Go to Sears and get the multimeter that has a duty cycle funtion (there is a thread in the last month or so with details).

Remove air cleaner and unscrew the cover of the diagnostic socket on the fender (on the fuel/ignition box there). It's the only one with a round screw-on cover with a retaining chain.

Set the meter for duty cycle and put the red probe in the #3 hole and the black probe in the #2 hole in the diagnostic socket.

Switch ignition on but do not start engine. Should read 70% (there are other readings -- see the DIY article by Steve Brotherton for details).

Start engine. Duty cycle will be 50% until the oxygen sensor warms up, at which point it will go way down to 30% or less if you are running rich and the electronics are working.

When the computer is controlling the mixture, it will read something other than 50% and will change all the time. At this point, you can use a 3mm Allen wrench to adjust the mechanical fuel mixture at the fuel distributor. Insert allen, then press down GENTLY to engage the key in the screw. Don't press very hard, you will shove the mass flow sensor flap down and add extra fuel.

I believe clockwise is lean, CCW is rich, but I'm pretty dyslexic and it could easily be the other way.

Turn the allen slowly and release the pressure, then wait a bit to see where the reading goes. At idle it will take 10 sec or so for the change to show up, and if you get carried away, you will have trouble getting the correct reading.

Adjust in small increments, 1/16 turn or so at a time, until you get a reading close to 50% duty cycle. This should set the EHA current at close to 0, giving you proper fuel mixture control.

Now, the fun starts. If you cannot obtain anything but 50% duty cycle with no changes, the O2 sensor is bad or unhooked (connector is under the floor mat in front of the passneger side front seat). If it changes, but you cannot set it to 50%, something else is wrong.

You should get momentary enrichment (lower duty cycle %) when you open the throttle, going back to 50% quickly. If you get the opposite, you probably have a bad mass air flow potentiometer, so the computer doesn't get signaled you opened the throttle.

You may also have a bad throttle switch.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!