Check the switches on the brake pedal. #6 and #7, although I believe #6 is the one that affects the shifter interlock. Are they plugged in securely? Do the brake lights come on when the switch is activated?

Guys, can this switch be bypassed, as a test, or what is a good testing procedure?

 

Go to post #8 and start there.

 

My hunch is that this is where I'm going to find the problem. The battery voltage didn't seem like it was really the problem, especially since the car started fine before replacing the transmission solenoid and also because it started on a few occasions when I manually released the BTSI solenoid and put the shifter in neutral and/or removed the shift cable from the clip on top of the transmission and moved it around.

Just to review what I've already looked at in Post #8, I've checked various fuses in the underhood fuse box (and also the fuse box in the center console below the radio); I've checked the wires supplying power to the underhood fuse box and the have proper voltage; there's 12V at the starter on the red wire but no signal voltage on the purple wire; the dash lights come on as expected when I put the key in and turn it on.

It really seems to be some kind of problem with the park/neutral switch on top of the transmission. I tried the original switch in several different positions after reassembly of the transmission but couldn't get the car started. I replaced the switch with a new one from AutoZone and it would start with the shift cable disengaged from the clip on top of the transmission. I even bought the alignment tool to line up the switch and it wouldn't start after aligning the switch with the tool. I also tried adjusting the switch until the reverse lights came on and the car still wouldn't start when I tightened the switch down and moved it back to neutral or park. I'm starting to wonder if maybe there's some kind of broken wire in the harness going to the park/neutral switch or something.

I haven't checked the ignition cylinder yet but that will be next on my list, along with the brake switch. If I can verify both of those are working correctly that will allow me to concentrate on the rest of the system.

 

The ignition switch is digital (one wire), so doubtful since the electronics turn on.
It does start with the cable disconnected? That kind of narrows it down in my mind; something connected to the cable. The shifter is pretty cheap to replace, any year will work 2009 up is best.

 

Checking voltage can prove a battery is bad but not that it is good. Same goes for a load test. But a batter that can both pass a load test AND hold a charge in the high 12’s for weeks on is probably good. And in absence of a load tester, the ability to start the car several times is arguably a good enough load test. The important thing to understand is that passing a load test alone does NOT prove a battery is good. It can have plenty of power to start and run the car and still have a short in cell, and the battery will self-discharge to something on the order of 10.5V, be it in minutes or weeks.

 

At last—the long-awaited conclusion...

Back in May I was chasing down every possible thing that was suggested here on the forum. I checked the brake switch, replaced the battery, etc. but didn't get anywhere. After that, I got really busy with work which kept sending me out of town for a week or more at a time. In July I finally got tired of the car sitting there with no time to work on it so I had it towed to my local shop. They went through the car and replaced the park/neutral switch, put everything back together, and proclaimed it fixed. My wife picked the car up and started driving it and it only took a couple days for the problem to come back. I took it back to the shop and they replaced the shift cable. Apparently what was happening was that the shift cable was binding up and not letting the park/neutral switch go fully into park (or neutral) and that is why the car both wouldn't start and the shifter wouldn't release. With the new shift cable it's working perfectly and my wife no longer has to drive the winter car (2002 TrailBlazer) in the summer.

 

Thanks for posting the follow-up! Glad it's fixed!

 

Awesome! Thanks for the follow up