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kalamykid, Shop Owner/ Tech
Category: Chevy
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2005 chevy Tahoe 4WD. Recently replaced the brake master

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2005 chevy Tahoe 4WD. Recently replaced the brake master cylinder and hydroboost and BCM. got it all back together and now the brake lights are miswired somehow. with engine off, the brake lights are always on, press the brake and they turn off, so it is behaving backwards. I replaced the brake switch and the wiring harness connector and no change. turn signals, hazards, backup lamps all work normal, it is only the brake lights.
JA: Have you checked the brake fluid? How's the level and when was it last changed?
Customer: brake fluid is full and is brand new. new fluid went in with the new master cylinder. it is some kind of electrical problem.
JA: Are you fixing your Tahoe yourself? What have you tried so far?
Customer: yes I am. I replaced the brake switch and the wiring harness to it, but still brakes lights always on, when the brake switch is depressed they turn off.
JA: Anything else you want the mechanic to know before I connect you?
Customer: i think that's it. thank you.

Can you send me a pic or two of the brake light switch mounted please? It sounds like you have it connected to the brake pedal incorrectly.

Customer: replied 1 month ago.
It is not even mounted on the pedal right now. It is just sitting there with nothing touching it, brake lights are on. When I manually depress the button on the switch like the rod does when you push on the brake pedal, they go out. It was doing the same thing when I had it mounted. Push the brake pedal and they go out. Otherwise always on even when engine is off.

remount the switch and let me see how you have it mounted. The switch is working the way it should. The push rod for the brake booster pushes the switch closed when the pedal is not depressed. I believe that you are mounting the switch 108 degrees the wrong direction. makes sure the switches button is over the push rod, not over the clip that holds the push rod and switch on.

Customer: replied 1 month ago.
I had the pushrod in contact with the button when it's mounted, pushrod in between the two clips on the switch mount. So when you push the pedal the rod depresses into the button on the switch, is that how it's supposed to work?

No. The push rod should push the button when the pedal is not being pushed, and the button should be let out when the pedal is depressed.

Please reconnect the switch and send me a pic of it so I can see how you have it connected. If I can see it I can tell you how it should be connected.

With no pressure on the switch, the switch will turn the lights on. The push rod MUST push the button in to turn the lights off.

Customer: replied 1 month ago.
Ok I must be mounting it wrong then. Attached some pictures. I do not think the button is depressed with it mounted like this but I can't tell for sure.

Do you have it plugged up in this position? Do the brake lights still come on with the pedal not depressed?

Customer: replied 1 month ago.
They are always on and when I depress the brake pedal they turn off. So it's behaving backwards. I have two new switches one a delco and one a duralast and they both do the same thing.

I believe that you have the switch turned 1/4 turn to high. Can you turn the switch so it is going up and down and not straight across?

Can you take another pic this time with the push rod on the pin, but the switch not installed?

Customer: replied 1 month ago.
Here is with switch not installed. Which way do I turn it? Does it matter?

Turn the switch so the plug is on the bottom.

Customer: replied 1 month ago.
a half turn, still have it going straight across like it was and not up and down?


Customer: replied 1 month ago.
No change. When I line up the hole on the switch and the hole on the pushrod, the rod is not depressing the button. Like it is not long enough at the end. I would think I would need to push it a little to get them to align otherwise the button won't be depressed while it's sitting on the pedal. This pushrod is part of a new hydriboost unit I just put in, possible it came with the wrong rod?

That is possible. Do you have the old booster?

Customer: replied 1 month ago.
No, returned it for the core charge. I will have to see if the pick n pull has one. Can I just pull the pushrod out from the inside cabin without removing the whole booster again? I think it just pulls out and pushes back in if I remember right

I think the push rod that connects to the pedal is kept in the booster with a snap ring.

Customer: replied 1 month ago.
Ok. I kind of doubt I can buy only the pushrod. If I can't find one, what if I just glue a piece of plastic onto the switch button to make it 'taller' so the rod actually depresses it when it's mounted. Should work?

You would have to make sure that when you push the pedal the switch will open so the lights will work, but yes that should work.

kalamykid and 4 other Chevy Specialists are ready to help you
Customer: replied 1 month ago.
Ok thank you for your help and education on how the switch works. Looks like the switch is fine and the issue is with how the pushrod is contacting it. I can take it from here.

I believe you are correct. The push rod is incorrect for your truck. Let me know if you need any more help.