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Brake Assist Control Module Replacement
 
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Pelican Technical Article:

Brake Assist Control Module Replacement

Tom Morr

Time:

30 min

Tab:

$849

Talent:

*

Tools:

A small flat head screwdriver

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

New BAS Control Module

Hot Tip:

The center wiring harness is easier to unplug with the BAS removed from its mounting bracket.

Performance Gain:

Restores proper BAS functionality for added panic- stop boost.

Complementary Modification:

Clean front wheel-speed sensors, replace if necessary

The Brake Assist System (BAS) adds extra braking power during panic stops. The BAS control module receives signals from the wheel sensors, the brake-pedal switch and the BAS diaphragm travel sensor. When an emergency stop is sensed, the BAS module signals its solenoid to open a valve on the brake booster. This allows atmospheric air to enter the booster and apply extra force to the master cylinder via the output rod.

The BAS warning light on the instrument cluster is coupled with the ASR (acceleration slip regulation/traction control) indicator. They share some signals from certain sensors and switches.

If the Check Engine light is on, use a scanner to reveal any diagnostic trouble codes (DTCs).

A faulty front wheel-speed sensor is a common trigger for the BAS/ASR warning light. Even if it isn't fingered by a DTC, it can't hurt to clean the front wheel speed sensors and ABS sensor rings anyway. The speed sensors are behind the rotors, held on with Allen screws. Spraying these with electrical contact cleaner can solve the problem.

The charging system can also be the root of the problem. The BAS/ASR light will come on if the system falls to 10 volts or less. If the warning light is initially on but then goes out while driving, this indicates low battery voltage -- as the alternator charges the battery, the BAS gets the correct voltage signal and the warning light shuts off. Check the charging system's health in these instances.

Another affordable possibility is a bad brake switch (please see our article on replacing your brake switch). This switch has circuits for both the brake lights and ASR, so the ASR signal for brake-pedal position can be bad, but the brake lights will still work if that circuit is functional.

Although the BAS control module is pricey, replacing it only takes a few minutes and can be done at home. In fact, no tools are required if the plugs that connect to the BAS comes out without incident. Sometimes the problem is moisture or corrosion at the connections -- cleaning the contacts can restore proper operation.

Even with the BAS/ASR warning light on, normal braking and ABS will still be functional. Panic stops will require more leg power; ASC is unaffected. Finally, the new BAS module is self-calibrating.

A constantly illuminated BAS/ASR warning light (green arrow) indicates a malfunction in one of these systems.
Figure 1

A constantly illuminated BAS/ASR warning light (green arrow) indicates a malfunction in one of these systems. Although not inexpensive, the BAS control module is easy to replace when it goes out.

Begin by disconnecting the ground strap from the battery.
Figure 2

Begin by disconnecting the ground strap from the battery.

Pump the pedal several times to reduce vacuum pressure in the booster.
Figure 3

Pump the pedal several times to reduce vacuum pressure in the booster. Otherwise, the diaphragm travel sensor sealing ring can get sucked into the booster when the BAS module is disconnected.

The BAS control module is shoehorned under the master cylinder.
Figure 4

The BAS control module is shoehorned under the master cylinder. It has three plugs (center plug not visible; other two indicated by green arrows).

Remove the right-hand harness first.
Figure 5

Remove the right-hand harness first.

The center harness (switch/solenoid valve connector) might be easier to unplug after the BAS is slid off its mounting bracket and help perpendicular to the master cylinder/brake booster.
Figure 6

The center harness (switch/solenoid valve connector) might be easier to unplug after the BAS is slid off its mounting bracket and help perpendicular to the master cylinder/brake booster. The tangs on the module secure it to the bracket.

The module's ports are different sizes, so the plugs can't be interchanged.
Figure 7

The module's ports are different sizes, so the plugs can't be interchanged. Installation is the reverse of removal

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Comments and Suggestions:
Gary Comments: Hi I have slk 1997 auto.i have asr abs handbrake and a break disc light on dashboard. And I have lost power it's not changing gears as a automatic shud could you pls help me on this.thanks
November 18, 2016
  Followup from the Pelican Staff: First check the transmission fluid level to be sure it is correct. Then check the transmission ECM for fault codes. If the system is not working properly, a fault code will be set. This will be your best bet when diagnosing.
- Nick at Pelican Parts
 
Martin Comments: Hi have a 2001 slk230 6 speed manual can u tell me the location or mb part # of the bas/esp control module please. Thank you
October 16, 2016
  Followup from the Pelican Staff: I’m not the best with part numbers.

Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
SLK230 Comments: I looked up on the MB world site and most suggestion pointed to replacing the brake stop switch.
August 13, 2016
  Followup from the Pelican Staff: Thanks for the follow up. We appreciate it.
- Nick at Pelican Parts
 
Mercman Comments: A 'mate' gave me a jump start and reversed polarity for a few seconds before correcting it...! Have replaced a few fuses and all seems ok except the bas asr light comes on when I brake hard. Gentle braking lets it stay off! after being on then off when starting the engine. Is there a fuse I have missed? Why a prob only when brake hard??
May 2, 2016
  Followup from the Pelican Staff: would assume something electrical was damaged. Start by checking the fault codes. - Nick at Pelican Parts  

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Page last updated: Fri 12/9/2016 02:25:55 AM