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ABS Relay and Pump Replacement
 
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Pelican Technical Article:

ABS Relay and Pump Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$10 to $3,100

Talent:

**

Tools:

11mm flared wrench, 10mm socket, T15 Torx, turkey baster or fluid pump

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New ABS pump and relays

Hot Tip:

Clean around the brake lines before working

Performance Gain:

Better braking

Complementary Modification:

New brake pads

As the earlier ABS systems start to age they begin to have problems; thankfully it is usually the relays that fail and not the ABS pump. The relays are cheap and easy to replace whereas the pump is very expensive to replace. This article will show you how to do both. Note: if you open the brake lines anywhere in the system you will need to completely bleed the entire brake system before driving the vehicle again. DO NOT drive the vehicle without first completely bleeding the system

The ABS pump is located in the left front of the vehicle just behind the headlight assembly (red arrow).
Figure 1

The ABS pump is located in the left front of the vehicle just behind the headlight assembly (red arrow). The brake lines run into it from the master cylinder and then out to the calipers.

If you are going to be opening the brake lines in the pump you will need to remove as much brake fluid from the system as possible.
Figure 2

If you are going to be opening the brake lines in the pump you will need to remove as much brake fluid from the system as possible. Begin by using a fluid pump or turkey baster and removing as much fluid from the brake reservoir as possible (red arrow). Note: if you open the brake lines anywhere in the system you will need to completely bleed the entire brake system before driving the vehicle again. DO NOT drive the vehicle without first completely bleeding the system

The pump sits on three rubber isolators above the tray (red arrows, two shown).
Figure 3

The pump sits on three rubber isolators above the tray (red arrows, two shown).

To check or replace the relays use a T15 Torx driver and remove the single screw (red arrow) holding the cover plate on.
Figure 4

To check or replace the relays use a T15 Torx driver and remove the single screw (red arrow) holding the cover plate on.

Remove the cover and you have easy access to the wiring harness connection (red arrow), the 12 volt relay (yellow arrow) and the ABS pressure regulator (green arrow).
Figure 5

Remove the cover and you have easy access to the wiring harness connection (red arrow), the 12 volt relay (yellow arrow) and the ABS pressure regulator (green arrow).

The relays are easy to remove and replace (red arrow).
Figure 6

The relays are easy to remove and replace (red arrow). Simply wiggle them straight up and out. They can be in the housing pretty firm so try and grab them by the base as much as possible to avoid just pulling the cover off the relay.

If you are replacing the pump you will need to separate the wiring connection from the pump.
Figure 7

If you are replacing the pump you will need to separate the wiring connection from the pump. Use a T15 Torx and remove the two screws holding the wiring bracket (red arrows, already removed) and then pull and wiggle the connection of the posts on the pump.

You need to have an 11mm flared nut wrench to break the lines free.
Figure 8

You need to have an 11mm flared nut wrench to break the lines free. The wrench grasps the nut on the wrench more completely than a standard wrench and helps remove the nuts without rounding them off. The pump's lines are a tight fit so don't take chances. Use the right tool.

Clean the area around the lines, nuts and pump before opening the system up (red arrow).
Figure 9

Clean the area around the lines, nuts and pump before opening the system up (red arrow).

There will be some fluid that leaks out so place a couple of rags under the pump and remove the lines from the pump (red arrows).
Figure 10

There will be some fluid that leaks out so place a couple of rags under the pump and remove the lines from the pump (red arrows). Remember, brake fluid is deadly to your paint so do not get any on the paint or on yourself and then touch the paint.

This photo illustrates the end of the brake line and nut.
Figure 11

This photo illustrates the end of the brake line and nut. You can see how the end of the line is flared to fit into the pump (red arrow). The nuts are made of a soft metal and round off easily. Due to the flare on the end of the line you cannot just easily replace one of the nuts.

Use a 10mm socket and remove the ground wire from the pump to the mounting tray (red arrow).
Figure 12

Use a 10mm socket and remove the ground wire from the pump to the mounting tray (red arrow).

