1989 420 SEL ABS Hydraulic Replacement
 

On a 1989 420SEL with 265,000 KM the ABS light comes on after rolling about 10 yards. This happens consistently. My mechanic tested the ABS system with one of those special test boxes and indicated the ABS hydraulic unit is not performing properly. We replaced the relays in the unit, but the light still comes on. So it seems the base unit has to be replaced.

I just bought a used unit, bosch part number 0 265 200 043. It is that rectangular box on the drivers' side fender, with all of the brake lines leading into it.

Is there anything special I need to do to install it? Do just undo all of the brake lines, open the plastic cover, pull the connector, remove the grounding strap, and put in the new unit, reconnect brake lines and electrics, and then bleed the entire brake system? (brake bleeding with a pressure unit, starting with right rear, left rear, right front, left front)

I'm not there to test the system, BUT if the car has to MOVE before the lamp comes on then a BETTER diagnosis of the wheel speed sensors would be MY starting point.


VERY rare for the hydraulic system to cause that problem.

Sensors
 

I understand your point, here is a little more history.

I had replaced the sensors about 3 years, ago. Then when the light came on again, about a year later, my first guess was the sensors again, so I replace the sensors again. The light went off with the new sensors but then it came on. Then it stayed off for a few months, then came back for a few months, then on again and now has remained on for about 10 months. So I took the car to a mercedes mechanic who was an ex-shop foreman, 22 years experience, who now just fixes cars out of his house. He had a box he plugged into the cable between the computer, or maybe it was directly into the box, and then followed the book on the test procedures. He showed me the book, and the failure point, spinning the wheels etc. and the conclusion in the manual. I was sceptical too, but I am tending to believe him since he performed the tests twice, confirming his orginal conclusion. He said it is rare for the unit to fail, but they do fail. He had never changed one before but has heard of others failing.

We swapped the both relays within the box with spares he had, but that did not fix things.

So, any advice on changing the unit??

Here is an update. I changed the ABS pump unit, with a used one from EBay. Worked great, easy install. The light is out.

I took apart the old unit to see if I could see why it failed. Inside I found small black pieces of something in the brake fluid. The pieces could be rubber from the brake line hoses. I also found that some of the brake fluid had a viscosity of jello, very strange.

I highly recommend changing your brake fluid annually.

is it easy to replace? don't need using special tool?

I had a fine black sediment in the brake fluid of my W126 300SE. I changed the master cylinder and flushed the brake system. Since taking a brake class at NVCC Manassas campus, I found that you have to drain the accumulators in the ABS box, done by pumping the brake pedal about 50 times.

Tools
 

You can probably get the job done with no specials tools. I used and I recommend using flare nut wrenches, the brake lines were on very tight and I did not want to round the nuts. You will also need a Torx screw driver for that screw on the black plastic cover on the unit.

Just watch out for the brake fluid, which will lift paint.