Help Reading My Multi-Meter - Duty Cycle
 

I just picked up a Craftsman digital multi-meter which has the ability to read duty cycle.

I've been using it to trouble shoot my O2 system and set the mixture. When I started the car, the O2 system started operating right away. So either my 15 deg. temp switch is not working, or the engine was above 15C/59F which I guess is possible since the car has been sitting in a hot garage.

At any rate, I grounded the temp switch wire to see if the O2 system would go into closed loop, and it did. But the reading I got on my duty cycle meter was 35%.

It seems too coincidental that 65% would be the opposite side of that number. So is it possible my meter is reading the time the frequency valve is closed rather than open?

Sounds like you are looking for 65 % - don't know the duty cycle characteristics you are looking for, and am not familiar with the Duty Cycle capability of the meter - BUT, if there is a +/- trigger or slope button, that could account for the reading you got. IOW, in answer to your question - could be. :-)

Yes, the duty cycle is supposed to be a constant 65% in closed loop.

This is the multimeter I have:

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There's no +/- function or slope function (i don't even know what that means).

I wonder if I reversed the probes, if it would read opposite?

that's neat Matt - o-scopes will allow you to change the select the positive or negative going part of the signal with a slope 'button' so you can 'trigger' on the edge 'sense' you want. w/o flipping the leads.

I scanned the pdf manual, and I didn't see a warning - so you should be able to flip the probes. That will change the duty cycle to the compliment (35), but again, I can't comment on the reading validity - I simply don't know enough about the specific measurement you are taking. (I have to read a DME / motronic book?)

I think the meter is looking at 3 things, the time for the signal to go positive twice (that's the 100%), and the time from pos to neg - when compared to the 100%. When you flip the leads, you fool the meter.

I'll try pasting a fig - the second waveform is the same signal with leads (and polarity) flipped.
http://forums.pelicanparts.com/uploa...1312255360.jpg

Dan...thanks for looking into this, and the diagram.

I have CIS, not EFI, so maybe that's why you're not familiar with it. I'm hoping some of the CIS experts may chime in and explain it.

So I found some good information here:

Cabby Info - CIS

It looks like my multimeter reads the positive slope and CIS works on the negative slope. So I just need to subtract my reading from 100. Good to know.

Does your multi-meter read 100% with the probes shorted?

Or does it read 100% with the probes open?

It reads 0% with the probes open. The user manual says it reads the positive slope.