Where the brake light switch sits and what it does
The brake light switch on your 1990 Dodge Ram B350 is a small cylindrical component mounted on the brake pedal assembly under the dashboard. When you press the brake pedal, the switch closes an electrical circuit that tells your brake lights to turn on. When you release the pedal, the switch opens and the lights turn off. If your brake lights stay on constantly, flicker, or don't come on at all, the switch is often the culprit.
This switch is different from your anti-lock brake system (ABS) sensor or your brake fluid level sensor — it's purely electrical and controls only the visible brake lights at the rear of the truck. A faulty switch won't affect your actual braking power, but it will make your truck unsafe to drive because other drivers won't see when you're slowing down.
Key Takeaways
- The brake light switch on a 1990 B350 mounts directly to the brake pedal assembly and costs between $15 and $40 for the part itself.
- You can reach and replace it without removing the dashboard or getting under the truck — the job takes 15 to 30 minutes with basic hand tools.
- The switch has two wire connectors that pull straight off; take a photo before disconnecting them so you reconnect them in the right order.
- After installation, test the lights by turning on the ignition and pressing the brake pedal before driving the truck.
Identifying a failing brake light switch
A brake light switch fails in a few recognizable ways. Your brake lights may stay on even when the pedal is released, draining your battery over time. They may flicker when you hit bumps or turn corners. Or they may not illuminate at all when you press the pedal, even though the bulbs are good and your battery is charged.
To rule out other causes, check that your brake fluid level is adequate (low fluid can trigger a separate warning light but not affect the brake lights themselves) and that your brake light bulbs are not burned out. If the bulbs are fine and your battery is good, the switch is the most likely source of the problem.
What you'll need to gather before you start
You need a replacement brake light switch for a 1990 Dodge Ram B350. The original Dodge part number is 4874370, but aftermarket switches from suppliers like Standard Motor Products or Motorcraft will work and often cost less. Buy the switch before you start so you have it on hand.
Gather a small flathead screwdriver, a Phillips screwdriver, and a flashlight or headlamp. You may also want a small container to hold the wire connectors so they don't get lost. No special tools or a lift are required — you'll work from inside the cab with the door open.
Locating and accessing the switch under the dashboard
Sit in the driver's seat and look up at the brake pedal assembly. The switch is a small cylinder about 1.5 inches long, mounted on a bracket just above where the pedal rod enters the firewall. You'll see two wire connectors plugged into the back of it. The switch may be held in place by a clip or a single bolt, depending on the year and production batch.
If you have trouble seeing it, turn on your headlights and use a flashlight to illuminate the area. The switch is usually painted black or dark gray and blends into the pedal assembly. Once you spot it, you have clear access from inside the cab — no need to remove trim panels or get under the truck.
Removing the old switch and installing the new one
Before you disconnect anything, take a photo of the two wire connectors so you know which one goes where. The connectors are usually different sizes, which helps, but a photo is insurance against mixing them up. Gently pull each connector straight back off the switch terminals — they should come free with light pressure.
If the switch is held by a clip, pry it gently with a flathead screwdriver. If it's held by a bolt, remove the bolt with the appropriate wrench or socket. Slide the old switch out of its bracket. Insert the new switch into the bracket and find it with the clip or bolt. Reconnect the two wire connectors in the same order they came off, using your photo as a reference.
Press each connector firmly until you hear or feel a click — the connection should be snug and not pull off with light tugging. If a connector feels loose, remove it and check that the terminal inside is clean and not bent. Dirt or corrosion can prevent a good connection.
Testing the new switch before you drive
Turn on the ignition without starting the engine. The dashboard lights should come on. Now press the brake pedal and watch your brake lights in a mirror or have someone stand behind the truck and watch them. The lights should turn on when ready when you press the pedal and turn off when you release it.
If the lights don't come on, turn off the ignition and check that both wire connectors are fully seated. If they are and the lights still don't work, the new switch may be defective — this is rare but possible. Return it to the supplier and request a replacement. If the lights work correctly, you're done. Start the engine and take a short test drive to confirm the lights respond normally to braking.
Common mistakes to avoid
The most common error is reversing the two wire connectors. The connectors are usually different sizes, so they won't physically fit in the wrong spots, but if you force them or if your switch has connectors that are the same size, you can create a short circuit. Always photograph the original setup before disconnecting anything.
Another mistake is over-tightening the mounting bolt. The switch body is plastic and can crack if you torque the bolt too hard. Tighten it just enough that the switch doesn't move when you wiggle it — you should not need a wrench to explore heavy force. If the switch is held by a clip, make sure the clip is fully seated in its groove so the switch doesn't rattle loose over time.
Frequently Asked Questions
Can a bad brake light switch cause the brake pedal to feel soft or spongy?
No. A faulty brake light switch affects only the electrical circuit that powers your brake lights. It has no connection to the hydraulic system that makes your brakes work. If your pedal feels soft or spongy, the issue is air in the brake lines, low brake fluid, or a problem with the master cylinder — not the light switch.
Do I need to bleed the brakes after replacing the switch?
No. The brake light switch is mounted on the pedal assembly but does not touch any brake fluid lines or hydraulic components. Replacing it does not require bleeding the brakes or disturbing the brake system in any way.
What if the brake lights still don't work after I install the new switch?
Check that both wire connectors are fully seated and that the battery is charged. If the lights still don't work, the issue may be a blown fuse, a corroded wire, or a burned-out bulb rather than the switch itself. Consult your owner's manual to locate the brake light fuse and check that it's intact. If the fuse is good and the bulbs are good, have a mechanic test the wiring with a multimeter.
How long does a brake light switch usually last?
Most switches last 100,000 miles or more, but age and moisture can cause them to fail earlier. A 1990 truck is now over 30 years old, so the original switch may straightforward be worn out. Replacement switches are inexpensive and should last another decade or longer if installed correctly.