How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Mike S. Your Own Question
Mike S.
Mike S., ASE Certified Technician
Category: GM
Satisfied Customers: 7431
Experience:  ASE Certified with 25+ years experience.
20625509
Type Your GM Question Here...
Mike S. is online now
A new question is answered every 9 seconds

GM Sierra: Hey Mike, can you answer a question ? I

Customer Question

Hey Mike, can you answer a question for me?
I have a 20a ECM B fuse that continues to pop.
I've pulled both the fuel sensor and fuel pump individually, still pops.
I don't see anything with the wiring harness running from the junction box to the fuel pump.
Is there anything it could be that cause the fuse to blow?
Submitted: 2 years ago.
Category: GM
Expert:  Mike S. replied 2 years ago.
The ecm b fuse feeds not only the fuel pump relay, but also the pcm and the transmission control module.
All are orange wires and 2 of them to the pcm pins 20 and 57 on connector c1 and the other orange wire to pin 3 of connector c1 to the tcm.
Customer: replied 2 years ago.
Where should I look to see if there is a short? I've already ruled out the fuel pump.
Customer: replied 2 years ago.
I don't even need to start the engine. As soon as I turn the key, the ECM B 20a fuse pops.
Expert:  Mike S. replied 2 years ago.
Disconnect each of those orange wires one at a time and hook up an ammeter in the fuse's spot so you don't have to keep popping fuses.
Customer: replied 2 years ago.
Do I disconnect nearest from the junction box? Or is there an end point I should connect from?
Expert:  Mike S. replied 2 years ago.
Disconnect them at those pins I told you. Here is the wiring diagram.
Customer: replied 2 years ago.
I guess I'm a little confused on where exactly I'm suppose to disconnect from. From the junction box or from the ECM? Where is connector C1?
Expert:  Mike S. replied 2 years ago.
The connectors on the pcm and tcm are marked. Just disconnect the C1 connector on both the pcm and the tcm and see if the fuse still pops.
Customer: replied 2 years ago.
PCM pins 20 & 57 pulled. Didn't pop the fuse.
Expert:  Mike S. replied 2 years ago.
try one at a time
Customer: replied 2 years ago.
i put those pins back. going to pull the #3 from C1 of the TCM
Expert:  Mike S. replied 2 years ago.
why? you already found the problem. did you put those fuses back in one at a time? if so, when did the fuse blow this time?
Customer: replied 2 years ago.
I did not. I gotta pull away. family matter. can I pick this up tomorrow?
Customer: replied 2 years ago.
The truck still didn't turn over. Any idea of what it could be?
Expert:  Mike S. replied 2 years ago.
you did't mention anything about it not turning over
Expert:  Mike S. replied 2 years ago.
yes, till tomorrow
Customer: replied 2 years ago.
Mike, you there? So when I pulled the PCM pins the fuse did not pop, but because those pins were pulled, the truck still did not start. Still looking for an explanation and resolution.
Expert:  Mike S. replied 2 years ago.
You are correct though about the not turning over because the pcm was disconnected. I wanted you to hook those pins back up one at a time and see exactly which one it is causing the fuse to pop. Chances are though, it's going to end up being the pcm anyhow.
Customer: replied 2 years ago.
I pull each individually on the PCM and the ECM B fuse popped both times. Only when I pull both at the same time, the fuse doesn't pop. I've checked the wires from the PCM to the Junction box, nothing that would be 'burnt', 'frayed or grounded. I also ruled out the wires to the fuel system. What next?
Expert:  Mike S. replied 2 years ago.
I would check and make sure the pcm is grounded good and if so I am afraid to tell you the pcm is at fault and needs to be replaced.
Customer: replied 2 years ago.
How do I check to see if the PCM is grounded good? It's one of those things that get hardly touched. Which wire do I need to check?
Expert:  Mike S. replied 2 years ago.
There are 4 ground wires. All of them are colored black/white. The pins on the pcm are as follows, pins 1 and 40 on connector C1 and pins 1 and 40 on connector C2. There all 4 splice together at splice S103 and then go from that single black/white wire to the ground G103.
Expert:  Mike S. replied 2 years ago.
You should be able to test each ground circuit but I have to think here for a second because the fuse is still popping with the pcm hooked up and in order to check the voltage drops, I need power to the pcm.
Expert:  Mike S. replied 2 years ago.
I suppose the only way it to test without the circuit live. So disconnect that ground G103. If you don't know where it is, I'll try to locate it but I need more info on this vehicle first, such as which Sierra is it, the 1500, 2500, etc and anything else such as 2wd or 4wd. Then disconnect the 2 connectors at the pcm where theses grounds are, that would be connectors C1 and C2. Then using an ohmmeter check for continuity between the end at G103 on the wire and each pin ends at where you disconnected them from the pcm. They would be pins 1 and 40 on connector C1 and pins 1 and 40 on connector C2.
Expert:  Mike S. replied 2 years ago.
If they all check out ok and the connectors were tight and no loose or backed out pins, then the pcm is at fault.
Customer: replied 2 years ago.
I think it might be time to call the dealer to see if I can get the ECM re-loaded or perhaps get a brand new one.
My question of concern is this...how can a truck seemly operates fine one day, absolutely no issues, then rest overnight, and the next morning starts blowing a ECM B fuse?
Customer: replied 2 years ago.
BTW it's a 2001 GMC Sierra 1500 Ext Cab, 5.3L, 4x4 with Canadian specs.
Expert:  Mike S. replied 2 years ago.
I can think of at least one thing, mice. I had to throw away 4-5 wires going to and from my computer because mice have chewed through them. Now my speakers work again and who knows what problems I could blame on them damn mice.
Customer: replied 2 years ago.
I'm in the panhandle of Florida and I have never seen a mouse on my property in all the years I've been here. I would think just wear and tear but you would think there would have to be some other outside force to cause the short.
Customer: replied 2 years ago.
Would the oil pressure sensor cause any of this?
Expert:  Mike S. replied 2 years ago.
Oil pressure sensor? No, I don't think so.
Who knows what may have caused it. Perhaps a lightning strike, rust, another animal, saltwater, age, factory default, leaving the key on while doing electrical work, forgetting to tighten grounds after doing work, adding amplifiers, adding security systems, etc.
Customer: replied 2 years ago.
Cool. I'm going to connect with a dealer to see if I can get the ECM re-programmed or, if needed, get a new one. Just trying to diminish a repair bill.
Expert:  Mike S. replied 2 years ago.
Ok, let me know what they quote you. Parts and labor.
Expert:  Mike S. replied 2 years ago.
Well, I think I am done here. At least we narrowed it down. If you would rather be in contact with the dealer from now on that is up to you, but don't forget to rate me.