You don't supply the year and model of the car you have.

Perhaps this is your issue:

"For California cars, starting in 1988: Press the push button switch on test connector X92 (passenger side, between the two firewalls) for two to four seconds. The LED will flash once, indicating no fault code is stored in the system. Now press the switch again for two to four seconds. The LED will be on continuously, and the injection control is switched over to an on/off ratio output. "

These instructions come from the following helpfull website: http://www.landiss.com/mixture.htm

If you don't get the system in test mode your multi-meter may be reading noise in the circuit if it is auto-ranging. Be sure to also manully set the multi-meter to the appropriate range (~12V).

Another suggestion: First adjust the CO mixture without looking at the on/off ratio. You should be able to get somewhere close when the engine smooths out.