I have diagnosing the ABS brakes.
The ABS light turns on after a 2-3 min drive.
If i recycle power then it is almost instantly on again.
When the light is off within the first minutes the ABS functions ,OK.
I have checked the OVP and i have the correct duty cycle on pin 3. diagnose socket.
When i checked the two front sensors with my scope then the one on the right did not give any signal.
Got a new one and installed it and when i first checked it seemed okay ,but after diagnosing the battery and alternator output i figured i should measure the sensor which i just installed.
It was totaly open ,compared to my old one and the left side sensor which are 1KOhm.
Reinstalled my old sensor which i managed to get going with some angle correction (it didn´t go deep enough to detect anything).
I have checked the solenoid relay and the pump relay.
I also tried the pump by manually hotwire it for a few seconds.
Did not sound that nice but started.

Where can i found the connector for the rear sensor ?
The amplitude from the left sensor is a bit lower then from the right sensor is that a problem ? (0,2v difference)
Is it common with a new sensor that is defective ?
I have disabled my 4matic since it is almost impossible to run with defective ABS.
The control boxes i have are from the battery and backwards
1. KE-Jetronic
2. 4matic (intel inside)
3. the last one which i assume is the ABS control box.

I have opened up and examined all the boxes and the only thing i am worried about is that the last one which i think is the ABS control module have a smell of that something have been hot in there.
Since i am quite experienced in electronics i visually checked the components but could not find any obvious reason for the smell.
I wonder if the controlbox can be overheated because something draws to much current.
It looks like the box controls the ABS pump motor and the solonoid valve(s).

My next thing to check is the rear diff. ABS sensor but where is the connector ?
Any idea of how similar the voltage from the sensors have to be would be appreciated.
How to properly test the solonoid valve(s)

Of course i can take it to my local professional service but i am not ready to give up yet.(saving my money for a new diesel)


Pelle

300E 4M -89 5 speed

Rear ABS sensor connector is under rear seat.

__________________
MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner

The rear sensor is okay ,very nice curves on my scope and similar voltage as my right front sensor.

I was wrong about my EHA values.
The fuel cut off function works when the car is cold.
When engine temperature gets higher and the KE is going to closed loop mode then my fuel cut off function fails.
Fuel cut off is easy to monitor and feel when you have manual transmission.

When the engine is cold the ABS and EHA is okay ,but when the engine is hot then my problem begins.
The problem is probably a combination of a bad OVP or the harness to or from it.
When the engine is running and twist the relay around with the cables i can hear the engine not running as smooth as it should.
The OVP fuses voltage is 13.45 volts when engine is running so i guess the power to the relay is okay.
OVP relays are overpriced in my country.
I am also going to check my front left wheel bearing as i think this can trigger a fault especially when the left front sensor signal i weaker.

Pelle

300E 4M -89 speed

I am going to testdrive the E400CDI and feel if that could be my next car.

I installed a new OVP today and the fuel cut off and the EHA is now working but the ABS still fails.

I am cutting the batterypower and try again tomorrow to see if it is some memoryproblem.

Still have a bad feeling about my left front sensor.

Does anybody know if there is any electronics underneath the relays on the ABS unit.

Pelle

300E 4M -89

The battery disconnect did not solve my problem.
I am going to try and test the ABS pump relay one more time.
Also heard that the signals from the sensors must be very similar in voltage.
Last stop is the ABS control Unit.

Also attaching a picture to explain the sensor and hub to anybody that is new to the subject.

Pelle

Attached Thumbnails

 

even if i´m not moving my car the ABS light stills comes on after a while. 4-5 minutes.
What are the ABS controller realy doing ?
There are 3 power transistors inside but what ABS function are they controlling.
The Solenoid relay or the solenoid power ?

I am going to see if i can find a used ABS controller.


Pelle