Shift to reverse issue. Car won't fully turn off, won't start, and reverse lights don't work. I believe I've tried everything suggested on the forums to no avail. Any help is appreciated. Please read my steps taken list before responding.

Steps already taken:

• (2) New reverse switches in transmission
• Tested reverse switches as operational. Reads open and closes when ball is pressed.
• Jumped both reverse switches - nothing happens. No reverse lights, and shift to reverse stays on
• Jumped across reverse lamp relay (backside of BCM) - this turns on the reverse lights successfully
• Aligned shifter linkage
• New battery (less than a month) - lives on battery tender, reads 12.9v
• Disconnected battery overnight
• Disconnected battery and touched + and - for an hour

How can I test the wiring between the BCM and the switches? How can I reset the ECM/BCM to not immediately turn on to ACC mode?

Any other tips?

 

It's years since I had this issue for the first time but I recall both of my switches were testing open and closed correctly but one of them had an internal short to ground. Tested good on the bench but when fitted grounded out. It doesn't sound likely to be the exact same if you have jumped them etc but have you tested the appropriate circuits for a short to ground?

When I found my ground issue and jumped it the column lock did it's thing and the car shut down straight away for the first time in weeks lol. I'm sure you know but it's the column lock preventing the car shutting down and because it can't shut down it can't start back up. The switches in the transmission are fed by a common 12V - when they are made by the act of selecting reverse one sends 12v to the back up lamps - the other to the SCLCM which then does it's thing with the lock etc. It is completely ridiculous design wise as a short to ground on either shorts out both.

If you haven't already i'd check you are actually getting battery voltage at the switch connectors and they aren't shorting..

Good luck

 

Thanks for chipping in! How did you go about testing for the 12v or a short? I hooked up my voltage meter to the reverse switch plugs and tried grounding the negative on the meter, but didn't get any reading.

 

Do you have the connector pin outs for the switches/BCM/SCLCM etc?

Forgive me if i'm hazy, thankfully it was almost a decade ago I had this issue, Chevy had the car for a week and couldn't fix it without my willingness to fire the parts cannon for $3k with no guarantees! I had it towed home and got stuck in with DMM and service manual. It was a $40 fix in the end.

If you have the pin out for the switch connector you need to be seeing battery voltage on whichever pin needs it. To check for continuity and ground faults I made a long thin gauge wire up with croc clips either end, this allows you to check wiring from end to end.

If the car is stuck in the 'shift to reverse' loop and the wheel hasn't locked then you should still see 12V on that circuit, I still had it after weeks and the car being towed about. That is where I would focus initially, proving/disproving 12V from the BCM to transmission circuit.

 

I do have the wiring diagrams and currently digesting them. According to the docs, C2 on the BCM has pin 49 (dark blue reverse switch supply voltage) and pin 69 (light blue reverse lamp relay control) - I've verified 12v on pin 49 to ground and tried jumping across the 2 at the BCM and get nothing. No lights and still says shift to reverse. I've also jumped across a second light blue wire which is the wiper motor relay and it works as it should.

I'm not sure what this is telling us at this point...

Next I'm going to try jacking up the car again and unplugging both reverse switches to make sure nothing is grounding out like you mentioned and then trying my above steps again.

What I can't find on the wiring diagram is a second reverse switch. Are they wired in parallel to the same terminals on the BCM?

 

2005 T56 has two trans detent pin sensors.

One is for the back up lights, while the other is the one telling the BCM if the trans is in reverse or not.


So would start by putting the trans in reverse, disconnect both detent sensors, and make sure that you have continuity through both sensor pins. If no, and both sensors are funny seated, then trans needs to come apart to correct the linkage that presses the sensors in when the trans in in reverse.

If you do have continuity, then time to chase the wires back up line to see where one is cracked.

If wires are fine, including checking the ends of the BCM connectors, them problem is in the BCM. So at that point, BCM board gets pulled from its case, and you start checking header pin soldering joints to the board, as well the rest of the board for problems as well. When pulling the board out, you need to drill the lips off the two metal gromits. the one that the red connector is going to thread into, make sure to drill the top grommet lip, while on the other one, drill the bottom lip to pull the rivet grommets. when the red connector is threaded back in, will be pulling up on the grommet and it needs the bottom lip, while the other one is to prevent moisture from coming in the top, so what the top grommit lip in play.

If you find that the BCM board has major water damage and its just too far gone to repair, then will need SPS to firmware flash marry in another BCM to the car. When using SPS, make sure to select "replace and reflash, and not just firmware up grade. The replace option will make sure during the flash/remarry, that all the modules are on the correct vin security code.

 

Dano, I think you mean 2005 so I'm just correcting that for future people who find this thread.

I did take the BCM apart and found no obvious cold solders or cracks.

I'm jumping pins 49+69 at the BCM C2 and get nothing. This should essentially take the switches and wiring out of question, correct?

 

Got her to shut down! Broken wire at the ECU in the passenger fender well. Looks like someone patched it with a wire extension and just twisted her together. I soldered it back up and good to go.

Still doesn't start, but at least she's shutting down fully.

Currently when I try to start, the dash just goes black for ~5+ seconds and then comes back with sweeping gauges. Also when I shut down and get out of the car, the car will go back into accessory when I shut the door.

 

49 to 69 is the back up light switch (reverse switch 2) to the BCM for the back up lights.


Its the other reverse switch 1 that goes back to the ecm that is the problem isntead. Hence ecm not telling the BCM that the trans in in Reverse, so the BCM can put the modules into sleep mode.

When I get a chance, will pull up gmsi to give you reverse switch 1 wiring diagram with it connector point back to the ecm.

 

Well now you have Dano on the case you're in safe hands, the man is a legend here.

I may have a massive misunderstanding here but hear me out! Pin 49 to 69 is gonna prove absolutely nothing in relation to your problem as that is the park signal related to the auto. My understanding is as follows -

You select 'off', BCM is looking for several signals to actually shut down the car. One of these is the signal from the SCLCM (behind IP) that the lock has successfully locked. The SCLCM gets it's 12V to do so from one of the two detent switches on the transmission (the other being for back up lamps). The 12V feed for the switches is common and splices just before the transmission switches and originates at pin 49.

I do not think you can jump the signal at the BCM because it is logic and time controlled.

So if you have 12V at pin 49 the next thing to do is to check the circuit from there to the reverse switches at the transmission for continuity and shorts to ground? If those check out I would then jump across each pin on those two connectors, not between connectors to simulate a successful pair of switches 'making' and then go from there.

All this talk of stripping down trannys and opening up BCM's is giving me the heebee geebees lol!!

How long you had the car? Do you know it's history well?

Honestly, if it were as easy as jumping across the bcm pins to simulate the column lock we'd have had a solution to that piece of crap a hell of a lot sooner.

Good luck

 

Pin 69 doubles as the park sensor for auto or the reverse sensor for manual. You can definitely just jump across them as it completes the 12v circuit which is all the sensor is doing. Same as I can jump across the windshield wipers or the turn signals to engage them at the BCM connections.

My issue was a broken wire in the harness in the passenger fender well. Soldered it back in and good to go.

I really appreciate all the brainstorming help. I'm now back in business so time to finish some race car things and hit the track.

 

Good to hear! Enjoy the track, i'll be out again soon too...

 

 

My MN6 2005 is doing the same thing. Persistent Shift To Reverse - won't go out of accessory mode. Where can I find drawings and info on the pin numbers for the modules mentioned above? BCM and SCLCM