ABS kicks in on 1996 c2220D

[poseidon]

Hi,

I can feel my ABS is activating when stopped and holding brake. This only happens when completely stopped (at trafic lights) and otherwise brakes operate normally. ABS also operates normally when skidding. There are no fault signs on control panel.
Is this a simple sensor matter that I can fix or a whole ECU needs replacement?

Poseidon

 

[jberks]

Are you sure it's the ABS?. I have not heard of the ABS kicking in when stationary. The ABS system is pretty good at self checking so any fault would normally switch it off. It would have to think a wheel is still turning so a sensor would have to be givving a false signal. If this were the case, it would detect it and trip or it would be working all the time you are braking, not just when stationary.

When stationary, the braking system will be pumped back up by the engine, then sit there fully pressured with the pedal depressed. I don't know enough about how the braking system works but presumably a dodgy bypass valve could have a similar feel.

 

[poseidon]

Hi,
Thanks.
I believe it is ABS since it has characteristic kicking in the brake pedal. However, when stationary the period of kicks is longer (say 1-2 seconds). During skidding it works much faster. Also, after sitting static with brake depressed, brake becomes softer, which I think is the result of brake fluid getting released by the ABS. This oil gets pupmed back to the brake fluid reservoir by ABS so there is never loss of brake fluid.

The bypass valve might be good sugestion (although not sure if this is the ABS valve), but still I expect it would operate (falsely) also during normal braking.

 

[jberks]

I'd say the 1-2 secs is the tell tail. ABS won't (probably can't) run this slowly, whereas a valve probably would. I'd go with a valve on the power asistance side somewhere, poss something in the vacuum side.
Has it always done this? I have some recollection that a previous merc 'clicked' every now and then when held on the brake. It was subtle though.

 

[poseidon]

Hi,
I had a good look at haynes manual and at the car this evening. There is not much on hydraulic valves on brake system. The master cylinder comes as a single piece. The vacum servo check valve is related to vacuum system and presumably would not give clinking noise.

It has been annoying since I bought the car (around two years ago, 60k), but it has been worsening. The car can move while waiting on a downhill trafic light, because the brake fluid pressure drops and brake pedal softens.

Otherwise the car has used more money than my wife. (cluch warped and needed replacement, rusted brake pipes, ball joints, always pulling to the left despite all MB "specialist" electronic adjustements).
Regards,

 

[jberks]

Sorry - I don't know enough about they system, aside from the fact that I still doubt it's the abs.
Does sound serious if it's letting go on a hill. You don't need it doing the same thing on a motorway exit. I'd get at least a diagnosis from the specialist, even if you source and fit the parts yourself.

Odd you describe it as a money pit. Our 1995 C has cost almost nothing over the last 7 years. On the up-side whatever it is should be cheaper than the ABS!

 

[poseidon]

update and possible solution

Hi,

I have not done any repairs regarding the clinking noise from the brakes and it has been annoying since (sometimes better sometimes worse).

However, recently I did good bleeding of the brake system and this has improved situation. There is no clinking noise, or it is very vague.
I used the presurised bleeding system (uses spare tire air pressure), which I bought because of the recent cluch bleeding. This seems to be much better method than the passive method with a plastic pipe and rubber valve, which I used earlier.

I had done good bleeding with the passive method before the problem occured, but my impression is that some air was still trapped and that this was causing the problems.

Pose

 

[television]

If the pedal still sinks it will be the brake master cylinder U/S its the Tandem type for independent front and rear and can give the impression that there is air in the system, the piston for the rear is nomally the first to go as it only works at 28% of the presure of the front.

Pleased you found the presure bleeding was good,it is aid that this is the only way to get all of the bubbles out of the ABS pump

Malcolm

 

[poseidon]

Malcolm,

The presurised bleeding system is far better and i agree it is actually the only way to properly bleed the whole system. (I wish I knew this years ago).

Furthermore, with presurised system you do not even have to open all niples since under the high pressure air gets pushed up and exits through the open fluid-supply valve.
With the clutch, as an example, I found that just by connecting high pressure at the fluid bottle, the system was bled. I opend the nipple at the slave cylinder afterwards but there were no bubbles. There was no need for any nipple-bleeding even though I put new master and slave cylinders.
With the brakes there were some bubbles when I opened nipples, but very little.

Otherwise my brakes look OK for now.

Pose