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Pelican Technical Article:

Brake Switch Replacement

Tom Morr

Time:

1 hour1 hr

Tab:

$15 to $32

Talent:

*

Tools:

Phillips screwdriver, small flat head screwdriver, flashlight

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

New brake switch (pedal travel sensor)

Hot Tip:

Move the driver's seat all the way back to improve access

Performance Gain:

Working brake lights

Complementary Modification:

Replace brake light bulbs if they don't illuminate after replacing switch

Some good news: Replacing a defective SLK pedal travel sensor (brake switch to laypeople) is relatively easy and inexpensive. The job's most difficult part is working in the limited space under the dashboard.

The not-as-good news: Identifying the brake switch as faulty can be a process of elimination. The switch actually has dual functions: It controls the brake lights and is also part of the ASR traction-control system.

Non-illuminating brake lights can have a variety of causes. The obvious starting point is checking the bulbs to see if they're burned out, then cleaning the contacts if the bulbs appear to be good.

Mercedes Benz issued an R170 taillight recall due to melting bulb sockets. If the taillights are still original, the dealership should replace them under warranty with the updated lights. Next is checking the brake light fuse in the under-dash fuse panel. If all these check out, the brake switch is possibly faulty.

Even when the brake lights function properly, the brake switch's ASR circuit can still be bad. Symptoms of this include the ASR light (on the center console) illuminating while applying the brakes during normal driving. If the Hazard triangle warning light comes on with the ASR, then another switch or sensor (or combination of them) could be to blame.

No special tools or skills are necessary to replace the brake switch. However, old plastic can be brittle: be careful when bending the accelerator cover panel out from under the carpet. Also, take care with the ASR plug (the lower one on the brake switch) when prying it off the switch.

Two Phillips screws secure the accelerator panel cover to the lower IP cover (arrow).
Figure 1

Two Phillips screws secure the accelerator panel cover to the lower IP cover (arrow). Once they're removed, the black accelerator cover can be carefully untucked from behind the carpet and removed.

We removed the lower IP cover for a clearer view at the brake switch (arrow).
Figure 2

We removed the lower IP cover for a clearer view at the brake switch (arrow). The switch has two plugs: the upper one goes to the brake lights; the lower plug connects to the ASR.

The switch has a plastic tang on its outboard side.
Figure 3

The switch has a plastic tang on its outboard side. Depressing this allows the switch to rotate in its mounting bracket.

Rotate the switch about a quarter turn clockwise to release it from the bracket.
Figure 4

Rotate the switch about a quarter turn clockwise to release it from the bracket.

The manual recommends unplugging the connections before removing the switch.
Figure 5

The manual recommends unplugging the connections before removing the switch. However, the plugs are more visible and accessible: particularly if the lower IP cover is still in place: if the switch is removed first. Then the assembly can be lowered closer to eye level.

The switch will only fit one direction in the mounting bracket (arrow).
Figure 6

The switch will only fit one direction in the mounting bracket (arrow).

Plug the brake light and ASR harnesses into the new switch.
Figure 7

Plug the brake light and ASR harnesses into the new switch.

Depress the brake pedal while inserting the new switch to allow plunger clearance.
Figure 8

Depress the brake pedal while inserting the new switch to allow plunger clearance. Then spin the switch counter-clockwise until it seats in position. The switch self-adjusts.

The new switch is affordable and relatively easy to install.
Figure 9

The new switch is affordable and relatively easy to install.

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Comments and Suggestions:
MapleLeaf Pilot Comments: The best web site I have come across 'on how to do it' The Extra Large Image facility is great.

Oh yes I am based in the UK and nothing here in Europe Uk and the rest of Europe incl Germany comes up to your standard.

Well done.
April 8, 2015
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 

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