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