Brake light switch intermittently works

JDEVRIES64

MEMBER
2007 CHEVROLET EXPRESS
6.0L
V8
2WD
AUTOMATIC
150,000 MILES

Brake lights (left, right and CHMSL) stopped working a few days ago.
I replaced all of the bulbs, and fuses (uhbec and bbec). Nothing.
Tapped into the right rear brake lamp (brown wire) and injected 12vdc directly from the battery. All of the brake lights came on. So confirmed that they work.
Installed new brake switch. Brake lights worked. Then became intermittent.
If I vigorously manipulate the brake pedal the brake lights will work occasionally.

Background: about a year ago I replaced the brake booster and while doing so I notice that the brake booster linkage to brake pedal pin fitment is relatively tight. Not much more than 0.010-0.020" of clearance. The brake lights wouldn't work and the transmission shift lock wouldn't release the shifter. I changed out the brake switch at that time and it all worked, but I was concerned about the tightness of the interface then.

Back to today: in taking the older brake switch (one I just replaced) apart I see that the total switch travel is closer to 0.080" and I see that the switch has to travel around 0.040-0.050" before the contacts either make or break.
I believe that the normally open contact inside the switch is for the transmission shift lock and the the other two normally closed contacts are for the brake and something else (not sure).
I can confirm that the transmission shift lock is releasing the shifter when I depress the brake so the switch must be working.
I also can can confirm that if I depress the brake pedal relatively hard the brake lights (right, left, CHMSL) will come on, but then they turn off while holding brake pedal position. If I depress the pedal further the brake lights come on and switch off again. I can make this happen 4-5 times while depressing the pedal further each time.

Thoughts?

7 Replies

JACOBANDNICKOLAS

MECHANIC
108,269 POSTS

I have to be honest, it sounds like the switch is bad again. As far as the clearance, it doesn't seem off. Are you sure nothing has come loose? (I realize you seem to know what you are looking for, but I need to ask). Here are the directions from Alldata showing how to install. Make sure nothing on the switch retainer is loose.
________________________________________
2007 Chevy Truck Express 1500 RWD V8-5.3L
Stop Lamp Switch Replacement
Vehicle Sensors and Switches Sensors and Switches - Lighting and Horns Brake Light Switch Service and Repair Removal and Replacement Stop Lamp Switch Replacement
STOP LAMP SWITCH REPLACEMENT
STOP LAMP SWITCH REPLACEMENT

REMOVAL PROCEDURE

1. CAUTION: Refer to Battery Disconnect Caution.

Disconnect the battery negative cable.

Pic 1

2. Disconnect the electrical connector from the stop lamp switch.
3. Remove the retainer from the brake pedal pin.
4. Remove the stop lamp switch and the pushrod to the power brake booster from the brake pedal pin.
5. Unsnap the stop lamp switch from the power brake booster pushrod.

INSTALLATION PROCEDURE

pic 2

1. Snap the stop lamp switch onto the pushrod to the power brake booster.
2. Install the stop lamp switch with the pushrod on the brake pedal pin.
3. Install brake switch retainer.
4. Connect the electrical connector to the stop lamp switch.
5. Connect the battery negative cable.

___________________________

Let me know.
Joe

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Saturday, February 29th, 2020 AT 8:35 PM

JDEVRIES64

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21 POSTS

Joe,
Thanks for the quick reply.
Yes, everything is tight including the Brake Switch retaining clip and bolt that retains the clip.

Here’s what I’m going to do:
1. Modify the contacts on the year old switch and test.
2, get another switch and test.

I’ll report back my findings.

Thanks, Jim

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Saturday, February 29th, 2020 AT 8:49 PM

JACOBANDNICKOLAS

MECHANIC
108,269 POSTS

Jim,

I took a look through schematics. Interestingly, there are only two wires shown from the switch. Once gets power and the other sends. Thus, you have to have a splice in them at some point. Since the interlock actuates, the brake light switch is getting power. I'm questioning if something is corroded near the switch and causing the issue. I realize it changed based on pedal pressure, but could you be moving any wiring under the dash? Also, at this point I can't tell you why, there are two fuses related. The interlock gets power from the BTSI fuse (pic 1) and there is another fuse for the brake lights (pic 2). Is this a cutaway van by chance?

Let me know.
Joe

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Saturday, February 29th, 2020 AT 9:15 PM

JDEVRIES64

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21 POSTS

Joe,
I think I solved it.
Looking at the photo of the switch (attached), I bent contact "C" a small amount close to its base in the direction of the plunger finger that moves the contact so that when the switch plunger moves it actuates contact "C" sooner. The plunger now only needs to move 0.020" before the contact opens.
When I plugged it in the brake lights come on every time I press the pedal.

Side Note: I hadn't mentioned it before, because I wasn't sure, but when I pressed the brake pedal I could hear a relay cycling. I'm thinking that contact "C" was making an breaking so much and was causing the relay to cycle. I believe this to be the relay that combines the turn and brake signals. Now with the brake lights solidly coming on the relay cycling is gone.

Answers to your questions:
All of the wires seem are tight and none are chafed so I don't think that's the problem.
The van is a 3500 LS 12 Passenger, so not a cutaway.

So here's where we're at:
Brake Lights Work (Right, Left, CHMSL)
Turn Signals work
Turn Signals work when Brake Lights on
Transmission Interlock works correctly (engine off and on)

Thanks for your help!
Much appreciated.

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Sunday, March 1st, 2020 AT 8:48 AM

JACOBANDNICKOLAS

MECHANIC
108,269 POSTS

I'm glad to hear you got it fixed. By the way, looking at the pic (great job by the way), it appears that the schematic I was looking at was a bit different. Regardless, take care and please feel free to come back anytime.

Thanks for getting back to me as well.

Joe