Good to see that you haven't given up yet! This reading may not necessarily be influenced by the wrong connection since the multimeter actually is calculating duty-cycle by looking at the "on-time" of the signal. Anyway what you should do is to check your reading with ignition on (not starting the engine). If you see a 70% reading you are ok! If not swap the test connectors to see if it makes a difference.

If your reading was fixed 38 % it could be a fault indication. It is close to 40% which indicates a problem with the connection to the airflow sensor. Also a 50% reading is indication of an inactive O2 sensor, which is what you will have all the time during warm-up (unless you have another sensor problem). Try pulling the coolant sensor connector and you will have a 30% reading.

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Tore Bjorndalen

1990 300E

Thanks Chicago and ToreBj,

I reversed the connectors and tried the following:

With engine off, ignition on, I get nothing - 000.0

With engine on the numbers jump from around 0 up into the 400 range.

I am more confused now. I must be doing something wrong.

Any more ideas??
Thanks, Mike

And this was looking so promising! Seriously I think this is a matter of hooking things up the right way and setting the test equipment right. First are you 100% sure that your multimeter has this "Duty-cycle" selection capability? If so, it might be that you need to select a mode for the duty-cycle. Some meters will have options for negative or positive level detection. Check this.

Since it worked the first time, try it that way again with ignition on. If you get this 70% reading continue the test by starting the engine and let it warm-up to reach closed-loop operation. Hmmm, unless your car is a Californian version??

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Tore Bjorndalen

1990 300E

Could be completely wrong about this but I put one lead into pin 3 of X11 and the other to chassis ground. Not sure about using Pin 2, it may work fine as others have said. Pin 1 AFAIK is RPM.

You need to check your duty cycle with the engine running at temp over 80 deg C. You should see the value fluctuate by around 10% say 45%-55% on the meter. You can also use a dwell meter to read duty cycle, search the archives for how.

There are many reasons you won't go into closed loop here's a few:

Zero position on the Air flow meter incorrectly set.
Basic Position incorrectly set / Mixture way out of adjustment
Malfunctioning sensor - 02/Coolant temperature/Air flow meter sensor etc

Look for my thread about setting the 190 16V Air flow meter, should still be on the forum. Talks about checking zero/basic positions.

Report back what you see above 80 deg. For instance, take her for a drive warm her up then look at the meter.

Cheers,
Neil

Pin 3 to ground is the way to do it. Not sure what pin 2 is. It could be ground.

If you don't read 70% (or 30% which is the same thing with a different point of reference) when checking (key on engine off), try reading voltage. A ratio of measured voltage to battery voltage times 100 will give duty cycle. In other words if you measure 6.5v and battery is 13 the ratio 6.5/13 equals .5 times 100 equals 50%.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Ok,

Tired again- With temp gauge at 87 degrees, I get readings which hover around 36%, with an occasional, meaning every 8 seconds or so, flucuation to 44 or down to around 26. Almost always around 36% .

This is the tester in pin 3, using pin 2 as a ground.


Mike

ToreBj - its def a multimeter with duty cycle - picked up the one from sears which was recommended on the forum. Thanks for all of your help!

Well you made it! It seems that you have closed loop operation. It's just set a little too rich, but that doesn't really matter since the KE Jetronic ECU brings it back where it belongs! I set mine to run at 45% aprox.

So your problem may be somewhere else?

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Tore Bjorndalen

1990 300E

Your reading should be obviuosly cyclical. If you are just wandering around in that area it isn't working. Did you get 70% key on engine off? Do you get 50% with O2 sensor disconnected , engine at idle? Does the duty cycle slowly go toward 0% if one puts the O2 sensor input to ground? Does the duty cycle go toward 100% if one grabs 12v with one hand and the O2 sensor input with the other?

Do this intergrater range test with O2 sensor disconnected and the battery through body and ground circuits hooked to the wire that goes to controller (not the sensor - hook where sensor hooked).

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Finally had a new O2 sensor put in...had it done at the shop as I have zero time for car play right now.

I used the Bosch Ford Mustang version and my mechanic hooked it up. I get obvious 50% readings now! Fuel mileage up to 20-21mpg also.

Hope this thread helps some other guys.

Thanks for all the advice, It was a learning experience.

Mike