Brake Switch - Park Lock

jpowel29

Member
Feb 1, 2008
780
0
16
Bentonville, Arkansas
I swapped in the LML hydroboost unit in my LBZ (which made a noticeable improvement in brake pedal feel). There seems to be far less pedal travel which I definitely prefer. The only downside that I have is that the Park Lock no longer wants to disengage properly. I changed out the brake switch and that made no difference. Anyone else run into this issue. The brake switch is working (brake lights work and I can see the switch signal working on my Solus Pro). I can turn the key forward one position (not into the run position) and the gear shifter works fine.

The brake switch is not adjustable. What else within the column/harness could be adjusted to help this?
 

jpowel29

Member
Feb 1, 2008
780
0
16
Bentonville, Arkansas
Not sure if this is BCM related or not. I found out that the Park Lock releases fine when I am not sitting in the driver's seat. I can apply the brake pedal standing outside the truck and the park lock releases fine. I've scrolled through most of the available sensor data using my Solus Pro but I guess I'll need to start troubleshooting at the seats or BCM.
 

DangerDog

Member
Jul 11, 2010
76
0
6
Mesa AZ
I thought I was the only one who ran into this problem with the LML hydroboost in the LBZ's... I don't know what exactly causes it, but the best me and my guys could tell is the switch isn't able to make/break the proper contacts in the switch itself.

What I've started doing is taking the switch out and opening it up, then bending the individual pins a little at a time, re-install the pin and manually press the switch portion just a little bit and see what happens. I mean a tiny tiny amount, once it's tweak to where all three of the contacts do what they should with minimal pressure, reassemble and reinstall. It's worked flawlessly for the trucks I've done it on. I'll see if I can get an extra switch or get pics of mine if you need them.
 

jpowel29

Member
Feb 1, 2008
780
0
16
Bentonville, Arkansas
I thought I was the only one who ran into this problem with the LML hydroboost in the LBZ's... I don't know what exactly causes it, but the best me and my guys could tell is the switch isn't able to make/break the proper contacts in the switch itself.

What I've started doing is taking the switch out and opening it up, then bending the individual pins a little at a time, re-install the pin and manually press the switch portion just a little bit and see what happens. I mean a tiny tiny amount, once it's tweak to where all three of the contacts do what they should with minimal pressure, reassemble and reinstall. It's worked flawlessly for the trucks I've done it on. I'll see if I can get an extra switch or get pics of mine if you need them.

That would be awesome - thank you
 

citadel

New member
Jan 2, 2008
46
0
0
TN
I'd be interested to see how this works for you. I have a similar issue with my 05 LLY and eventually pulled the fuse for a work around.
 

Trussman

JAFO
May 22, 2008
9
0
1
Wisconsin
I've been dealing with the same issue for a couple of years, but it only happens once in a while. And then it may happen nearly every time for a couple of days, then back to normal.
We have found that pushing the brake pedal to the right will disengage the lock. Have a new switch to try, but have not installed yet......
 

DangerDog

Member
Jul 11, 2010
76
0
6
Mesa AZ
First off, this is using a LB7 brake switch I had here that was floating around, the LBZ's are different IIRC but the idea/principals are the same.

First you have your brake switch, there are 5 tabs around it that hold it together. I gently apply pressure between the ears that go around the brake booster rod and start popping the tabs closest and work outwards. Light pressure and careful not to break the tabs.
9oV1P77.jpg


Here's the brake switch disassembled. Your first two contacts are normally closed(NC) and open when the brake pedal is pressed, the third contact is normally open(NO) and closes when pressed.
aQJErK6.jpg


The problem is the white/tan freckled plastic part doesn't actually touch the copper contacts that it's supposed to open and the LML hydroboost rod doesn't have as big of a step to push this part all the way compared to the LBZ IIRC.
fDBnWiG.jpg


This is about as far as the LML hydroboost rod can push the switch, you can see the NC contacts are still closed but the NO is now closed as well. If you hold it like this for 15 seconds I believe, you can get a break switch correlation code.
KYExoXk.jpg


