M103 100% duty cycle
 

Little by little I have been getting my m103 back into good shape.



I went today to check the duty cycle on the x11 port (pin 2 and 3). It is setting at a constant 100% cold or hot.




Whats the deal?


I do have a pinhole leak in a 4matic line so I have it in bypass mode.


1991 300E 4matic M103 3.0L

m:

Possible causes:
> OVP relay defect. Is the idle speed too high? Is the ABS light on?
> Mixture very lean (outside the range of compensation).
> O2 sensor shorted to ground.
> No current to EHA, or EHA unplugged.
> Defective CIS controller.

Idle speed is normal. No ABS light.
EHA is plugged in. When reading the voltage on the x11 port it does change, and there is a marked difference between when the eha is connected and when it is not.




Like and idiot I was messing around with the mixture without a proper multimeter. I probably leaned it out way too much.

"The signal provided at pin 3 of X11 is called the "lambda on/off ratio" signal. It is convenient to use pin 2 of the same connector as a ground reference.

This lambda signal is not a replica of the voltage from the oxygen sensor. Instead, it is a constant 100 Hz pulse whose duty cycle indicates the hunting of the fuel injection for an ideal mixture by switching back and forth between very slightly rich and very slightly lean.

Mercedes has chosen the less common definition of "duty cycle" in this case. They are referring to the percentage of the entire pulse period during which the voltage is zero, not the time when it is near battery voltage (mine was +13.6 V when the battery was +14.0 V). In other words, if the pulse rests at ground for 7 milliseconds and then rises to +13.6 V for 3 ms, the duty cycle is considered to be 70% (see oscilloscope trace at right).

If you have an oscilloscope to measure this timing, fine. If not, it can still be estimated with a voltmeter. Since continuous "0 volts" would be considered 100% and continuous 13.6 V would count as 0%, just measure the voltage between pins 2 and 3 of X11 and divide that by 13.6. Next subtract that ratio from one, and convert the result to percent.

For example, if the meter reads 4.0 V, first divide 4.0/13.6 = 0.294. Subtract 1.0 - 0.294 = 0.706, or 71%. In equation form:

Duty Cycle = [1 - (V{pin 3}/V{max})] x 100%

Remember that if the oxygen sensor is doing its job and the system is operating closed-loop, the reading will jump around, so you might see readings from 5 V to 7 V and have guess at an average."

Ismalley:

When we publish the work of another, citation is customary.