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Problem:

Premature pad wear ranging from mild to severe.

Cause:

While there are many causes of premature pad wear (see our previous Blog posts on this), stoplight switch adjustment can be a common one. It is common enough to have been included in several TSB’s written over the years.

Incorrect stoplight switch adjustment can prevent the brake pedal from returning to a point that allows the cup seals in the master cylinder to open the vent ports as shown in Figure 12.1.

Figure 12.1 Vent Ports

The vent ports allow for fluid expansion when the brakes get hot. If they are not open a residual pressure will be created due to the expanding fluid. The residual pressure will cause the calipers to apply the brake pads slightly and will result in premature pad wear.

Solution:

Vehicles with adjustable stoplight switches will use one of two varieties, adjustable body type or self-ratcheting plunger type. Each is covered below:

Adjustable body type

1. Verify complaint – if there is drag on both front wheels (or all four wheels on 4-wheel disc brake systems) start by loosening the master cylinder away from the vacuum booster by at least .”. This step checks for anything in front of the master cylinder that would be preventing the master cylinder pistons from fully releasing.

2. If the wheels release the stoplight switch adjustment should be checked. Either follow the manufacturer’s procedures or continue with the next step.

3. Duplicate the wheel drag and then loosen the locknut on the stoplight switch. (Figure 12.2)

Figure 12.2 Locknut Style

4. Back the locknuts off to move the stoplight switch away from the brake pedal. Check the brake drag. If wheels released then stop light switch was source of problem. If wheels did not release see page 96 for more information.

5. Adjust the switch by using the locknuts to position it to a point where the plunger is depressed (verify by checking if brake lights are off) making sure pressure is not being applied to brake pedal that would prevent release. (Figure 12.3)

Figure 12.3 Properly Adjusted Switch

Ratcheting Plunger Type (Figure 12.4)

Figure 12.4 Ratchet Style

1. Verify complaint – if there is drag on both front wheels (or all four wheels on 4-wheel disc brake systems) start by loosening the master cylinder away from the vacuum booster by at least. This step checks for anything in front of the master cylinder that would be preventing the master cylinder pistons from fully releasing.

2. If the wheels release, the stoplight switch adjustment should be checked. Either follow the manufacturer’s procedures or continue with the next step.

3. Remove stoplight switch from mounting bracket. This usually involves rotating it 90 degrees in one direction or the other.

4. Pull the plunger out to its full length. (Figure 12.5)

Figure 12.5 Ratchet Style Extended Plunger

5. Insert the stoplight switch back into its bracket while holding the brake pedal in the full released position.

6. Verify proper operation of brake lights.

NOTE: There are other variations of switch adjustments used so it may be necessary to consult the service manual. Always verify proper brake light operation.

NOTE: Cruise control switches can also be the source of the same problem on certain vehicles.

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