# Brake Light Issues

So my brake lights aren’t working on my 1997 Buick Park Avenue, not even my middle one. All my bulbs, fuses, relays are good. My tail lights and hazards work fine. The circuit has continuity when pedal is pressed and power going into the brake switch when pedal is not pressed, but when pressed it has no power going in or out it just cuts off. I figured it was probably a ground then but I checked and all of the grounds are good I’m at a loss and was wondering if anyone had any suggestions

This is a technical point, but you do not have power going to the brake light switch when it is not pressed, you have voltage. When the brake pedal is not pressed, the switch is open so there is no current flow. Power is voltage times current and when current is zero, anything multiplied by zero is zero.

There is a reason for making this distinction and it is not to embarrass you, it is to under stand what I am going to say next. Next we have to discuss Ohm’s Law. The first paragraph was a discussion of part of Ohm’s law. Other parts of the law include formulas that include resistance. One of those formulas is Voltage = Current X Resistance. If for example, I have the positive terminal of a 12 volt battery connected to one end of a 12 ohm resistor and the other end of the resistor is connected to ground (as is the Negative terminal of the battery), then one amp of current will flow through the resistor.

Now if I take a multimeter or voltmeter and connect one lead to the top of the resistor and the other lead to ground, I will read 12 volts. If I place one lead on the bottom of the resistor and the other lead to ground, I will read 0 volts as the only thing between the bottom of the resistor and ground is a short piece of wire, It would be like placing both leads on ground.

If I place one lead on the + terminal of the battery and the other to the top of the resistor, I will also read 0 volts as again the only thing between the leads is a short piece of wire, essentially the same as placing both leads on the + terminal.

Now lets make this a little more complicated by using two resistors. If both resistors are the same, lets say 12 ohms each and they are connected in series, that is + terminal to one lead of the first resistor, then the other lead of the first resistor to one lead of the second resistor and the remaining lead of the second resistor to ground. The total resistance is 24 ohms so only 1/2 amp of current flows.

Now if I place one terminal of the volt meter to ground and leave it there and then place the other lead to the top of the resistors, I get 12 volts. 1/2 amp X 24 ohms = 12 volts. If I place that lead to the junction between the resistors, I would read 6 volts. 1/2 amp X 12 ohms = 6 volts. If I placed the leads of the voltmeter, one to each side of the first resistor, I would again get 6 volts. Each resistor in this case would drop 6 volts for a total of 12 volts dropped between them.

Now lets go to an extreme. Lets say the first resistor is the wiring and fuses from the battery to the brake light switch and it is all good, so it reads 0 ohms and the second resistance is your brake light switch and brake light filaments combined. When the brake pedal is pressed, the total resistance of this circuit might be around 3 ohms. Since the brake light switch is 0 when closed, the only resistance left is the brake light filaments so a volt meter on either terminal of the brake light switch and ground will read 12 bolts.

When the brake pedal is not pressed, the switch becomes infinite resistance so no current flows so the supply voltage is felt on one side and 0 on the other. Now insert a very high resistance somewhere in the supply circuit. A blown fuse would be an infinite resistance, but since no current could flow through the fuse, 0 volts would be felt at either terminal of the brake light switch.

But if the resistance is very high, but not an open circuit like a blown fuse, but lets say on the order of 3k ohms of resistance. If you put the leads of the voltmeter between ground and the hot side of the brake light switch when open, the infinite resistance of the brake light switch drops all the voltage because the 3k ohm resistance is insignificant. But when the brake light switch is activated, now you have 3k ohms between the brake light switch and the battery, and 3 ohms between the brake light switch and ground.

12 volts divided by 3k ohms of resistance is only going to result in 0.004 amps of current flow. So from either terminal of the brake light switch to ground, the voltage drop will be 0.004 X 3 = .012 volts. Unless you select the millivolt range, you may see this as no voltage.

Bottom line, somewhere between the battery and the brake light switch, you have a bad (high resistance) connection. It could be a fuse with a hairline crack in it, the corroded blades of a fuse or a corroded terminal on the brake light switch. It would help to have a schematic of the brake light wiring.

My suggestion is to first find the fuse closest to the brake lights, or the fuse labeled brake lights and check the tops of the fuse. On all fuses, whether blade type used in all cars today or the round fuses used in older cars, you can touch the terminals on each side of the fuse when it is in place. Do the same test of checking the voltage between ground and each of the terminals of the fuse with both the brakes off and brakes on. If the voltage stays at 12v on both terminals with brakes on/off, the problem is between the fuse and the switch. If the voltage drops to zero on both sides of the fuse with brakes on, then it is between the fuse and the battery. If only one terminal drops to zero, then the fuse is bad.

I know that stuff by power i ment volts, there is 12v going into the hot end, and the other end is testing normal, when the switch is open. When closed aka pedal pressed from releasing the brake switch, the voltage for the hot wire goes to zero, and the other wise is zero there is no current. the switch is good and the circuit has continuity while pedal is pressed. So what your saying is, even tho the circuit is complete there is an interference that can still stop it?

Yes and it is between the switch and the battery.

Yeah it stays at 12v on both sides of the fuse with brakes on or off, i tried looking quite a bit i couldn’t find a brake light wiring schematic for my car. Any suggestions for locating the high resistance spot? I may be a complete idiot here, but would it just be easier to buy a fuse tap and add it and just put it in on a new fuse with a new hot wire ran to the brake switch? i don’t think that would have any issues working but im not exactly experienced with this type of stufff

If both sides of the fuse remain at 12 volts, then the high resistance is between the fuse and the switch. I would need a schematic to find all the possibilities, but I think the next thing is to disconnect the connector at the switch and check both sides for corrosion, bent pins or just crud.

You can spray them out with MAF cleaner and then reconnect and see if that makes a difference. Also check the wires going into the connector for a broken wire at the top of the pin. Tug on each individual wire.

If all looks good here and cleaning it doesn’t help, then you will need to find out if there are any more connectors between the switch and the fuse. Sometimes there is a wire bundle connector near where the wire bundle goes through the firewall, but that doesn’t apply if the fuse panel is under the dash.

If the fuse panel is under the dash, the next step after the brake light connector would be to pull the fuse panel down to where you can inspect the back side to see if a wire is almost broken off there.

From this point on, it only gets harder.