80SC - Testing duty cycle for donkeys

[Glenfield]

I know this has been gone over many times. The work of Andrew Cologne, Tim Irwin and Tony (Boyt911SC) (to name but a few) has been invaluable and they rightly deserve their place in the Pantheon of Pelican Parts. Can lead a donkey to water though …

For the life of me, I am still unable to test duty cycle. Bear with me. The test port in my car is missing the green/white wire. Would love a hand with two questions as I tackle this:

1. There is a floating green / white wire, which I believe to be it. I’d attached a male bullet connector to it (see in the photo). Could this be my long lost friend?

2. If the answer to the above is “YES”, and I attach black cable of MM to it, where is best location to attach red cable to get ground? I realize I’m giving my donkey-ness away with that Q.

Cheers,
Tom

* Edit - ignore the unplugged 02 sensor. I’d temporarily pulled out while fidgeting back there but always run it in closed loop

[boyt911sc]

Tom,

Is this your test port? Your test port should have three (3) wires namely:

• Green/white from pin #17 ECU.
• Brown wire to ground.
• Red wire from fuse #22.

Do not connect the red wire to the ground.



Tony

[Glenfield]

Thanks Tony.

I hadn’t intended to mess with the red wire in test port. Red wire I was referred to was red lead on multimeter.

I do believe that’s the test port but it’s missing the green / white wire.

Regards,
Tom

[boyt911sc]

Tom,

The green/wire from the test port for your SC goes to pin #17 (ECU). Use a continuity tester to confirm that the green wire you located is the one that goes to the ECU @ pin #17. Most likely that’s the missing green & white wire. Test and confirm.

Tony

[AndrewCologne]

Hi Tom,

sorry for replying today to your PM
I think its anyway better discuss this official here in the public area :-)

As Tony said, follow his advice for the correct wiring towards pn 17 at the ECU.

Attached you see the wring diagram which should help you.

[Glenfield]

Thanks guys.

Had just enough cat6 lying around to span from engine to ECU and test. With wire to the furthest most pin (see photo), I have continuity. Seems the stray green/white is my test wire.

With that established, tried to test duty cycle. Black test chord to green/white and red test wire bolt where test port is grounded out. I get very odd results:
- either jumping all over the place
- 90%
- 20%

AFR under full load is 13.0 so probably a hair lean.



Cheers,
Tom

[AndrewCologne]

Originally Posted by Glenfield

Black test chord to green/white and red test wire bolt where test port is grounded out. I get very odd results:
- either jumping all over the place
- 90%
- 20%

AFR under full load is 13.0 so probably a hair lean.

Maybe the the test port ground is not ok.
Attach the black from the DMM to the green white wire and the red to the vehicles ground.
What are you using for reading the Duty Cycle?
What Hz value is shown when connected to that line/pin? Should be around 70 Hz.

[mysocal911]

Read here; 82 SC Fluctuating Lambda Duty Cycle

[Glenfield]

Originally Posted by AndrewCologne

Maybe the the test port ground is not ok.
Attach the black from the DMM to the green white wire and the red to the vehicles ground.
What are you using for reading the Duty Cycle?
What Hz value is shown when connected to that line/pin? Should be around 70 Hz.

Andrew, I’m using a DMM set to Hz%.

I’m attaching the red lead to the bolt on the ground by the CDI box with an alligator clip. Often I don’t get a reading. If I do get a reading, it bounces from 17%-70%.

*edit: the test port is a mess. Challenging to make out which is ground.

[AndrewCologne]

That's odd.

Try a different ground connection with more distance to the CDI unit as it could be responsive for interferencies during the measurement.

If that doesn't work, then remove the ecu and check it.
Use the approach explained by mysocal911 in this thread from page 5 on and see if your ECU is flawed or not.
https://forums.pelicanparts.com/porsche-911-technical-forum/1055502-high-dwell-911-sc-81-lambda-us-5.html

[Glenfield]

Thanks for the help fellas. Know there have been more interesting threads.

Tried grounding to the battery ground in the frunk using a span of wire. Have posted a video of the test here: https://youtu.be/IANnPn94vlc?si=5T-ScFP0laurzTOT

You’ll see in video that when I first connect it, the dwell is a fairly stable high 30s to low 40s. Had come off a long drive and briefly switched the car off to make the connections then back on. It then starts bouncing from low 20s to low 40s. 30 second later, it is reading 0%. Checked the connections and they are all in tact. Tried to repeat the test and got 0%. Odd.

Appreciate a sanity check on my method. If method isn’t flawed, there are two culprits (both can be true at once):
1. Set pig rich per mysocal911
2. Faulty ECU

Cheers again,
Tom

[AndrewCologne]

Do another test with ECU still installed .... at idle disconnect and lead the wire at the 15c temp switch (located at the right chain housing) towards to engine/ground. The duty cycle must switch to 65% (or 35% if DMM is connected wrongly inverted) and keeps static, means no up'n down swinging.