I'm doing some CIS testing, trying make sure all the components are working as they should be.

When I checked the Duty Cycle with a dwell meter, my reading was 21% (18 deg.) constant with and without the O2 sensor plugged in.

Any ideas what can be causing that?

 

A bump for iceman... He may know

 

Hell I don't even know what dwell is !!! .. i do have a timing light but honestly have never hooked it up .. !!

 

It has to do with the O2 sensor system. You have a 78. You don't have to worry about it.

 

Dwell is usually associated with the ignition - specifically points. Dwell is how long the points are in contact. The O2 system may "duty cycle" as it oscillates, but not really. The O2 is one of only a few components that CREATE energy(voltage)on these cars. A knock-sensor is another creator of voltage on newer cars. The alternator, crank and referance sensors are others. The O2 is measured by simply using a volt meter. An O2's output sweeps back and forth from about .2 to .9 volts continuously and should average about half(.5)volts when measured with a true RMS meter.
I'm not sure if this helps or if I am confused with terminology. Do you have a problem you are trying to isolate? or is this just an exercise in discovery?

 

One more thing. O2 failure modes manifest themselves as a "lazy"(slow to sweep) situation, "bias" where the average voltage is in the upper or lower registers, or just caput - no volt.
Against industry hype, O2 sensors are pretty robust little units and usually last a long time in well maintained cars. One of the biggest killers of O2s is silicon. Often people will use silicon paste or spray on the intake boots etc. Silicon molecules pass right thru the combustion process and can attach themselves to the O2 sensor killing it quickly. Don't use silicon around the intake, especially spraying it around the motor when it is running.

 

I don't believe my O2 sensor system is functioning properly. So I'm trying to determine if it is.

 

Dwell for inductive spark ignitions is only critical at the higher engine RPM level when there often isn't enough time between sparks to "charge" the coil. With solid state, transistorized inductive ignitions 18% dwell at a low RPM is perfectly acceptable.

 

I think we've gotten off base. I'm having a problem with the duty cycle not dwell. Duty cycle is measured with a dwell meter.

 

Duty cycle does not really apply to an O2 sensor. It does not pulse. An O2 emits a constant, yet varying, DC voltage. How are you measuring the "dwell" of your O2 sensor? You should be able to unplug the sensor, connect a voltmeter between the output and ground to see whats happening.

 

I'm not. I'm measuring the duty cycle from the test port in the engine bay.

 

Okay well i don't feel like as much of a dunce as i did a couple days ago . the only dwell i new about was on my points motorcyle . But I am not going to "dwell" over it"

I think we are just talking terminologies .. and I think I understand what is happening now .. are you really trying to determine the duty cycle of thr frequency valve ?

 

Correct. I've determined it to be 21% constant in open/closed loop operation...which is out of spec.

 

Oh, I see.

The early oxygen sensors had a binary output, oxygen was either present in the exhaust or it was not. The engine ECU uses a duty cycle to slightly vary the A/F mixture plus and minus around the ideal mixture and then "watches" the oxygen sensor output to be sure it swings back and forth in synchronization.

Actual duty cycle is likely meaningless, the importance is that the oxygen sensor output is following the A/F mixture "modulation" indicating that on the average the A/F mixture is correct.

 

You are correct, you should only see the duty cycle waveform from the oxygen sensor output in closed loop operation, idling or relatively constant speed cruising. The mixture should be enriched beyond the sensing point of the oxygen sensor anytime the engine is under a serious level of pulling load, acceleration, towing, uphill, etc.