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2005 TrailBlazer Power Distribution issue...

6K views 22 replies 4 participants last post by  jsusnaim 
#1 · (Edited)
Good afternoon and thanks for taking the time to read my message today!
I am a long time reader, first time poster so.. here goes!

2005 Chevy Trailblazer

  • Occasional issue with Passlock symbol coming on dash, car not starting..
  • Bought a Passlock bypass unit and installed by the BCM
  • Bypass works great... except car still doesn't start sometimes and wasn't getting any power to the BCM / Bypass Module when this occurs..
  • Figured out no power was being sent back there
  • Figured out that if I remove the negative battery cable from car battery, wait a moment, and put it back on.. power starts flowing back there to the BCM & Bypass Module and car starts right up after doing this
(During the time that there is no power for the BCM / Bypass unit, also getting no power to the radio (unless its already turned on), interior lights, headlights (unless turned on manually), front windshield wipers, power door locks/unlock, and power window switches)

Reached out to support team of the bypass module and explained to them this trick of resetting the battery and they said the problem is not with the passlock or the bypass unit, but rather a "Power Distribution Issue"

I am thinking I will just bring it to the shop, as I have been dealing with this whole problem and making it work for over a year now... I'd be fine using the car in it's condition but I am having trouble getting a State Inspection done due to computer being reset all the time..
I always thought it was my modest electrician work with installing the bypass unit....hell I guess I've had it right this whole time lol.

Checked fuses under the hood and all seemed good... Do I need a new fuse box? Any advice on where this power that goes to the BCM comes from? What should I tell the mechanic? Just explain like I did here?

Thanks all!
Have a great day!
 
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#2 ·
There are many power lines to the BCM. Some are for the operation of the BCM itself, some are merely signals to the BCM so it knows the current key position, some are for powering devices the BCM operates, some are on all the time, some are on in accessory key position, some are on in the RUN key position, and so on, and so on.

57267
 
#3 ·
There are many power lines to the BCM. Some are for the operation of the BCM itself, some are merely signals to the BCM so it knows the current key position, some are for powering devices the BCM operates, some are on all the time, some are on in accessory key position, some are on in the RUN key position, and so on, and so on.

View attachment 57267
Well I had given in for a moment but now I am back!
Thank you @TJBaker for your reply and provided diagram.

The wire in question is the red/white "12VREF" wire going into Pin E9 to the Anti Theft System. During the times that the passlock light is enabled on the dash and the car won't start - I am getting no power through this wire checking it near the BCM. When the car does start I get power through this wire. I feel like I need to trace this wire back to the source? Is there any electrical diagram to help with this? I am having trouble locating electrical diagrams. I picked up a Haynes Repair Manual which has a "Starting and charging systems" diagram with some information but I can't seem to find the BCM on this diagram. I will check the starter relay this weekend just to make sure. Any additional advice is immensely appreciated. Thanks again.

Rectangle Schematic Font Slope Parallel




Additional thoughts after studying your provided diagram a bit further..

Trying to learn how to / make sure I am reading the diagram you provided.

So, from what I think I understand is:

1. Power from the battery goes into the under the hood fuse box
2. TBC 1 FUSE 31 10A Fuse is provided power which goes to the BCM via the ORG (Orange?) wire.
3. This powers the BCM.
4. BCM Sends power out of Pin E9 via "12V REF" Red/White wire -> to the Anti-Theft System?

Is this right? This whole time I was just assuming power was going into the BCM using that red/white wire. Or is it the other way around.. Sorry for my low level understanding of these electronics.
 
#6 ·
There appears to be a discrepancy between your written instructions and the hand drawn schematic. In the latter you show terminal C2, E8 and the brown wire from the bypass module going to ground, but in the connection directions no mention of a ground connection exists for these two wires. Factory wiring for C2,E8 is to the low reference terminal of the Passlock sensor. A low reference is not the same as a ground. It is close, but not equal.
 