Remove the single 10mm bolt holding the front rubber grommet in place (red arrow).
Figure 13

Remove the single 10mm bolt holding the front rubber grommet in place (red arrow).

Slide the rubber bracket off the post on the front of the pump (red arrow) and then slide the pump forward off the two remaining brackets.
Figure 14

Slide the rubber bracket off the post on the front of the pump (red arrow) and then slide the pump forward off the two remaining brackets.

Use care when removing the pump from the vehicle so that you do not damage any of the brake lines.
Figure 15

Use care when removing the pump from the vehicle so that you do not damage any of the brake lines. Installation is the reverse of removal. Note: you will need to completely bleed the entire brake system before driving the vehicle again. DO NOT drive the vehicle without first completely bleeding the system


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Comments and Suggestions:
KJZ78701 Comments: Re Codes: May I suggest that before you pull codes you try this. ABS light on before the car is moving? Check the relays in the write up above and the OVP relay, ask Google. Fix the relays and I bet you solve the "light always on" problem. ABS light comes on after you start moving? Check the sensors, see above. If either or both don't resolve your issue, then go to code reading. :-
April 15, 2017
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it.
- Nick at Pelican Parts
 
KJZ78701 Comments: Re Bleeding: If your reservoir has two sensors the one in Figure 2 only has one sensor, fill the empty reservoir ALL the way to the top. If you do not do this, you may only be filling one reservoir, leaving either the front or rear system without fluid i.e. no brakes
Re Relays: Ohm between the 85 and 86 pins on both relays. If you get zero ohms, apply a 12 volts differential 12 V and ground across those same pins and listen for the click. Now ohm between 30 and 87 while applying a 12 volt differential to 85 and 86 again. If you got the click but you still have an open circuit across 30 and 87, then take the relay apart and clean the contacts 200ish wet/dry sandpaper works for me and apply voltage again. Clean until you get a closed circuit across 30 and 87 with voltage applied to 85 and 86. I then spray the relay with Nano Protech to waterproof it and put the cover back on. Good for another couple decades.
April 15, 2017
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 
reddog Comments: In the parts section only pressure relay is listed? I would like both.

my 93 300d has very hard brakes with no fluid going to front wheels even after new booster and master cylinder.
October 15, 2016
  Followup from the Pelican Staff: I am having trouble locating it online as well. But, 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
 
Bill Ben Comments: I have a 1995 Mercedes E300 Diesel w124 chassis. The ABS light is now on continuously. Are there Obs1 fault codes I can check ? If so which pin to use.
If not, how to I troubleshoot this without just swapping parts ?
October 6, 2016
  Followup from the Pelican Staff: You will need to read the fault codes. Benz has a special tester and test kit for testing. I would see if a local shop can read the codes for you. - Nick at Pelican Parts  
bigshineybike Comments: Hi Nick thanks for the reply. I got to the bottom of the pump running all the time problem on my 1996 W124 E300D it was the power relay in the ABS unit. the contacts had stuck, I managed to free it but a replacement is on the way.
August 15, 2016
  Followup from the Pelican Staff: Thanks for the follow up. We appreciate it.
- Nick at Pelican Parts
 
bigshineybike Comments: I have a 300,000 mile/ 1995 W124 estate the ABS pump runs when the car is switched off. Is the pump running because its not building up enough pressure or is it because the relay is jammed? if I remove the relay the pump stops?
August 9, 2016
  Followup from the Pelican Staff: With the key off, the pump should not run. It is likely malfunctioning, or the control module. - Nick at Pelican Parts  
Kyselijä Comments: Is there any special bleeding procedures for the new abs-pump unit.
November 3, 2015
  Followup from the Pelican Staff: Yes, you will need a Mercedes-Benz scan tool. - Nick at Pelican Parts  

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Page last updated: Fri 6/23/2017 02:41:43 AM