Fuse #10 PCM B blows instantly

[Sparky]

This morning I started the truck and started jotting down time, odometer reading, etc for my work travel log. While I was doing that the truck suddenly just shut down as if I had turned off the ignition. I turned the key to off, then back to on and noticed that the fuel gauge was on E and the check gauges light came on, although I know I had a half tank in there. Also noticed the SES light didn't come on. Tried turning to start and had nothing. OK fine, since the cluster is acting wonky I guess my original ignition switch finally died after 10+ years. Thankfully I hadn't put away my summer car quite yet and it wasn't snowing.

On my way back this evening I dropped into Autozone and bought a new ignition switch. I just installed it and it does the exact same thing. Figures. I start digging more and found that fuse #10, labeled PCM B (20A) is blown. Fuse #28 is PCM 1 and it is OK. If a PCM fuse is blown that can't be a good thing... Swapped it with a new one, turned the ignition, and the SES light came on for the test as it should. Tried to start, nothing. Released the key, no SES. Checked the fuse, blown. Also I smelled an burnt electronic smell. Awesome.

It is dark and I can't really see much so I'll have to come back to this in the daylight, but does anyone have any ideas what I should be looking for? I looked at the PCM itself and the wiring and it all looks OK.

For what it is worth, I started having a slow coolant leak from somewhere, but I have no idea where. It is just dripping off the bellhousing underneath but I don't know the exact source. Maybe it leaked onto something important...

I hit 166k miles with this truck and all of a sudden it goes full retard on me and starts giving me fits

 

[Sparky]

OK for anyone who comes across this, if you have a problem with fuse #10 blowing like this, odds are good that you have a fuel pump or pump harness issue. Pull the fuel pump relay, replace the fuse, and try cranking the vehicle again. Obviously won't be able to start but it will let you know if the pump/wiring is the cause. In my case I can smell burning from underneath the truck too right as the fuse pops.

 

[Mooseman]

Man, you've been having the worst luck with your truck lately. Was the pump ever replaced? If so, what brand? What about the relay itself? Maybe it's not making full contact and creating resistance. Maybe try swapping it. Next test would be to unplug the pump at the tank and see if it blows again. If not, then the wiring is probably OK and the pump has crapped out.

 

[Sparky]

As far as I'm aware the pump has never been replaced. I know it isn't the relay since the burnt circuit stench coming from the underside of the truck near the tank can't be caused by a bad relay.

If I could get to the pump I'd try unplugging it and putting a new fuse in, but I can't. I do think it'll be pretty clear what is bad once the tank is dropped however - a burning smell like that is bound to have a fairly obvious source I would think. Since I can smell it so strongly however it makes me suspect that it isn't the pump itself that is the problem. Would be a heck of a lot cheaper if I didn't have to replace the pump and just repair a connector or two! But then, if the connector is melting down it could be because of a shot pump with far too much current draw I guess...

 

[gmcman]

Have you traced the wires back to the fuel pump and checked for any chafed or shorted wires?

 

[Sparky]

Well it just happened today and this weekend was weird and I had to work, so I haven't had a chance to try. I can't really jack up the truck in gravel though. I'm not trusting the truck up on jackstands on gravel with me underneath

More than likely I'll have AAA come and tow it to the shop around the corner and have them deal with it. I'm tired of screwing with this thing and all these recent stupid little issues, I just want it to work properly darnit

 

[The_Roadie]

1) Electrical burning smell in the vicinity of the top of the plastic fuel tank would get my attention pretty fast.

2) If you're going to drop the tank to work on wiring, I'd do the pump at the same time. Don't want to be in there twice.

 

[Sparky]

Yeah, first time I smelled it I wasn't sure where it came from as I was looking inside the rear fuse box. Second time I realized where the odor came from. I considered just sticking a 50A fuse in there and seeing if that worked but thought that was a bad idea

 

[Adjustso3]

Hello. I am having the exact same problem with my 03 Trailblazer. What exactly was the problem ? Was it the wiring or the fuel pump or both ?

 

[Sparky]

Didn't realize I never posted the result of this.

It was the harness itself, the pump was fine. The harness runs along the driver side frame where there is a plug that then runs to the fuel pump. That plug had corroded, got warm, then shorted the contacts. Instead of replacing the whole harness I just cut the plug out and connected the wires directly, and weather sealed it. If I ever had to do the pump I could just unplug from the pump. Not sure the reason for the extra harness plug. Either way, never had a problem with it again, and never had to replace the fuel pump. The vehicle is long gone now though. I do miss it sometimes.

 

[Adjustso3]

Hello Sparky and thanks for the reply. So there is another plug other than the one that connects directly to the fuel pump. Where about is it located ? After reading your reply I assume you don’t have to drop the tank to access this plug ? Thanks again.

 

[Sparky]

Right. The main harness runs along the driver side frame, then somewhere around the rear door area there is a plug that another harness plugs into that runs to the fuel pump and plugs in there. What went bad on mine was the plug connecting those two harnesses. It is readily accessible without dropping anything.

Can't say for sure that's what went bad on yours, but it did on mine. Looked like the weather seal had gone bad, salty water got in, started corroding it, which then led to heat due to a poorer connection, which melted the plug until the pins shifted enough to touch and short out.

 

[Adjustso3]

Thank you very much Sparky for the reply. I will be getting my borrowed tools, jack, and jack stands this weekend, so I’ll be able to take a peek under the TB. My TB has running boards so it’s kinda hard to just crawl under there. The last time I had a 28 inch waist was about 30 years ago LOL ! Hopefully it’s just something simple like the TB you had. As soon as I get her running I’ll have to figure out why the key won’t turn back and come out and then figure out all the other electrical problems. Hopefully this will stop that #10 PCM B fuse from blowing. Thanks again.

Just in case you are curious about all the electrical problems I’ve been having, I have a thread titled “Electrical problems”. Thanks again. I appreciate all your time.