#7 ·
Ah, I simply assumed the black wire from C2 E8 went to ground from one of your last posts. That's why I did draw it that way

As I think you figured out now the red/white wire (Connector 2, pin E9) supplies the 12 volts from the BCM to the Passlock sensor circuit to power that device. The black wire (connector 2, pin E8)
(sometimes orange/black) supplies the ground.
 
#8 ·
Now I am wondering.. the passlock sensor has 3 wires, a yellow (disconnected), a orange/black, and the red/white. I believe the red/white is the 12V ref discussed from C2 Pin E9 but is that orange/black the one you are talking about? It is only black coming out of the BCM
 
#11 ·
wontonstrips, you need to get yourself a set of the following - they make back probing easy! I wish I had purchased them years ago!!!


or


Good Luck!
 
#17 ·
Finally getting around to post... So I did get a set of backprobes and was able to do some tests -
I backprobed the orange wire going into BCM Connector 1, Pin A3 and it had consistent power at all times, even while I was experiencing the problem... So all good there.


I found my problem at the passlock sensor near the ignition.

During the time I am having the problem with the passlock light illuminated on the dash I am getting no power at the red/white wire going into the passlock sensor near ignition.
I suppose this means my issue is with the red/white wire getting power from the BCM but could also be a ground?


Not sure what I will do next but I feel I am getting a little closer to the source of the problem haha. Thanks and happy holidays!

Here's a few pictures from when I was testing.


Image below: 0.00 Volts at passlock sensor - Engine: Off, Key Position: On, Problem: Yes

Speedometer Vehicle Trip computer Odometer Tachometer




Image below: 11.18 Volts at passlock sensor - Engine: Off, Key Position: On, Problem: No

Speedometer Vehicle Tachometer Trip computer Odometer







Image below: 13.08 VDC at passlock sensor - Engine: On, Key Position: On, Problem: No

Speedometer Trip computer Vehicle Tachometer Odometer
 
#18 ·
Edited while composing because I saw something while writing a response!!
--------------------------


Not likely a ground since I assume you are testing with the same ground point each time?? A chassis ground?

I would now check for 12 volts at the BCM end of that red/white wire. If you find that at the same time you have power to the BCM and have no power at the Passlock sensor but you do have power at the BCM end of that same wire then clearly there is a bad connection or wire damage somewhere between the two ends.

When you tested for power to the orange wire at the BCM did you use a chassis point for a meter ground or use a BCM terminal for ground? If using a chassis point when testing for power at that orange wire that still leaves the possibility of the BCM having a faulty ground connection you see.


--------------------------

So I was composing a response when something clicked. I looked back at your first image where you have the issue of no Passlock sensor supply voltage. Look at the battery gauge. Does not show a voltage!! That gauge is driven by serial communications from the BCM to the Instrument Cluster. Your BCM is not active in the picture! With the key in any position other than OFF that BCM should be up and running but the lack of a voltage gauge reading at the very minimum suggest the BCM is not awake and operating.

Back at message #2 in this discussion there is an image. On the top there is a power source labelled "TBC Ig Fuse 50 3A", "Hot in ACC, RUN, OR START" . That leads to terminal A18 on connector 3, "Ignition 0 Voltage". This is the power signal that alerts the BCM to awaken and get ready for business. You may have an issue there in that ribbon cable. You can test for voltage at the BCM end of that flat ribbon cable. Here in this image I am probing terminal A11, a ground. You want to test A18 for 12 volts whenever the key is not OFF. In particular, test when the issue is present.

Circuit component Green Automotive lighting Electrical wiring Hardware programmer
 
#19 ·
First off - Can't express how grateful I am for all of your responses @TJBaker - Thank you!

Yes when testing the passlock sensor I used the same ground each time - chassis ground
When testing the orange wire at BCM I used a chassis ground as well.


Response to your edit:
Yes when the problem is happening the battery gauge goes down to 0.

