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2006 2500HD Duramax LLY 184,000 Miles – I was having a problem with a bad steering fluid leak and initially changed all 4 hoses which helped but did not completely fix the issue. Inspected and found that the Hydro Boost was leaking between it and the Brake master cylinder and went ahead and replaced the Hydro Boost unit.

That is where the problems began, after finishing the install, flushing the fluid and bleeding the system it was time for a test drive. I started the truck and could not shift out of park, I researched and found info about the brake light switch and checked that the brake lights were working, but decided to purchase a new brake light switch and give it a try.

When removing the stock switch it came apart and at that point I thought no big deal I have a new one. I installed the new switch a BWD SL2043 from Advance Auto and not only would it not shift out of park, there were no brake lights. So, I grabbed the OE switch and put it back together and installed and now we have brake lights but still cant shift out of park. I then tried a Duralast from AutoZone and a SMP from O’reilly and the same thing No Brake lights and can’t shift out of Park.

So I reinstall the stock switch and have been driving it the past few days with no problems except having to put it in neutral before starting it until yesterday when I tried to use the cruise control which would not work and then this morning the P0504 Check engine light came on. I just ordered both an AC Delco and a GM OE switch but to be honest I think I’m missing something and any help would be greatly appreciated.
 

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I had same problem. Changed booster, no cruise. Then, not going in gear, ect. Changed switch, then it worked awhile. Then cruise went out and then only brake lights when pressing real hard.
I finally figured this out. First, check the spring clip. Make sure you have it clipped in the clip groove top and bottom. It is common to have it clipped on bottom but not on top. This allows switch to move to much. Causing various problems. Next if that doesnt work, the booster rod is just a little different from your original. You will have to adjust the points inside the brake switch to open and close differently....just a little different. To do this, remove switch. Put it on the bench, unsnap the case to open the two brake switch halves open... this reveals the spades and 3 sets of point contacts. The first set of points nearest the clip end, is the shift interlock points. These are normally closed. When you push the pedal hard, they open, interrupting power to interlock solenoid. Thus releasing the shifter. Since the distance your new booster rod is moving the switch.... it isnt quite opening these points. Be careful and tweak these blades so less travel is required to open these points. Just dont tweak them to far.... cause then interlock function will not safely limit movement of shift lever. .... next set of points ( in middle) is the cruise control interrupt. These can be adjusted so they will open with less travel to interrupt cruise. ( this was my main problem) once i adjusted these, cruise started working. The last set, are normally open. They are the brake light points. When you press pedal, they close, turning brake lights on. They can be adjusted also to close with less travel. Once done, you can carefully close switch halves up, install it, make sure spring clip is in place good and test it. I tested mine with a ohm meter between terminals to see ifeverything was working with ease. Fix my problem completely.
If you need my diagram or pictures i can post some of inside of switch and terminals.
 
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