Start off I'm a tech at a shop that specializes in wiring issues. A customer has brought in an 04 Trailblazer with a really odd issue and I'm wondering if any of the deep knowledge people have heard of anything like this. Let the truck sit for a period, get in turn key no start no spin. Doesn't matter where you put the shifter same thing. Headlight switch has an LED that's amber. Turn counter clockwise and LED turns green. Release switch, now turn key and starts as normal. Allow time to sit and returns to no spin/amber LED. It had an aftermarket alarm, that's removed now and wires repaired. Also had a code/health reader on OBD2 port, no longer in place. Anybody had similar happen?
2004 trailblazer no start till turn headlight switch
- Thread starter Lil' b
- Start date
Blckshdw
Moderator
That's a new one on me. So after removing the alarm system and code reader, you still need the headlight override switch to start the truck? What happens if you disconnect the plugs on the back of the headlight switch assembly? Does the starter crank at all then?
Mooseman
Moderator
I thought I had heard of a similar issue but it was actually the other way around where it would stall when the high beams were activated:
gmtnation.com
Turned out to be a grounding issue. Maybe the same on this one?
05 trailblazer stalls when high beams turned on
Heard of this problem online but having trouble finding solution. 1. Trailblazer stalls when high beams turned on. 2. Turn off beams, truck restarts. 3. Drivers side turn signal wont blink but will dimly light up. 4. Drivers side low beam won't work when truck is driving but will when in park...
gmtnation.com
Turned out to be a grounding issue. Maybe the same on this one?
In that video you can see display before and after light switch is turned. The body control module has been replaced, does same thing with either.
Seems to be not completing a cycle unless the switch is moved.
Maverick6587
Member
It definitely sounds like a wiring issue. I'm wondering which system it's grounding out with though. The anti-theft system or the starter. Can or did you verify that you hear the fuel pump priming?
I'm not sure if grounding out with the anti-theft system would or would not turn the security light on though.
I would check to see where/if there is a 5v reference loss in one of those systems though to narrow the search down. Just some thoughts of mine. Seems like a rough one to troubleshoot.
I'm not sure if grounding out with the anti-theft system would or would not turn the security light on though.
I would check to see where/if there is a 5v reference loss in one of those systems though to narrow the search down. Just some thoughts of mine. Seems like a rough one to troubleshoot.
I'm not the only tech on this one either. Just got confirmation that cluster and light switch were also swapped before I was called on. Yes sir this is a good one. It takes about an hour to set. As long as you are driving it or messing with the key it won't set. Turn the key off and walk away for an hour it does.
Maverick6587
Member
I was just looking at the ground distribution page (pages 65-76) in the service manual. It looks like (from what I've found so far) the headlamps and the ignition switch alarm share tan wire. The headlamps also share a ground with the park/neutral safety switch.
It looks like that headlamp switch is running off of the G201 ground. I would bet that it's somewhere in between the ignition switch and the headlamp switch though.
I've seen that before from South Main Auto on a trailblazer. The wires that connect to the ignition switch connector become lose fairly frequently from what I've seen at least. When it's a no start/no crank issue that is.
It looks like that headlamp switch is running off of the G201 ground. I would bet that it's somewhere in between the ignition switch and the headlamp switch though.
I've seen that before from South Main Auto on a trailblazer. The wires that connect to the ignition switch connector become lose fairly frequently from what I've seen at least. When it's a no start/no crank issue that is.
"oh my kingdom for a multimeter" 
you might be wise to check for some voltages (and grounds) at some places along the start circuit. My guess right now is that the pcm is not getting the proper voltage conditions to satisfy its operational checks which then prevent it from sending a ground to pull up the start relay.... but that is just a guess without ANY info on electrical conditions in the circuit.
As for the other observations with the light switch, check which voltages are showing up where on the switch and compare to a circuit diagram to see what would be normal at what time / state where they come from. It might give you a clue as to where things sit.
further... of course, there is also the "usual suspect" of the ignition switch is bad... a common failure for this vehicle along with a less than good battery... the initial indicator at key on with the "battery" lit isn't a good sign.... probably a bad connection at the battery terminals.
you might be wise to check for some voltages (and grounds) at some places along the start circuit. My guess right now is that the pcm is not getting the proper voltage conditions to satisfy its operational checks which then prevent it from sending a ground to pull up the start relay.... but that is just a guess without ANY info on electrical conditions in the circuit.
As for the other observations with the light switch, check which voltages are showing up where on the switch and compare to a circuit diagram to see what would be normal at what time / state where they come from. It might give you a clue as to where things sit.
further... of course, there is also the "usual suspect" of the ignition switch is bad... a common failure for this vehicle along with a less than good battery... the initial indicator at key on with the "battery" lit isn't a good sign.... probably a bad connection at the battery terminals.
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To assist you with Wire Tracing to and from the PCM... View Post #4 in this Link:
gmtnation.com
...and if a Parasitic Draw becomes involved:
gmtnation.com
...and if you are deciphering the removed wiring of an off-brand Remote Starter or Alarm System:
BCM pinout
Looking to pick up on the reverse lamp circuit at the BCM. I've found sources that indicate it may be a light green wire, but there are several of them back there. One of which is a com wire that didn't take kindly to having a test light on it... anyone have a pinout for the connectors or a...
gmtnation.com
...and if a Parasitic Draw becomes involved:
Tutorial: The BEST Procedures for Performing a Parasitic Draw Test
If you keep suffering from the "Dreaded Dead Battery" problem at the most inconvenient of times...which means ALWAYS... THIS Video has laid out the very best procedures and links to related Fuse-Type vs, Voltage Drops vs. Amperage Loss Tables and the Best means to seek and find any Parasitic...
gmtnation.com
...and if you are deciphering the removed wiring of an off-brand Remote Starter or Alarm System:
With the headlight switch unplugged there seems to be no problem, it doesn't set. Cluster goes through all the cycles and it starts. There is another switch coming but I'm going to look into the ground and power in this area in more detail.
Blckshdw
Moderator
That's good, then at least now you have a more targeted list of circuits to trace through, if the power and ground sources don't reveal anything.
Sorry I am new here not sure if this thread is even active anymore but long story short I am having the same issue was wondering if anyone figured out the problem. Thank you
Blckshdw
Moderator
The OP said the problem went away after disconnecting the headlight control switch, and had a new one on the way. Since they haven't been back to the site since that post, I assume that was the issue. Have you tried that test yet?
Check out this entire Epic Thread... but pay particular attention to Post #5 concerning the attached image showing the Shared Ground of G-107 the Left Headlight, the Left Turn Signal AND the Fuel Pump Relay that all conspire to cause the Fuel Pump Relay to Drop Out and Stall the Engine if the Ground at G-107 on the Driver's Side of the Engine Block is either Corroded or Loose:
gmtnation.com
Check the related Under Hood Power Distribution Center Fuses and Fuel Pump Relay Pins for any signs of Electrolytic or Galvanic Corrosion as well.
Electrical Ground locations
While troubleshooting a bad ground I came across some scans of the service manual regarding electrical ground locations for our platform posted on another site. This includes both short and long wheel base, as well as both 4.2L and 5.3L. Credit goes to Wex...
gmtnation.com
Check the related Under Hood Power Distribution Center Fuses and Fuel Pump Relay Pins for any signs of Electrolytic or Galvanic Corrosion as well.