Frequency defect in Lambda diagnostic port, '89 300E W124 M103
 

When I read duty cycles from pin 3 at the diagnostic port there's a very strange thing happening. With KOEO I get 85% on a frequency of 100hz (California car). When I start the car the duty cycles go all over the place and the frequency jumps to approximately 500hz +/-50hz. And here's the strangest part: as the RPMS increase so does the frequency, up to around 1500hz @ 2500 RPMS. This happens when the multimeter is grounded on the chassis or on pin 2. This also happens when I bypass the OVP relay. At KOEO a deflection of the air sensor or the throttle does not cause the duty cycle reading to drop to the designated 10%. I tested the TVS and the air mass sensors and they are to spec.

I believe the car is actually running with the Lambda to spec as the amps on the EHA dance around 3ma and the readings on the O2 sensor are between .1 and .9 volts.

I have another W124/M103 that reads appropriatly. I'm not above swapping parts until the answer is found :o .

Any ideas? I am clueless...

Thanks,
Robert

On CA models you have to place the system into "test mode" with the button on the connector in the battery compartment. The following page describes the procedure.

www.landiss.com/mixture.htm

Duke

Thanks Duke! I'll give that a shot.

Robert

Thanks Duke, that fixed it, and the car is now around 35% and the EHA pulls approx -3ma.

I noticed some differences between the '89 and the '92, both of which are california models.

The '92 doesn't require entering test mode and runs at 80hz

The '89 requires entering test mode and runs at 100hz

Is this right or is there something wrong with the '92?

Thanks,
Robert

I don't know how the system changed after the late eighties. Details should be in the 124 CD. It you don't have it you can essentially download the whole thing from:

http://mb.braingears.com/

Duke