Duty cycle and dwell testing

[joe912/62]

Don't know if I'm doing this right but I'm trying to get a reading for duty cycle of the Lambda unit I think. I connected my multimeter to the green white wire from the test connector. Don't really understand the readings I'm getting. Please refer to video.
https://youtube.com/shorts/YP6ogef3jkM
https://youtube.com/shorts/5LwKaZ8FMy8

[Schulisco]

You need to cross the measurements wires:

Measuring the clock rate (duty cycle)
This can best be displayed with a multimeter (or oscilloscope – see above), which supports a duty cycle measurement. I use a Voltcraft 175 for this, which can display the duty cycle (%) in addition to the frequency (Hz), there are also many models from other manufacturers that support this as well. The red cable from the multimeter goes to ground here and the black to the green/white cable of the test port, on the back left in the area between the HKZ and the fuse holder.

Taken from Andrews fabulous page about the 930/16 (US model with catalyst) equipped 911 SC:
https://nineelevenheaven-wordpress-com.translate.goog/abgasregelung-beim-sc/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp

Thomas

[Damon88]

Joe912 thanks for posting this and asking these questions..I'm trying to figure this process out myself with my 83..

[joe912/62]

So I have the cables switched?

[troth]

I bought a multimeter with a duty cycle function specifically for this test, but got similar results to you Joe. I reverted to using my digital timing light that has a dwell function on it which works well. I'll have to try Thomas' suggestion of swapping the test leads.

[Funracer]

I was never able to get a duty cycle reading using a voltmeter even if it had a duty cycle setting. The only way I could ever measure it was with a cheap oscilliscope

[Damon88]

Originally Posted by Funracer

I was never able to get a duty cycle reading using a voltmeter even if it had a duty cycle setting. The only way I could ever measure it was with a cheap oscilliscope

Would you be able to help with some info on how you used this oscilloscope. I was looking at buying this same unit to set my ARF on my 83. complete newb when it comes to using a unit like this.. Cheers

[Schulisco]

I purchased also thiis tiny and cheap oscilloscope and I can recommend it for this purpose. The frequency of the signal is roundabout 70Hz, it varies on duty cycle only. You can measure in parallel to the output of the frequency valve. No swapping of wires necessary.

Thomas

[AndrewCologne]

The oscilloscope model is called "dso150" and is available at Amazon, Ebay and other shops in the www.

[ahh911]

Same scope, weak point is there is no battery compartment.
Used a spare connector and hooked up a 6x1.5V AA battery.
Found the pin under the seat that corresponds to FV duty cycle, slipped in a thin wire.
Also could control the 15C switch that drives the duty cycle up to 65%.
I could then monitor the duty cycle at all times while driving and change the enrichment on the drive to problem solve. Worked very well, but when the battery runs even a little low, the zero calibration goes out of range and caused some confusion.

Phil

[Funracer]

Originally Posted by Damon88

Would you be able to help with some info on how you used this oscilloscope. I was looking at buying this same unit to set my ARF on my 83. complete newb when it comes to using a unit like this.. Cheers

I have an 83 as well and had never used one either.

It’s actually really easy to use. Set the dials on the bottom of the scope as you see in my picture. Plug into 120VAC extension cord and rest it on the bumper or out of the way of moving parts. I think it was red clip to ground (I used the wing nut on the rear fuse cover) and black to the green wire pin in the test socket. You can then watch the FV change as you move the 3mm mixture adjustment screw. Shooting for 42-45 range.

Others with more experience please correct me if needed.

Good luck

[mysocal911]

Read here at post #15; High dwell on 911 SC ‘81 lambda (US)

It doesn't get any more detailed than that!

[joe912/62]

I don't know but reading past post #15 it just gets more confusing to me. Plugged unplugged pin# and so on.

[Damon88]

Just ordered an oscilloscope dso150. Thanks for the info Funracer and Andrew !

[joe912/62]

I ordered one also. I also switched out the relay for the Lambda under the passenger seat because I can't feel the FV buzzing or vibrating. I don't know if I'm getting power at the plug. Do I need to jump the fuel pump to test the FV plug?

[mysocal911]

Originally Posted by joe912/62

I ordered one also. I also switched out the relay for the Lambda under the passenger seat because I can't feel the FV buzzing or vibrating. I don't know if I'm getting power at the plug. Do I need to jump the fuel pump to test the FV plug?

Buy and learn how to use a test-light.

[joe912/62]

Simple question do I need to jump the FP relay to test the connection at the FV plug?
I know how a test light works thanks.

[Damon88]

So I just received my oscilloscope.on my 83 engine My question is.

Should I unplug O2 in engine bay before I try adjusting AFR. I’m running a cat bypass with 02 . And should I be trying to get between 50 and 60 % duty C at idle when engine is a running temp. Any help would be greatly appreciated. Cheers

[Schulisco]

Yes. See more details on how to adjust the mixture here: https://jimsbasementworkshop.com/CIS/pages/test_mixtures.html
Be aware that the exhausts will flow a bit faster with the cat bypass. This impacts the mixture as well as it can lean the mixture, esp. on higher revs.
Make some test drives and monitor the duty cycle while driving and plugged O2 sensor to make sure that the engine won't run too lean. Refer to the Porsche workshop manual for the lambda models from 81-83 for the exact CO values...
In general: As long as the duty cycle remains under load below 50% I tend ro say that it's okay and the lambda control has some reserves to enrichen the mixture in special cases. The CIS is designed to enrichen the mixture on higher revs and load so that it will enrichen it by herself if it works correct and all components are the correct ones fitted for your engine.
If you see 70% (higher duty cycles the stock control unit cannot give) the mixture is too lean. But don't forget:This all assumes that the O2 sensor works perfectly.

Thomas

[Damon88]

Originally Posted by Funracer

I was never able to get a duty cycle reading using a voltmeter even if it had a duty cycle setting. The only way I could ever measure it was with a cheap oscilliscope

Funracer I copied your set up on the oscilloscope and gave it a try. Engine was at operating temp. The % seems to jump around quite a bit while idling and adjusting. But here is mine. I'm Feeling like a deer in headlights a bit.. But car seems idle fine at 950rpm.