Hi All,

I have read a lot about EGR Duty cycles and I understand what a duty cycle is (I am an engineer - but not in automotive), but everywhere I go for EGRs I see good explanations on the Duty Cycle RATIOS but NEVER the FREQUENCY of the cycle (how often it reoccurs).

When I am told that an EGR Duty Cycle is running at 50% I know that it means the Solenoid is activated 50% of the time and deactivated 50% of the time. But nobody ever says anything about how long (in orders of time magnitude) does the Duty Cycle repeat. Is it on the order of minutes, seconds, fractions of seconds?

Also, if the Duty Cycle is at a rapid rate, does that mean that the EGR Valve responds to the average of the On/Off ratio? That is, if at a very fast 50% Duty Cycle, would that mean the valve is 50% steady open (fast Duty Cycle frequency). Or a physically noticeable open 50% of the time and closed 50% of the time (slow Duty Cycle frequency).

Think of a light bulb driven by a 50% duty cycle at a low frequency - you will see the light flicker from full on to full off. But at a 50% duty cycle at a high frequency - you would not see the light flicker but it would appear to be at a steady half brightness. In either case, the light consumed basically is the same amount of light power - it just how much it "flickers" in either case. So, the Solenoid and EGR Diaphragm can work in a similar fashion - so it is a LOW or HIGH frequency duty cycle? - except we are talking vacuum rather than light power.

Look at an Air Conditioner for example, when I say it is duty cycling at 50% it will be on for say 10 minutes, then off for 10 minutes. So, you see, mentioning the Duty Cycle is only half the info - how quickly the duty cycle repeats is an important fact of its overall operation. An air conditioner that was at 50% duty cycle on for 1 second and off 1 second would not be energy efficient and destroy the compressor quickly.

So I gave two examples on knowing the frequency of the duty cycle as an important factor in knowing exactly how the EGR works.

Has anyone actually monitored the EGR Solenoid signal to see at what rate it goes on and off as well as its duty cycle ratio changes?

Thanks

------------------
Paul Romsky

[This message has been edited by Romsk (edited 10-02-2022).]

Paul,

Just in case you haven't seen this, the Service Manual has a section on EGR operation and it talks about what Sensors are consulted for EGR operation. My guess (which is probably wrong) is that the answer to your question is 'it depends,' that is, on what the Sensors are doing.

The EGR is activated by a vacuum solenoid, which is responding to signals from the ECM, so the 'Duty Cycle' in a Fiero is, in effect, the refresh speed of the computer. I recall reading that the computer refresh is 'really slow' compared to newer cars, but IIRC it still was a few times a second.

edit: I suspect the software has some sort of buffer written into it to limit the on/off frequency.

[This message has been edited by Gall757 (edited 10-13-2022).]

you've got me curious of what importance this is

quote Originally posted by Romsk: Hi All, I have read a lot about EGR Duty cycles and I understand what a duty cycle is (I am an engineer - but not in automotive), but everywhere I go for EGRs I see good explanations on the Duty Cycle RATIOS but NEVER the FREQUENCY of the cycle (how often it reoccurs). When I am told that an EGR Duty Cycle is running at 50% I know that it means the Solenoid is activated 50% of the time and deactivated 50% of the time. But nobody ever says anything about how long (in orders of time magnitude) does the Duty Cycle repeat. Is it on the order of minutes, seconds, fractions of seconds? Thanks

My Fiero is in storage so I can't throw my scope on it for you, but I can tell you I own a 1986 vintage copy of the GM service tech training for 1.8/2.8/3.0/3.8 PFI engines and it says
"The EGR vacuum control as a vacuum solenoid that uses pulse width modulation. This means the ECM turns the solenoid on and off many times a second and varies the amount of "on" time (pulse width) to vary the amount of EGR.

If you don't have a scope you might get lucky and be able to measure the frequency with a multimeter equipped with a frequency setting.

That said, you won't see any pulsewidth at idle. The book suggests (for automatic vehicles) putting the vehicle in drive and accelerating up to 1800 RPM while holding the brakes.

ZaraSpOOk,

The reason I want to know exactly how fast the PWM pulses occur is because I am diagnosing my Fiero and as I understand the finer details I post them in power point slides on my home page. It turns out my EGR Solenoid coil was reading in the mega Ohms when it should be around 80 to 100 Ohms at the A and B pins (which has a RC filter inside the potting material under the Vacuum Switch. So I rewound the coil. Knowing these details and with the procedures I am writing, people will be able to test, diagnose, and repair the entire Fiero EGR system. Those EGR Solenoids are not being made anymore, so we are all going to need to know how to repair them. My slides are almost done and very comprehensive.

So if the PWM frequency is on the order of seconds, the EGR valve will be fully opennor fully closed due to the slow changes in vacuum to the valve, but if the PWM frequency is fairly high, then the PWM pulses would act more like an average vaccum to the valve and it may then be partially open in that case.

Everyone talks about it simply opening or closing, we got that, but is the system designed to "partially" open the EGR valve and thus use a high frequency PWM.

Thanks.

[This message has been edited by Romsk (edited 11-14-2022).]