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I saw a video on YouTube where a guy was checking for power at the red and orange wires leading to the ignition switch. I also saw another video where a guy was checking for power at the orange white and yellow wires.

Anyone know the proper way to test the ignition switch? I'm trying to hunt down some electrical grimlins. (AC blower inoperative/ wheel speed sensor codes) all happened at the same time. After reading up on the issues..I'm leaning towards the ignition switch being a possibility. The AC blower resistor looks good (no burnt wires or connectors) but I haven't actually tested it yet.
 

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2002 chevy trailblazer_lt
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There are two power inputs to the switch. One goes to terminal F and the other goes to terminal B. With the switch OFF only these two should have power.

There are 5 different load circuits in the switch.

When the key is turned, the very first contacts closed are from terminal F to terminal E. These close even before you get to the ACCessory position. When these close a IGN0 signal power is sent to both the BCM and the PCM and power is also sent to the shift solenoids and the automatic transmission shift interlock switch. The battery light in the dash turns on and the PRND321 display and odometer lights up.

Next circuit to close is terminal B to terminal A. This sends an ACCesssory voltage power to the BCM and power to the front wipers, a DVD player if so equipped, and outside moisture sensors if equipped. Also the BCM sends out a network control message that alerts the modules in the vehicle to wake up and enables things like the dash lights, radio, etc.

That completes the switching from OFF to Accessory.

As the switch is moved toward RUN from ACCessory the next contacts to close connect terminal G to terminal F along with the already powered terminal E. So F and E and G are all together now. This sends signal power to the HVAC module, the TCCM, the HVAC actuators, the front axle actuator, and the turn signal multifunction switch.

Next up is terminal C which connects with terminals B and A. This sends and IGN 1 signal to the PCM and BCM. Also powers the fuel injectors, ignition coils, cooling fan relay, heated O2 sensors, evap system solenoids, SIR module, another terminal on the shift lever, stop lamp switch and a boatload of other things. Again the BCM sends out another network control command telling modules what state of operation is desired.

That gets you to the RUN position.

When the key is turned to CRANK, terminal A DIS-connects from power at terminals B and C, terminal G also DIS-connects from terminal F and E, and terminal D connects to power at terminals F, E. Terminal D is the crank signal that goes to the PCM indicating a request to start. The two terminals that disconnect do so to allow maximum power to go to the starter.

It is worth a mention here that this terminal D crank signal is only that, a signal. It does NOT supply any power to switch anything on. It is only an indication sent to the PCM that you would like to startup. In fact, there are a lot of these connections that are not supplying operating power to anything. They are just signals indicating the desired mode of operation. All the modules like the PCM, BCM, TCCM, the HVAC, the Radio, the Amp, the various door modules etc all have their own unswitched power ALL THE TIME, even with the key out.

There are things that the ignition switch supplies the operating power to, like the coils, fuel injectors, shift solenoids and so on, but not all the ignition switch circuits supply operating power.




Order of connections
OFF------
1stF-E---
ACCY 2ndF-EB-A
3rdF-E-GB-A
RUN 4thF-E-GB-A-C
START FinalF-E-DB-C
PoweredF= Red, Ign A fuse #34B= Red/White, Ign B Fuse #36
LoadsE= White, Acc, Run, StartA= Brown, Acc, Run
LoadsG= Orange, RunC= Pink, Run, Start
LoadsD= Yellow, Start
 

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A small sidebar question TJ:

I have noticed that I can quickly flip my key to the START position and I can release it and the starter will continue to crank until the engine is running.

Is that normal?
 

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A small sidebar question TJ:

I have noticed that I can quickly flip my key to the START position and I can release it and the starter will continue to crank until the engine is running.

Is that normal?

That is normal except for very early models like mine that didn't have that programming. I have read that this functionality can be added to my own, built in October of 2001, by updating the PCM calibration but I think I actually prefer mine the way it is. Since the crank signal from terminal D, the yellow wire to the PCM through the crank fuse is only a request of sorts there's no real need for it to be continous during startup.

However,,, wonder about the other circuits having those two otherwise disconnected circuits reconnected during cranking.. hmmm...
 

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2006 Chevrolet Trailblazer LS
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Interesting .......................
You want to really get freaked out?
Probably four times over 15 years, my 2006 TB 4.2L LS has misfired on startup, stalled, and (instantly) re-cranked and re-fired itself!
Now that's freaky!
 

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I guess "something "wasn't satisfied and insisted on completing what had been initiated.

No "failure to launch" mebee?
 

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I guess "something "wasn't satisfied and insisted on completing what had been initiated.

No "failure to launch" mebee?
No idea what, but obviously there is something the PCM looks for to determine the engine is running.
 
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