Reverse lights don't work after manual trans swap

[RussellZ]

Hello folks, 

 

First post here. I know I'm treading in fairly uncharted waters, but I am hoping someone here may have some advice for me.

 

I have a 2003 Cadillac Escalade that I have turned into something of an offroad toy. I've swapped out the IFS for a D60 solid front axle and the 10 bolt out with a full float 14 bolt, 3 link suspension front / rear with king coilovers and air bumps. I removed the LQ9 6.0L/4l65e/AWD t-case and installed a replacement 2005 LLY Duramax diesel engine with a ZF6 six speed manual transmission and a 29 spline NP241 manual transfer case. I am very familiar with swapping these engines around, my last build was a 95 Chevy Tahoe with an 02 LB7 Duramax / ZF6 combo. After some effort involving ripping the entire dash out and re-working the entire I/P harness to convert it to manual, relocating the Bose amp to the rear quarter panel above the spare tire tools to allow for a jump seat and installing the pedals I wound up with an install that you could more or less take to GM and have them service it. The dash has no warning lights on at all and everything works right down to the VATS system. 

 

Except for my reverse lights. I am totally stumped.

 

Using my Tech 2, I can see that both the ECM and the BCM report the manual trans rev switch as ON when I put the trans into reverse, and OFF when I take it out again. I've verified that there is 12V at the BCM and at the ECM input pins with the transmission in reverse. I've also verified that the TBC-B fuse in the I/P fuse block is good (my dome light works fine). I also checked all the other battery, IGN 1, 2 and 3 inputs to the BCM along with all the grounds. When I unlock the truck with my remote and the perimeter lighting is on, the reverse lights turn on. I can also turn the reverse lights on manually with the Tech 2 and they work just fine. The only thing I can see wrong with the Tech 2 is that the Reverse light call PID remains INACTIVE when I put the transmission into reverse.

 

I was having some issues with the BCM not sending out the power master mode commands properly which was causing a battery drain. I figured perhaps the BCM had a fault internally which was causing it not to turn the reverse lights on. I installed a replacement BCM and programmed it using TIS2WEB tonight. This fixed the battery draw and got the PMM stuff working properly again, but the reverse lights still don't work. 

I should mention that the lights worked fine with the auto trans before I started the swap.

Does anyone know if perhaps the BCM is waiting for a status flag from the PCM before it will turn the reverse lights on? Is there any other conditions that need to be met to turn them on outside of the key being in RUN and the transmission in reverse?

 

Any suggestions would be highly appreciated!

[Doug_Scott]

May sound stupid, but, have you tried checking with the engine running, trans in reverse, and either a second person to check or done at night close to a building and checking for reflection? 

 

Don't know what has driven GM to screw with light functions.  Try checking brake lights by yourself.  

[RussellZ]

Thanks for the reply Doug. I can confirm that I had the same thought and checked it with the engine running just in case the TBC was looking for a run flag to enable the reverse lights. No joy...

[Doug_Scott]

Is it possible that there was a switch on steering column that was used for backup lights that is not being used now?  Have you tried putting the column shifter into reverse? 

You must have done some major wiring changes to have the missing ECM and TCM to be accepted by the BCM. 

How did you get the clutch pedal mounted, or is it simple because they had the 5 speed still behind the v6? 

[RussellZ]

On ‎2018‎-‎08‎-‎02 at 2:11 PM, Doug_Scott said:

Is it possible that there was a switch on steering column that was used for backup lights that is not being used now?  Have you tried putting the column shifter into reverse? 

You must have done some major wiring changes to have the missing ECM and TCM to be accepted by the BCM. 

How did you get the clutch pedal mounted, or is it simple because they had the 5 speed still behind the v6? 

The truck was completely stripped down and the I/P harness reworked using the manual trans wiring diagrams from GM. I used the donor truck's I/P harness for all the wiring and connectors.

 

The donor truck's pedals were a bolt in less one support bracket on the inside of the firewall. I cut the bracket out of the donor and welded it into the caddy.

 

The caddy's original NSBU switch was on the transmission, not the column. All that wiring has been completely removed and replaced with the donor truck's wiring as well.

 

I did find a GM tech who was able to find out that the reverse light enable flag comes from the PCM to the BCM over the serial network. While I have not heard of it being an issue for anyone else, I guess the Duramax PCM writes that bit to a different address than the LS PCM used to.

 

With no way to confirm that, I have decided to install a relay to operate the backup lights. I will hook the wiring out to the backup cam, sensors and lamps up to the common contact, the NC contacts to the BCM output and the NO to a fused ignition source. The coil will be powered by the reverse switch.

 

This will allow the BCM to control the reverse lights whenever the truck is not in reverse for perimeter lighting etc, but will let the reverse switch disconnect the wiring from the BCM and connect them to an ignition supply to turn them on when it is in reverse.

 

 

[Doug_Scott]

I was going to suggest just tapping into the reverse indicator wire from transmission and using it to trigger a relay to power the backup lights.  Didn't they used to also turn on the cargo lights when backing up? 

Don't know what prices are like in the states for relays, but in Canada they are not readily available cheap.  I use Aliexpress to get them now.  You can get 5 pin relays(with NO and NC pins) for under $2 each with free shipping.  I ordered a half dozen the last time.