NEED HELP No power to fuse 47 but starts and runs fine

[Scubasteve]

Hello,

New here but been reading a lot lately for my recent problem(s).

Title pretty much sums it up where it sits right now. I can't find what feeds power TO fuse 47. I've been getting the 5 transmission solenoid codes and stuck key intermittently for a week, now constantly. Recently replaced BCM, ignition switch with AC Delco, ignition lock cylinder (broke when replacing switch) all due to random no-starts . I had power to fuse 47 the other day when the codes first came up. Non-DD 2002 Trailblazer 4x2. We've had all kinds of electrical gremlins with this thing. I'm a a retired avionics technician so no problem tracking it down with a schematic or a hint of where to go from here. Thanks in advance.

Steve

 

[TJBaker57]

Hello,

New here but been reading a lot lately for my recent problem(s).

Title pretty much sums it up where it sits right now. I can't find what feeds power TO fuse 47. I've been getting the 5 transmission solenoid codes and stuck key intermittently for a week, now constantly. Recently replaced BCM, ignition switch with AC Delco, ignition lock cylinder (broke when replacing switch) all due to random no-starts . I had power to fuse 47 the other day when the codes first came up. Non-DD 2002 Trailblazer 4x2. We've had all kinds of electrical gremlins with this thing. I'm a a retired avionics technician so no problem tracking it down with a schematic or a hint of where to go from here. Thanks in advance.

Steve

This will get you going. You can download manuals with wiring diagrams by following the link in @Mooseman signature.

 

[Scubasteve]

Thank you. I was just digging through those and saw you replied. I guess I need to go to the ign switch. Per the schematic and symptoms it could only be the switch or wiring between. I'm going to try to manually feed 12v into fuse 47 and see if everything works... it should.

 

[Mooseman]

That same leg also supplies power to fuse 50, which would cause a lot of things to not work.

 

[Scubasteve]

Comfirmed. Tapped a constant 12v source and fed it straight into the load side of fuse 47 socket. No codes, key releases.

Mooseman, just saw your last post and confirmed no power to fuse 50 either. I guess I'll try a switch, i need to have one around anyway and I don't feel like taking it apart to troubleshoot wiring without one on hand.

 

[Scubasteve]

Okay, new development... I can't find anything NOT working even though there's no power at fuse 50. Everything I can think of related to the BCM works fine. That makes no sense.

I just test drove it 2 miles with power injected into fuse 47 and everything's fine, shifts perfect, no codes, key turns off & releases.

 

[budwich]

BUT if you are feeding power into fuse 47 then unless you pulled fuse 50, you are backfeeding circuits on fuse 50 so in essence, you are also feeding fuse 50. Ultimately, it would appear that you have a problem feeding that rear fuse box location.

 

[Scubasteve]

Agreed. It was late and I was tired so I can't recall if I checked power to fuse 50 while I had the jumper in 47. I'll check again tonight. Ign switch will be here tomorrow and I'll troubleshoot from that end as well. What SHOULDN'T be working with no power to fuse 50? I can't find that info anywhere as there's multiple power inputs to the BCM. All interior accessories, lights, radio, etc worked even before I jumped the power over to 47. I'll check again and update later. Thanks all so far!

 

[Mooseman]

There are multiple power feeds to the BCM. It may just be a status or voltage monitor for it. But still, you'd think it would throw a code but then you likely don't have a scanner capable of scanning B codes.

My money is on the ignition switch. Check the output of the current switch to confirm.

 

[budwich]

Its likely a "status line"... probably for "start" status. Again, checking fusing AND ignition switch power pin stati, you need to go thru each switch position and verify the power condition on each pin out at the switch as paritally shown in the posted diagrams. Go from the result. I bet the "start" position / contact is good, otherwise you would not get start but the other positions at that switch area is broken.

 

[Scubasteve]

Mooseman - Correct, only scanner I have is a cheapie OBDII.

Mooseman/budwich - That seems like the only logical explanation. I'm going back in the column tonight to test the switch now that I have one on the way.

 

[TJBaker57]

Correct, only scanner I have is a cheapie OBDII.

If that cheapie is a bluetooth ELM327 clone then that and Car Gauge Pro will read all your codes. Clunky interface, but it works.

I don't see it mentioned... Have you checked out IGN A fuse #34, a 40 amp J-Case fuse in the underhood fuseblock?

 

[Scubasteve]

If that cheapie is a bluetooth ELM327 clone then that and Car Gauge Pro will read all your codes. Clunky interface, but it works.

I don't see it mentioned... Have you checked out IGN A fuse #34, a 40 amp J-Case fuse in the underhood fuseblock?

I have that but using a plug in unit right now. No problem reading standard obdII codes, hence how I wound up looking at fuse 47. I did check all ign fuses in the underhood box last night & they were good, including 34. Makes sense since everything else is working. Hopefully the weather holds long enough to get into the ignition switch tonight.

 

[Scubasteve]

Well... I'm a big dumb dummy. Seems when I replaced the ignition switch, I was off a tooth. It worked for a while & then the weird stuff happened. That, coupled with what seems like an intermittent start relay seems to cover all of my problems. I started whacking relays the other day when it wouldn't start and it cranked right up. That relay is probably the main cause of everything. I do think the BCM was bad when I replaced it, that was due to random battery drains and random accessory/inst cluster not working. Its been a long road. The combination of all those things and my own failure to properly index the ignition switch had me chasing my tail. I'll be back with the next thing I'm sure. Until then, thank you all for the help and Mooseman for the awesome resources!