On my 94 SL600, I get ASR light on sometimes,
This is how it goes in every couple of days, I start the car in the morning and hear ABS/ASR pump run for like 10 15 seconds, then shuts off. within couple of minutes it starts and shuts off again, this happens usually 3 to 4 time, and then ASR light comes on and pump doesnt start anymore. Each and everytime after this incident I get code 22 "Pressure switch leakage" from socket 6 (ABS/ASR control module).

I have replaced the brake fluid, and checked all the hydraulic lines and didnt find any leaks whatsoever. After replaceing the brake fluid it did help a little bit but not too much. I followed alldatadiy instructions and tested the Wiring that goes between the control module and ABS/ASR pump, everthing is in accordance with specified values. I have tested the Hydraulic system and pressure values are also satisfying. Last option is replacing the ABS/ASR pump to see if it makes a difference.

On the other hand, I felt that while this incident is happening, I feel the brake pedal gets slightly firmer like Brake booster is filled with air. Or when you turn the engine off brake pedal gets firmer, its almost like that, its not that firm but I can feel a little firmness. Also, while this incident is in progress it usually lasts 1 to 5 minutes, Engine idle is erratic, like there is a vacuum leak or something like that.

My theory is, I am guessing there is a valve or something which controls the vacuum system. when it malfunctions it causes an erratic idle and causes the brake booster fill with air. and when I apply brake pedal, system recognizes that there is not enough pressure and turns the ABS/ASR pump on to pressurize the system. However, after cycling couple of times system is still not pressurized enough, then the control module thinks there is a hydraulic leak somewhere and turns on ASR MIL light and turns off ABS/ASR pump.

Does this make any sense! does this theory at least sound logical?

try bleeding the pump...there is a port there for such.

I have bled the pump and even checked for the PSI and everything is in specified values.

the wiring harnesses on this model (and many other mercedes) are notorious problem causers. i have seen the harness on the 600 throttle actuators cause simmilar issues like you are having, i would suggest that you physically inspect all engine harnesses for heat damage. specifically the harnesses to each throttle actuator. cut them gently with a razor and look for damage inside. be warned however if these units are bad they can cost thousands each. also if they are defective cutting them open to inspect may compound your problems and not allow the car to be driven. you could start your inspection at the main engine harness if is damaged, you can usually bet the actuators and air mass meter harnesses are as well. all can be a very costly repair.

Thanks for the advise Carson, however, ETA s are rebuilt by me, last month vehicle was in limp home and there was 20 different errors now they are all gone after rewiring, wire between the control module and ABS/ASR pump checked out ok I dont have any problems with wires at all, no cracks nothing at all and consistancy is pretty good.

I was thinking maybe the item number 101 in the picture below, has something to do with idle and pressure switch error.

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the booster hose? that sound probable, do you think it has a leak? or the valve is bad?