I discovered yesterday that the brake light switch on my '72 Supreme is hit or miss. Everything else is working as it should (turn signals, taillights and hazards), so I'm pretty sure it's the switch. Basic troubleshooting revealed that the plunger is working correctly, so I'm thinking that I need to replace the switch.

Am I able to remove the lower portion of the dash to make the job easier, or will I need to bend and twist my body in unforgiving ways?

 

Before you order a new one, make sure the adjustment is correct and the plunger is fully engaged at rest. You do not need to remove the dash, but you do need to position your body so you're not a contortionist. Kneeling on the ground with the door open, laying on the front seat or floor works for some. It's a fairly easy job.

 

Yep, checked the plunger too, and it's fully engaged at rest. I was also thinking that maybe I just need to clean the contacts in the switch and the connector.

 

They aren't repairable, but you may be able to spray some contact cleaner inside through the plunger end.

 

I will try that and order a new one in the meantime. Would you happen to have an AC Delco part #?

 

Removing the front seat will make the job alot easier.

 

Replacing the stop light switch is about as simple a thing as you can do. I've done it on several different cars. You will need to contort yourself to get down by the brake pedal. Just go to the nearest auto parts store and tell them you need a stop light switch for your car. Whatever brand it is is fine. These are very simple devices. One thing they will need to know is if the car has cruise control because, on cars with CC, there is also a vacuum line attachment on the switch that breaks vacuum and disrupts the cruise control when you step on the pedal.

Here is what Autozone shows for your car. They do not show an ACDelco part number for cars without cruise control. For that, you would have to settle for a house brand. I don't know if other vendors would have a Delco part.





This is huge overkill and completely unnecessary. It would take ten times as much effort to remove the seat as it would to just remove and replace the switch.

 

Would it be possible to remove the lower dash to make things easier?

 

Not necessary. The switch is way down low under the dash. Find the brake pedal and look closely at it. You'll see the switch mounted right up against the pedal arm. As has been noted, there is a plunger on it which is on a spring and which contacts the pedal arm and moves in and out as the pedal is pressed and then let up on.

Note the photo of the switch. The end is a long, threaded tube. The threaded part screws into a hole in the bracket, and you unscrew it to take it out. You adjust the switch by turning it in or out on that threaded shaft. You want it close to the pedal so that it engages the moment the pedal is pressed, but not so close that the brake lights stay on all the time. There should be a locking nut on the existing switch that you transfer to new switch. Once adjusted, tighten it against the switch to prevent it from going out of adjustment,

It really is quite simple once you get down there and take a look at everything,

 

I took a look at it last night when I was troubleshooting and my concern is the tight quarters...lol.

 

Everyone who is saying how easy it is to access clearly has not worked on a 70-72 olds A-body with AC. On just about any other car including a non-AC car, that switch is very easy to access. On the your Supreme however, (as I am sure you have seen already) the A/C ductwork is directly behind and under the brake switch. To answer your question, yes you can take off the "lap-cooler" vent panel on the lower area of the dash very easily. The problem however is the large plastic duct running under the steering column delivering air to the driver side dash vent. That too can easliy be removed, but it can be difficult getting the rectangular hose connecting the duct to the dash vent back on when you put it all back together. Hope this helps.

 

Just for knowledge sake, how does the switch work?

 

The brake switch itself is a normally closed electronic switch. Meaning, if you have the switch plugged into the harness but not mounted in the car; the switch will make a connection and the brake lights will be on. If you press in the plunger, the lights turn off. Once the switch is mounted, it makes contact with the brake pedal lever and turns the lights off. Once you step on the pedal, the plunger is allowed to come out and the brake lights turn on.

 

When you say there is a problem, what exactly is the problem? If the lights never come on, you likely have a bad switch. If the brake lights are on all the time and not going off, the switch is most likely too far away from to the pedal. If this is the case, you need to thread the switch in further toward the pedal. Note than the brake switch mount may be spot welded on, or held on with a 3/8 head self-tapping bolt. If held on with a bolt, the mount can get loose and move away from the pedal causing the lights to be on. One way or another the switch needs to be close enough to the pedal to turn off the lights. Also note, it is NOT possible for the switch to be "too-close" to the pedal as suggested above. The switch can be completely bottomed against the pedal lever and the lights will still be off. You may just have to press the pedal down further before the lights come on.

 

I get that part, but how does the plunger coming out complete the circuit?

 

If you truly understand what a normally closed switch is, then you just answered your own question.

 

The problem is that sometimes the brake lights come on and sometimes they don't. I know the plunger is fully depressed when I'm not on the brakes, so I'm pretty sure it's not an adjustment problem. How far does the plunger need to open for the brake lights to come on?

 

Apologies in advance for going into nerd mode.

It’s an electrical switch. There are no electronics in it, just mechanical contacts.

exit nerd mode.

 

Normally closed completes the circuit, correct? So, in my mind, when the plunger is fully depressed, that's the "closed" position, correct? Or am I wrong?

 

The plunger moves a piece of metal in the switch that makes and breaks contact with the two wires (opens and closes the circuit). It’s the same principle as any mechanical switch.

 

So instead of pushing the switch to close the circuit, in this case, the plunger releases and completes the circuit, is that right?

 

Yes.

