the fuse, it immediately blows out again as soon as I turn the key?

Any ideas on what is shorting this out?

Thanks!!
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ItsStopher
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« Reply #1 on: 05/25/10 03:46 PM »

That fuse powers the fuel pump relay and the PCM. Pull the fuel pump relay and try to replace the fuse again.
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WestNewton
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« Reply #2 on: 05/25/10 04:00 PM »

Pulled the Fuel Pump relay and tried a new fuse.

All was good (except for no fuel of course)

I guess I will replacing my fuel pump tonight Sad
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ItsStopher
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« Reply #3 on: 05/25/10 04:08 PM »

not yet.... let me look up what to do next
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ItsStopher
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« Reply #4 on: 05/25/10 04:18 PM »

The next thing I would try is to disconnect the wires on the pump to be sure it is not the wires. Are going to drop the tank or cut an access hole? After you get it disconnected try it with the relay again and if all is good then it looks like you pump  Sad
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WestNewton
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« Reply #5 on: 05/25/10 05:10 PM »

 banghead banghead I put a 25 amp fuse in just to see if I could limp home, and it started immediately.

Put the 20 back in to verify it will blow the fuse, and it works.

Everything is back to normal...sitting still.  I am going to drive and see if something shorts out again.

Would having the truck sitting on a slight incline at idle have starved the pump for fuel and caused this problem temporarily?
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WestNewton
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« Reply #6 on: 05/25/10 05:29 PM »

Or....

When the pump is hot, it draws more and blows the fuse, and since it has been sitting for hours, its cooled down and runs fine?

So this is a sign of a failing, but not dead pump? 

88k miles is end of life for this thing?
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WestNewton
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« Reply #7 on: 05/25/10 05:38 PM »

Or #2, the relay is bad, and shortages out when hot...

Do relays slowly fail, or are they one of those things that either functions 100%, or is dead?
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ItsStopher
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« Reply #8 on: 05/25/10 05:51 PM »

The relay does not get power from that fuse.... it only switches the power on to the fuel pump.

It could go bad, but I dont that is you problem.
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WestNewton
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« Reply #9 on: 05/25/10 06:24 PM »

The relay does not get power from that fuse.... it only switches the power on to the fuel pump.

It could go bad, but I dont that is you problem.

I must have misread the ecm-b wiring diagram wrong then.

Is there a fuse just for the fuel pump?
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ItsStopher
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« Reply #10 on: 05/25/10 06:32 PM »

The power passes through the relay to the fuel pump but the fuse does not power the relay.
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WestNewton
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« Reply #11 on: 05/25/10 08:25 PM »

UPDATE:

Replaced the Fuel Pump relay just in case.  Drove fine around the neighborhood for 15 minutes or so.  Came back to it about 45 minutes later and it died while taking a corner after about 2 minutes.

I am going to go re-test the relay again by:
1. Remove the Fuel Pump Relay
2. Install good 20 amp fuse
3. Turn key on and verify the fuse is good.
4. Re-install Fuel Pump relay, turn key on and verify fuse is again blown.

This "intermittent" failure of the fuel pump circuit seems to point to a bad fuel pump.  Does the fuel pump have any internal wiring that can short out?

This keeps pushing toward the fuel pump and not at all to the VCM/PCM portion of the equation which would seem to indicate a random short.  I hope this isn't it, because I don't want to start tracing all the wires and check for ground shorts Sad
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« Reply #12 on: 05/25/10 08:34 PM »

I think you have a issue I would highly recommend you try what Itsstopher posted


The next thing I would try is to disconnect the wires on the pump to be sure it is not the wires. Are going to drop the tank or cut an access hole? After you get it disconnected try it with the relay again and if all is good then it looks like you pump  Sad
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WestNewton
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« Reply #13 on: 05/25/10 08:41 PM »

I think you have a issue I would highly recommend you try what Itsstopher posted



I have been reading off and on tonight on the best way to do this, and I am going with the hole cutting approach through the backseat.  I have been in denial and hoping it will just fix itself...but alas.

I don't have a dremel bit to cut through the metal, but I will first thing in the morning.  I can set the depth and use the measurements others have so graciously shared on this forum and try again.

