Benzmac

I Originally posted my problem in alt.auto.mercedes and will try to explain more clearly what my problem is.

The car is a 1988 300E with 120,000 miles on it. I recently checked the fuel injection system because it is due for inspection. I used my Fluke 88 tester which has pulse/duty cycle setting, plugging it into the test socket X11. My understanding is that if the value moves within a range between approximately 38% to 42% the system is functioning properly. But if the value stays on a specific number that is the number (or code) of the problem, these numbers being listed in the engine repair manual.

In my case from a cold start the reading is a constant 50% until the engine warms up. When the operating temperature is reached the number starts to move between 38% and 42%. But if I drive the car and stop and take another reading the number is a constant 60%. If I then turn the engine off and immediately start it the reading returns to a correct range between 38% and 42%. If I again drive the car the same results occur.

I have two different listings of duty cycle readings (codes) and neither one shows 60%. I have cleaned the contacts on the control box as well as the various sensors on the engine. I have also replaced the oxygen sensor in that the old one was three years old and figured I would have a good starting point. When the duty cycle reaing is within the correct range unplugging the coolant sensor results in a reading of 30%, unplugging the oxygen sensor results in a reading of 50%, and the altitude sensor a reading of 80%. But when the I am getting a reading of 60%, unplugging any of the sensors I just mentioned doesn't change the reading.

I hope I explained the symptoms clearly, I have started to test each individual sensor but so far have found nothing abnormal.

Thank you for any assistance you can provide me with.

There are 2 seperate fault code pages. 1 is for ignition on/ engine off and the other is for ignition on/ engine at idle. I'm guessing yours is ignition on/ engine at idle. The 60% means "speed signal at CIS-E control unit present". This is not a fault code that is stored when there is a problem, but rather it tells you when this system is actually working. I think that you do not have your Fluke 88 multimeter connected properly, so here's how to do it...

1. Connect the test leads: black to ground - red to pin #3 of the diagnostic socket x11.

2. Set the meter to DC volts.

3. Push the "Hz" button 2 times. This converts the volts scale to %.

4. Push the "))))" button 1 time. This changes the polarity indicator from positive to negative.

If it oscillates you are O.K. If it is holding 1 percentage steadily, let us know and we'll go from there.

Benzmac

I attempted to use my Fluke meter as you described and I think my meter is slightly different than what you are describing. By pressing the "Hz" button twice the meter goes into the RPM mode for 2 cycle engines, by pressing the polarity button at this point does nothingand the reading is in the thousands.

My fluke meter has a %Duty/ms-Pulse button which goes directly into a duty cycle reading when pressed once, which is what I am doing. I was told that to check whether or not the reading is right pull off a coolant sensor and the reading should be a steady 30%, which is what I am getting after starting up a warm engine.

One thing I did notice when hooking my meter to socket #3 is the voltage is different when I have a constant 60% reading and ALL other times (even when I pull off the coolant sensor plug. I get a constant reading of approximately 5.5 volts before pressing the %duty cycle button when the percentage is 60% as opposed to approximately 9 volts at all other times. I checked the input voltage at the control box and in all cases it is 13+ volts (engine running).

I am testing the sensors as per the Mercedes manual and so far the following seem OK Full Load Contact OK Coolant Sensor OK Airflow Sensor OK O2 Sensor OK TD-Signal OK Altitude Sensor OK Idle Speed Contact Am trying to test but so far am unable to make the engine "Hunt" when activating the swicth with the engine running at 2500 rpm. Also when testing the current at the Electrohydralic Actuator the reading doesn't swing to -60ma when releasing the throttle from 2500rpm.

I hope I am explaining things clearly. And thank you for your patienence with this problem.

I will let every one I know about this great site

Benzmac

I attempted to use my Fluke meter as you described and I think my meter is slightly different than what you are describing. By pressing the "Hz" button twice the meter goes into the RPM mode for 2 cycle engines, by pressing the polarity button at this point does nothingand the reading is in the thousands.

My fluke meter has a %Duty/ms-Pulse button which goes directly into a duty cycle reading when pressed once, which is what I am doing. I was told that to check whether or not the reading is right pull off a coolant sensor and the reading should be a steady 30%, which is what I am getting after starting up a warm engine.

One thing I did notice when hooking my meter to socket #3 is the voltage is different when I have a constant 60% reading and ALL other times (even when I pull off the coolant sensor plug. I get a constant reading of approximately 5.5 volts before pressing the %duty cycle button when the percentage is 60% as opposed to approximately 9 volts at all other times. I checked the input voltage at the control box and in all cases it is 13+ volts (engine running).

I am testing the sensors as per the Mercedes manual and so far the following seem OK Full Load Contact OK Coolant Sensor OK Airflow Sensor OK O2 Sensor OK TD-Signal OK Altitude Sensor OK

Sounds like you could have a faulty control module. See if anyone has a test unit you can plug in and try.

I will go to my code book and see what it says on Monday morning. Thanks for your interest in our site. Tell your friends! Benzmac

DOES YOUR CRUISE CONTROL SHUT OFF AT TIMES?
POSSIBLY THE HALL EFFECT SPEED SENSOR ISNT SENDING THE CORRECT SIGNAL TO THE C.I.S. COMPUTER.