my 1990 420 se has terrifying intermittent brake failure, when pedal goes to floor and no stopping power. Then recovers,but can reappear later.

Separately, ABS light fails to extinguish until after mile or so of driving.

Have had car checked over by non-franchised MB dealer and they could find no fault but changed master cylinder as precaution.

However, happened again today..

Any suggestions/siminlar experiences?

thank you

Damianj

 

Quote:..."goes to floor and no stopping power"

Is this an accurate description of the problem or a reaction to the car stopping less quickly than you expected.

i.e. Does the brake pedal actually go to floor/carpet or just down further than normal?
Likewise NO stopping power or just reduced stopping power/ increased stopping distance?

The fact that the ABS light is coming on suggests that there is a fault in the ABS which could cause the ABS to operate under conditions different to those that it is designed to work.

Are you familiar with how (from a drivers perspective) ABS works. i.e. the car "pulses" the brakes during severe stopping on damp surfaces to ensure the wheels doesn't lock. This can actually INCREASE stopping distances BUT since it prevents the wheels locking you can still steer which you can't if the wheels lock.
Some people assume ABS means that the laws of physics regarding stopping are changed and the car will always stop in time. NOT the case!

Investigation and repair of the ABS controller is a specialist job. they are very expensive although they can be remanufactured if you do a search on Google

Suggest you get the car to an MB dealer and get them to read the diagnostic codes which will provide a status check on the ABS (and other systems). This check is different to getting the engine codes done which can be done at most garages. Reading ABS codes requires specialist equipment which some garages may have but the dealer certainly will.
In order to safeguard your wallet ensure the MB dealer only carries out the code read (for which they may make a reasonable charge based on say 15 mins labour. Once they have these codes and can advise what is likely wrong get a quote for the work but come back first and we will indicate if the explanation sounds "reasonable'.

If the car exhibits the symptoms you have advised you should NOT CONTINUE TO DRIVE THE CAR!
Stuart.

 

Value your rapid response, Stuart.

Am very aware of normal operation of ABS. This is alarmingly different. Pedal goes into carpet and braking power drops to 5% max. ABS juddering type sensation at same time. Pumping pedal rapidly brings some improvement. Then passes and then seems normal. ABS light not on during these episodes.

Very scary. Master cylinder replaced, no fluid leaks, vacuum tubing intact within engine compartment. Only other slightly odd symptom continuously for about 6 months is hissing sound behind speedo which becomes less with more throttle. I've presumed that may be vacuum hose leak to 'economy' gauge, but don't know if at all related to brake problem...

Regards

Damian J

 

IF THEY IS A VACCUM LEAK THAN UR BRAKE PADEL WILL BE HARD.UR PROBLEM IS PADEL CRIPIN DOWN VERY EASY TO FIND THIS FAULT OF YOURS,WAT YOU NEED TO DO IS GET ALL 4 WHEELS OUT, CLAMP 1 BRAKE HOSE AT A TIMEMEANING THE HOSE U GET ON THE CLIPER,WHILE CLAMP ON SEE IF PADEL CRIPIN DOWN,IF PADEL SEEMS OK NOT CRIPIN OR GOING DOWN YOU NO THE FAULTY SIDE EITHER CLIPER OR BRAKE HOSE....CHECK THE CLIPER OR BRAKE HOSE,HOPE UUNDER STAND WAT AM SAYIN THATS THE BEST WAY TO CHECK,ESPECAIL ON OLD BOYS DUE IT HAS NT GOT BAS OR ESP,OR DIAGNOSTIC..............

 

NAZ has added a bit more info BUT if ABS modulation is taking (noise and ABS "juddering") when the pedal goes to the floor I would go for the ABS modulator.

The question is why the ABS modulator working when it shouldn't. ABS is designed to fail safe from the point of view of the primary braking system. i.e. ABS stops working but full non ABS braking capability remains if something goes wrong!

As you are familiar with ABS you will be aware that the ABS modulator allows brake pressure to be "dumped" when a "lock up" is sensed and then reapplied subject to individual wheel de-acceleration.

Without diagnostics I'm out of my depth here but just as an idea I would inspect very carefully all electrical connections in the ABS system. i.e. Wheel sensors and ABS modulator connections. Any sign of water / corrosion in the connectors?

If not then the problem is going to be difficult to resolve without the cost of a new modulator...... which may not resolve the problem!!

If I had this problem I suspect my next line of attack would be to disconnect the modulator electrically since the modulator must likely have either a mechanical or electrical fault.
If there is no power, brake pressure can't be dumped due to an electrical problem. However it still could if the problem is a dirty valve which isn't sitting correctly.

AS I STATED BEFORE. BRAKES ARE SAFETY EQUIPMENT AND IF THEY ARE NOT WORKING CORRECTLY YOU SHOULD NOT DRIVE THE CAR AND SEEK PROFESSIONAL ADVICE.

The hiss behind the dash is very likely a vacuum leak to your gauge or heater controls. Vacuum is used to"power" the brake servo so if vacuum is leaking you may have insufficient vacuum to provide pedal assistance during braking but the servo hose connection is usually different to any other equipment for reliability reasons. No vac to the servo does not give you a looong pedal. Just heavy.