I pull the NC copper contacts up and out to bend them slightly.
zDe8e3u.jpg



This pic is how they are when removed.
NXY23n5.jpg



I give them a slight S bend if you will, just enough to move the middle portion closer to the white/tan plastic tabs, and then the bottom half to where it's just touching the contact and isn't spring loaded against the gold contact still in the switch.
lERD5FM.jpg



Side by side comparison, modified on the left, factory on the right. I'll bend them a little and put them back in and test the switch by pressing on it slightly to see if they open under minimal movement. Usually takes me pulling the contacts out 4-5 times to get it perfect.
iC0iFQr.jpg



Example of how they should look at rest and with slight pressure to move it.
PuMt8mN.jpg

RaLdcC6.jpg


Of note, this spring is incredibly stiff, be careful cause it can spring out with a lot of force. I've removed it in some of these pictures to make it easier to show you what's going on, but usually I don't remove it to actually perform the modifications. The contacts can be lifted up and out without removing anything else.

Mods, if the pics are too big, let me know and I can resize them.
 

SoCalMike

Member
Dec 11, 2010
748
12
18
SoCal
From what I read, most guys that do the conversion drill out the hole in the guide rod on the lml booster a little bigger.
So when you press the brake pedal the rod on the booster can contact the switch.
 
  • Like
Reactions: Leadfoot

JoshH

Daggum farm truck
Staff member
Vendor/Sponsor
Feb 14, 2007
13,216
160
63
Texas!!!
You need to drill out the end of the rod that slides over the peg on the brake pedal. a 41/64" drill bit is the exact perfect size.
 

Leadfoot

Needs Bigger Tires!
Dec 27, 2006
855
12
18
45
Western MA
www.matpa.org
I had the same issue with the park interlock not engaging (would have to shut the truck off to shift out of park) and did the modded brake switch at first. I found that doing this worked great for the interlock, but I got pulled over and given a ticket for my brake lights not working (all part of that same switch). I did some testing and found my brake lights only working part of the time. I then modded the brake light circuit contacts (just like the interlock) and was OK for a few months but the switch ended up popping apart (probably from opening it up and weakening the tabs). I bought a new switch and had the same issues with the interlock and brake lights. Instead of popping this one open and modding, I decided to get to the root of the cause. This was years ago and I did not see anything related on the forums, and after some testing I came to the conclusion the plunger was not traveling far enough with the new booster rod. Instead of drilling the entire hole bigger, I decided to elongate it with a dremel (was able to do it in the truck under the dash...although not fun)and that worked mint. This way if the switch ever dies on me 10 years down the road (especially on a tow trip), I won't have to try to modify a new one in a parts store parking lot.

For those that have modded to get the transmission interlock to work, make sure your brake lights are working. Mine were only working about 50% of the time.

Anyone installing a booster, elongate the hole before installing as it will save a ton of grief trying to contort yourself under the dash to do it.
 

SoCalMike

Member
Dec 11, 2010
748
12
18
SoCal
I had the same issue with the park interlock not engaging (would have to shut the truck off to shift out of park) and did the modded brake switch at first. I found that doing this worked great for the interlock, but I got pulled over and given a ticket for my brake lights not working (all part of that same switch). I did some testing and found my brake lights only working part of the time. I then modded the brake light circuit contacts (just like the interlock) and was OK for a few months but the switch ended up popping apart (probably from opening it up and weakening the tabs). I bought a new switch and had the same issues with the interlock and brake lights. Instead of popping this one open and modding, I decided to get to the root of the cause. This was years ago and I did not see anything related on the forums, and after some testing I came to the conclusion the plunger was not traveling far enough with the new booster rod. Instead of drilling the entire hole bigger, I decided to elongate it with a dremel (was able to do it in the truck under the dash...although not fun)and that worked mint. This way if the switch ever dies on me 10 years down the road (especially on a tow trip), I won't have to try to modify a new one in a parts store parking lot.



For those that have modded to get the transmission interlock to work, make sure your brake lights are working. Mine were only working about 50% of the time.



Anyone installing a booster, elongate the hole before installing as it will save a ton of grief trying to contort yourself under the dash to do it.
Thank you for the information!
 

Pat Begley

New member
Dec 11, 2021
2
0
1
First off, this is using a LB7 brake switch I had here that was floating around, the LBZ's are different IIRC but the idea/principals are the same.