The only issue I have with disconnecting the pump wires at this point, is the fact the pump works intermittently with everything attached.  If every single fuse kept blowing, and the truck never started, I would have done what ItsStopher posted first.

Since it worked intermittantly, I wanted to try a relay first.  Maybe I will cut the hole and just find some wires in poor shape.

On a side note:

Doesn't the fuel pump make a whining/humming noise when it is wearing out?  I know that a bad wiring connector has nothing to do with the health of the pump itself, but I would rather not replace then entire pump assembly when all it needs is a new wire harness.

Thanks again everyone!  The advice and wisdom is greatly appreciated  Thumbs up!
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« Reply #14 on: 05/25/10 08:44 PM »

I'll let Itsstopher address this. Thumbs up!
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WestNewton
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« Reply #15 on: 05/25/10 08:56 PM »

I did another search for "ECMB" and came up with some posts a few years old from http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,102935.0.html

One quote:

Ign switch faults are when the contacts become resistive or intermittent open circuit due to wear and/or contamination (dirty).

The ECM-B fuse in the 02 sources the fuel pump relay contacts and the powertrain control module. It is not part of any ign switch circuit.

If the fuse blows as soon as you replace it, pull the fuel pump relay and try it again. Then if it doesn't with the relay out, it's the relay itself (all while the ign is off).

If it blows when the ign is on only,  the fuel pump or the fuel pump wiring has a short.

There could be conditions where the relay can have a mechanical defect in a way that it could blow a fuse when it's activated only. Kind of rare but easier to isolate by swapping same p/n relays and replacing fuses or using test circuit breakers. `Get's a bit complicated to explain. Sad

HTH Smiley

This supports ItsStopher's original recommendation that it is in fact the fuel pump, and satisfies my suspicions that it's only the fuel pump.

I will definitely be cutting open my backseat tomorrow and hope to find some ugly and corroded wires.  I hope it doesn't come down to needing new wires ran back there.

I do not know enough about the the actual electrical connections to know what to expect or how this happens, but it WILL be fixed!
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« Reply #16 on: 05/25/10 09:13 PM »

I did another search for "ECMB" and came up with some posts a few years old from.

Yes sir, but did it have a answer. So all I'm saying it disconnect the connection wiring to the fuel pump and see if the fuse blows. And yes cut a hole in the floor
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WestNewton
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« Reply #17 on: 05/26/10 02:44 PM »

 Cheesy

New pump AND new body side harness (extra $50 from dealership) and good to go!

Thanks everyone, especially ItsStopher, for the input.

My next (hopefully never though) fuel pump will take 22.5 minutes  Thumbs up!
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« Reply #18 on: 05/26/10 03:17 PM »

 Cheers!
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WestNewton
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« Reply #19 on: 06/03/10 11:37 AM »

Quick update:

The Airtrix or Airtrex fuel pump from autozone has a 1year, 12k mile warranty.

If you get a Delphi pump from GM, it is a lifetime warrany, but you MUST change the filter and/or strainer at the same time to have the warranty be legitimate.

I went with the lifetime warranty option, and granted this only covers parts, the labor is now minimal for me Smiley
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« Reply #20 on: 06/03/10 05:48 PM »

Glad you got your av running again.

 hurl Id drop the tank 100 times before I would cut a hole in the floor.
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« Reply #21 on: 06/03/10 07:40 PM »

Wuzz  Just kidding

I would cut a hole in a heart beat to save 2 hours worth of work (time=money) just make sure you seal the opening

The reason I recommend to pull the plug off first was to determine if the harness was bad or the pump. I have seen posts where the wires on the fuel pump melted inside of the tank not a good thing to happen.

But I'm glad it's fixed.
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« Reply #22 on: 09/08/10 11:21 AM »

I've had the same problem with my 2002 Avalanche (145,000 miles) except it happens very infrequently, maybe 2 - 3 times per year. Usually on long drives. Any news on the subject? Was it the fuel pump?
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WestNewton
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« Reply #23 on: 09/08/10 12:09 PM »

I truthfully think that it was the body side harness going to the pump that was shorting out.  The fuel pump seemed great, ran quiet, and the filter was always changed.

But going through all the hassle of doing the harness, you might as well do the pump as well, if you haven't already.

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