 

That is called a "normally closed" switch vs. a "normally open" one.

 

The plunger is pushed in when the brake pedal is released and opens the circuit. The plunger is released by spring pressure when pressing on the brake pedal closing the circuit. If you removed the switch from its mounting bracket and left the wires connected the plunger would be extended and stop lights will be on. If you pushed the plunger in with your finger the stop lights will turn off. If it helps the stop light switch works the opposite way of a door bell switch.

 

Dave - As indicated by several members electrical switches are generally identified as NO (Normally Open) or NC (Normally Closed). You'll often see either NO or NC stamped/embossed/labeled on an electrical switch. A NO switch may (or may not) look identical to a NC switch. Many appear identical to each other. If I had a bucket of identical appearing switches I'd grab the type/style I needed (either NO or NC).

 

Here is a simple drawing showing the concept.

 

Makes sense now...thank you. How far does the plunger have to travel to close the circuit?

 

Keep in mind, the brake lights go thru the turn signal switch. So it’s possible you have a defective or intermittent fault with the turn signal switch.

Does everything else work consistently and reliably? How frequent is the No brake light issue? Hold down the brake pedal, wiggle and move the wiring under the dash while a helper watches the brake lights. See if you can get them to flicker.

 

Everything else works fine...no issues. As far as frequency, well, I dunno since I can't see them when I'm driving....lol. I'm going to do some more troubleshooting Thursday.

 

Troubleshooting is easy. Disconnect the connector from the brake pedal switch. The lights should be off at all times. Move the turn signal switch to right and left turn positions - only the lights on that side should flash. Now jumper across the terminals in the connector. The brake lights should be on. Again move the turn signal switch to right and then left. The selected side should flash, the other side should be on at all times. If this test works, everything downstream of the brake pedal switch is working correctly. If not, find the problem.

 

Everything else is working correctly Joe...I've already verified that. For now, I'm just trying to get an answer on how far the plunger should travel to complete the circuit.

 

Not far.

 

The weather cooperated today so I was able to dig a little deeper into the issue, and as it turns out, my hunch was on point.

I thought maybe when the car is shut off and no vacuum is going to the booster, I didn't have enough pedal travel for the switch to close. (That's why I asked about how far the plunger has to travel to close the circuit).

I started it up, and as soon as the vacuum kicked in on the booster, I was able to push the pedal down enough for the lights to work. I did it several times and checked both sides and all was good.

As a precaution, I did order a new switch, but as it turns out, I wont need it. Into my stash it will go.

So that's that with that, and once again, thanks to everyone for the advice.

 

 

While you may not need a new switch, I do not think all is quite good yet.

You should not need the engine started and the vacuum booster kicking in for you to be able to push the pedal down far enough for the brake lights to turn on. Those lights should go on the moment you even rest your foot on the pedal. In fact, they should go on when you start simply thinking about pressing the pedal.

While the pedal is difficult to press very far on a power brake-equipped car when the engine is off, it is normally possible to push it down at least a small amount, and that small amount should be enough to illuminate the brake lights. If they don't go on, adjust the switch so that it is slightly farther from the pedal arm. The plunger is pushed fully in when the pedal is not pressed and the brakes are fully disengaged. A slight pressure on the pedal moves the pedal arm slightly away from the switch, causing the plunger to move out slightly and turn on the brake lights. So to answer your earlier question about how much does the plunger need to extend to turn on the lights? The answer is very little.

 

I hear what you're saying, but as long as they come on when I'm driving, why does it matter if they don't when it's parked?

 

To reuse the figure above, you want the distance circled in red in this figure (distance between the plunger arms and the contact points) to be as small as possible when the brakes are not applied. It's made smaller by adjusting the switch position relative to the pedal arm, moving it away from the pedal arm to decrease the distance.

 

To extend this argument further, why have brake lights at all? You certainly don't need them to stop the car, right? Why do cars even have brake lights?

Brake lights are a SAFETY thing for you and the driver behind you. You want them to come on as soon as the pedal is tapped to signal to other drivers that you are applying the brakes, even just a small amount. You don't want to have to be standing on the brake pedal to be pushing it far enough to turn on the lights.

 

I appreciate the input, but I dont have to "stand" on the pedal for the lights to come on. They come on within 1/2"-3/4" of brake pedal travel when it'srunning, just not when I push on the pedal when parked and there's no vacuum assist. I've never had to adjust the switch, and it's been fine for 6 years since I've owned the car, so I'll leave it for now.

 

Again, vacuum assist should not be required to get the brake lights to come on. I've tested stop lights after replacing bulbs in the taillights many times, and I have my wife, who is 5' 3" and weighs 80 lbs, come out and sit in the car and press on the pedal while I'm behind the vehicle checking the lights for proper operation. We've never needed to start the engine for her to be able to press the pedal enough to turn the lights on.

1/2" of travel before the lights come on is fine. My point in all of this has been simply that brake lights coming on sooner rather than later is a good thing.

I looked in the 1972 Chassis Service Manual section on brakes, and there is no mention of pedal travel requirements. The only adjustment noted is to make sure that the lights are off when the pedal is fully released.

But this changed by the time of the 1978 manual, which I also have. It says that the lights should come between 3/8" and 5/8" of pedal travel. So your 1/2" is right square in the middle.