First you have your brake switch, there are 5 tabs around it that hold it together. I gently apply pressure between the ears that go around the brake booster rod and start popping the tabs closest and work outwards. Light pressure and careful not to break the tabs.
9oV1P77.jpg


Here's the brake switch disassembled. Your first two contacts are normally closed(NC) and open when the brake pedal is pressed, the third contact is normally open(NO) and closes when pressed.
aQJErK6.jpg


The problem is the white/tan freckled plastic part doesn't actually touch the copper contacts that it's supposed to open and the LML hydroboost rod doesn't have as big of a step to push this part all the way compared to the LBZ IIRC.
fDBnWiG.jpg


This is about as far as the LML hydroboost rod can push the switch, you can see the NC contacts are still closed but the NO is now closed as well. If you hold it like this for 15 seconds I believe, you can get a break switch correlation code.
KYExoXk.jpg


I pull the NC copper contacts up and out to bend them slightly.
zDe8e3u.jpg



This pic is how they are when removed.
NXY23n5.jpg



I give them a slight S bend if you will, just enough to move the middle portion closer to the white/tan plastic tabs, and then the bottom half to where it's just touching the contact and isn't spring loaded against the gold contact still in the switch.
lERD5FM.jpg



Side by side comparison, modified on the left, factory on the right. I'll bend them a little and put them back in and test the switch by pressing on it slightly to see if they open under minimal movement. Usually takes me pulling the contacts out 4-5 times to get it perfect.
iC0iFQr.jpg



Example of how they should look at rest and with slight pressure to move it.
PuMt8mN.jpg

RaLdcC6.jpg


Of note, this spring is incredibly stiff, be careful cause it can spring out with a lot of force. I've removed it in some of these pictures to make it easier to show you what's going on, but usually I don't remove it to actually perform the modifications. The contacts can be lifted up and out without removing anything else.

Mods, if the pics are too big, let me know and I can resize them.
This was a huge help. Just did the lml hydroboost upgrade in my LLY. Was stumped when I couldn’t get the truck out of park. Installed a new switch then found this post which got me through that issue.
However once I got the truck on the road I realized I lost cruise control. I know this switch is probably the cause. I messed with the switch a little more with no luck. I know I’m a little late in this thread but would love some advice
 

KyleC4

Tech
Dec 30, 2016
409
20
18
Bay Area
This was a huge help. Just did the lml hydroboost upgrade in my LLY. Was stumped when I couldn’t get the truck out of park. Installed a new switch then found this post which got me through that issue.
However once I got the truck on the road I realized I lost cruise control. I know this switch is probably the cause. I messed with the switch a little more with no luck. I know I’m a little late in this thread but would love some advice

You need a scan tool or just buy a new switch. It’s probably still seeing a brake pedal signal in one of the circuits and that’s not letting cruise control to engage. I’ve seen it also where it doesn’t allow tcc to engage. Josh’s mod to the pedal rod works for the half dozen trucks I’ve done this too. No need to mess with the brake switch as first done in this write up
 

Pat Begley

New member
Dec 11, 2021
2
0
1
You need a scan tool or just buy a new switch. It’s probably still seeing a brake pedal signal in one of the circuits and that’s not letting cruise control to engage. I’ve seen it also where it doesn’t allow tcc to engage. Josh’s mod to the pedal rod works for the half dozen trucks I’ve done this too. No need to mess with the brake switch as first done in this write up
Thanks! Is there a recommended brand for the switch or does it matter?
 

Leadfoot

Needs Bigger Tires!
Dec 27, 2006
855
12
18
45
Western MA
www.matpa.org
Thanks! Is there a recommended brand for the switch or does it matter?

I've bought from GM and an AC Delco from Autozone and they both worked great. Modding the booster push rod (quick with a drill or elongated with a dremel which is what I prefrer) would be your best bet as you won't have to fool with the switch and risk losing cruise control, Torque converter lock, or brake lights. It's a multi pin setup with several functions so just because one works, doesn't mean the rest do, that's why I advocate for the push rod mod vs switch mod.
 
  • Like
Reactions: 1TRIKHD