Sometimes the car starts fine no problems occurring - then while I am driving, bam battery gauge on the instrument cluster falls down to 0, passlock light pops on.
And as I continue driving the battery gauge turns on and off swinging up to 14V and down to 0 along with the voltage on that red/white wire. (and along with the other symptoms.. headlights, interior lighting, backlight on instrument cluster, rear power windows..etc.)

I will look into testing this Connector 3 thank you!!
 
#20 ·
While you are there testing for power at C3:A18 it may be wise to test for a good consistent ground at C3:A11. The diagram at post #2 above shows two grounds for the BCM. Yes, there are two wires for ground at connector C3. One is at A11 and the other is at B11. Access is easy to A11 but B11 is not so easily accessed. HOWEVER!!... Internally they are in fact the same as they immediately join the same PCB tracing as seen here...

Passive circuit component Circuit component Resistor Hardware programmer Electronic instrument


So for a quick test one can just check A11 for either continuity to chassis ground or test for power and ground across the large red battery power wire to the rear fuseblock and BCM C3:A11.

One would have to disconnect the flat cable from the BCM if one wished to verify both of the ground wires separately from the BCM flat connector #3 for a proper path to ground.
 
#21 ·
Got in the car yesterday and the problem was occurring so I got out my meter and went to work..

First I put the black lead to chassis ground and probed A18 with the red lead and got 12V

Then I put the red lead to that large red battery power wire at the rear fuse block and probed A11 with the black lead and it's kind of strange because I took a video while doing it and I thought for a second my meter read less than 1V, so i just recounted the pins made sure I was on A11 and probed again this time only getting 12V consistently..

Thought maybe I wasn't making a good connection with the probe I was struggling for a moment there holding the camera lol. But in the video it looks like I was on the correct pin each time and it seemed like I was making good connection so.. I don't know? Probably should try testing again... see photos below

Maybe I can try the continuity test on there but I'm not sure if I know the proper way to do it. To test continuity I should not do it on a live circuit correct?

So to test the continuity:
I should disconnect the vehicle's battery completely, put my meter to continuity, attach black lead to chassis ground, and attach red lead to pin A11?


Here are some snapshots from my work

Image below: Probing A18 - Result: 12V
Light Blue Electrical wiring Automotive tire Gadget




Image below: Probing A11 - Result: Inconclusive?? Not sure if I was making a good connection or not at the time??
Hood Automotive lighting Automotive mirror Gadget Finger



Image below: Probing A11 again after the previous image was taken, I readjusted the way I was holding my camera, counted the pins and retested several times and got 12V every time..
Green Gadget Electrical wiring Audio equipment Electronic engineering



After doing those tests the car still wouldn't start so I performed the magic trick of disconnecting my negative battery cable under the hood for a couple minutes, popped her back on and away I went to the grocery store. Lol


Next time I will try testing at the BCM end of that red/white wire for 12V and will try testing A11 again just to be sure. If I am correct in the way I described above about how to test the continuity of A11 then I will try that as well.

Thank you!
 
#22 ·
This is the nature of intermittent faults and why it is so difficult to conclusively repair them. Chasing ghosts. When you tested and got the non-12V reading between battery and A11 you may have seen the fault there for a moment. Or maybe not. Was it the positioning of the test leads or was it the fault?? So difficult to prove one way or the other sometimes. That flat green ribbon cable is somewhat easy to remove for closer inspection of the terminal ends. The upper part has two locking tabs at the ends and the lower portion has a locked slider that clamps it into place. I have seen two pictures of this cable where a fault was present at the lower ends but these were permanent faults. One had a burnt out trace and the other had some gooey foreign substance spilled into the connector. Sometimes you just have to keep looking and systematically disassemble, inspect, clean and reassemble each potential fault point both power and grounds. The red battery power connection seen in this first picture.... Has that been removed, cleaned, reassembled?

Electrical wiring Bag Automotive exterior Bumper Cable


Hand Automotive tire Automotive lighting Hood Motor vehicle


Motor vehicle Bumper Finger Automotive exterior Auto part
 
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