Help Reading My Multi-Meter - Duty Cycle

[arbita1]

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?

[steely]

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. :-)

[arbita1]

Originally Posted by steely

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?

[steely]

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.

[arbita1]

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.

[arbita1]

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.

[1990C4S]

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

Or does it read 100% with the probes open?

[arbita1]

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