Truck won't start after changing the transmission

Make sure battery has a full charge and connections all good, on all ends, not just at the battery. Make sure you check all grounds, engine ground--chassis ground--whatever. When you turn the key, do the dash lamps come on? Use a test light at your fuse box, check voltage at fuses, some fuses are hot all the time, some go hot with the key, check them both ways. You may need to check fusible links where applicable. With testing there are ways to bypass everything just to see if starter motor will crank the engine, that doesn't mean it will start. If you try that make sure tranny is in park or neutral and parking brake is set. As you can see in starter diagram, there are fuses to check. Also, check for voltage at fuses.

I just wanted to add, I'm not a tranny person. good luck

Now unless the transmission was locked up before installing which I doubt the problem most likely has something to do with the neutral safety switch install go over the install again making sure everything is hooked up correctly and securely

I was told by a friend whom used to work as a mechanic at our local chevy dealer that if I change my transmission out I will have to take it to have programed for my truck, not sure how true that is. I have a 2007 silverado

We have the same problem with our truck and we haven't figured it out yet.

First you want to check that something is causing it not to turn over put a socket and ratchet onto crank shaft bolt and turn clock wise should turn freely if not reverse ratchet and try to spin if moves a half turn then stops something is causing it to not turn over had this happen before check bolts that attach flywheel to torque converter one maybe loose and bolt hitting side of transmission may not be your problem but something is hitting causing not to turn over.

I just changed my transmission in a 2005 Silverado and it turns over but won’t start

Just changed transmission in 01 Silverado 4x4 put new neutral safety switch on truck wont start with key. Put wires and toggle switch to starter relay it will start and die immediately with toggle switch but if you hold key in start position and use toggle it will start and run what do I need to do to make it work right?

If you jump the starter solenoid it will start and run. Sometimes depending on the transmission the valve body will need to be swapped but out if your old valve body is good swap it and it will fire properly. BUT also the neutral safety switch also needs to be moved around by someone under the car to getnit positioned correctly once you’re able to start from key tighten the bolts down.

Just did mine refurbished valve body TCM assembly. 09 Suburban 6l80 trans. Started drove but park assistance reverse lights doors unlocked in park aren't working. Drive it home from the shop shifts great. Better than ever since I have owned it. Got home put tools away and started it back up drove and parked it out front. Got up for work and not Even a click. Battery is fill power everything lights up it's like it's not seeing park or neutral. I downloaded the tune from the old trans tcu before I removed it. Went to flash the old tune into the new valve body and keep getting the dreaded unlock time out fail. 06.5 to 09 are the same operating system 10 to 13 are the same but neither will work in the other years forward or backward. 6l80 has the shift selector switch in the pan attached to the valve body bolts in without adjustment.

Hello Finch_S, what ended up being your fix? I’ve also done a TCM/Valve Body swap and I am having the same issue now after driving the truck 2-3 times it is not recognizing gear selection so it will not start.