High dwell on 911 SC ‘81 lambda (US)

[Funracer]

Originally Posted by mysocal911

Ringing from the frequency valve pulses can distort DC measurements with some meters, here DC = 83%;



Closeup of ringing;



Proper DC (50%) using a test light;



Therefore, disconnect the FV and use a test light as the FV, alligator clip to +12V and other end to FV connection at pin 15,
and measure the DC.

Interesting idea. Maybe I just need a better DVM then.

What kind of test light? Oscilloscope? Digital timing light? O2 and AEC unplugged? You may be losing me here but if I have the equipment I will give it a whirl.
Thanks

[Funracer]

Originally Posted by mysocal911

Why are you now worried about the O2 sensor voltage? Focus on getting the DC to 50% without any inputs on pins 2,7,12,14
(pins 7,12,14 must be above 5 volts, pin 2 at about .50V).

My pin 14 was less than 1V. What does this mean?

[mysocal911]

Originally Posted by Funracer

My pin 14 was less than 1V. What does this mean?

It doesn't matter. All three of the ECUs I tested had that voltage. Did you check the duty cycle by disconnecting
the frequency valve and simulating it using a test light, to determine whether your meter is incorrectly measuring the DC?

[mysocal911]

Did you check the duty cycle by disconnecting the frequency valve and simulating it using a test light, to determine whether
your meter is incorrectly measuring the DC?

Test light; https://www.amazon.com/Jastind-automoci%C3%B3n-resistencia-extendidos-perforaci%C3%B3n/dp/B07Q3622FT/ref=sr_1_2?crid=EODM2XOI4V9R&dchild=1&keywords=automotive+test+light+12+volt&qid=1591672543&sprefix=automotive+test+light%2Caps%2C199&sr=8-2

A test light is hooked-up as the FV would be to simulate the FV connected to pin 15. The alligator clip is connected to fuse panel
next to the motor, and the pointed end is connected to pin 15. Before connecting to pin 15, touch the pointed end to engine to
check that the light functions. In some cases using a test light is better and quicker than using a voltmeter. Using a test light
in place of the FV, one can easily see how the control pins (2,7,11,12) affect (the brightness changes) the DC.

[Funracer]

I have one of those and will check it tomorrow. AEC and O2 disconnected? Looking for 50%?
Thanks

[mysocal911]

Originally Posted by Funracer

I have one of those and will check it tomorrow. AEC and O2 disconnected? Looking for 50%?
Thanks

Correct!

[mysocal911]

More Lambda ECU scope images for the OP's ECU (055):

[Funracer]

I have unplugged the FV connector in the engine compartment. Would it be easier to plug into the FV plug and clip on to the fuse or does it have to come from pin 15?

[Funracer]

Dave I think you have lost me on this test that substitutes the test light for the FV. I warmed up the car, unplugged the FV, O2 and AEC, clipped to the engine compartment fuse and added two alligator clip extensions to get the test light tip to pin 15. Does that all sound right? The light flickered but I do not know what I was looking at. Was I supposed to check the DC at the same time? If so I will need a helper next time.

Originally Posted by mysocal911

Was this done? With that done and with the O2 sensor disconnected, the duty cycle should be close to 50%, if not the Lambda ECU is bad.
Again, the Lambda system is extremely simple to diagnose!

I plugged and unplugged the AEC several times. DC never moved at all stayed at about 80%

[mysocal911]

Originally Posted by Funracer

I warmed up the car,

You don't need to warm the car up.

Originally Posted by Funracer

unplugged the FV, O2 and AEC, clipped to the engine compartment fuse and added two alligator clip extensions to get the test light tip to pin 15.

Good, but pin 7 (oil temp signal) needs to be disconnected too.

Originally Posted by Funracer

The light flickered but I do not know what I was looking at.

The light should not flicker, but should be dimmer than when the test light lead is grounded instead of being connected to pin 15, (about 1/2 as bright).

Originally Posted by Funracer

Was I supposed to check the DC at the same time?

Yes, it should be 50%. As you ground pin 7 or pin 2, the light will get brighter and the DC will increase.

[Funracer]

Given that the DC does not go to 50% when the AEC is unplugged, can I conclude that either my
ECU is bad or I need a better DVM?

[mysocal911]

Originally Posted by Funracer

Given that the DC does not go to 50% when the AEC is unplugged, can I conclude that either my
ECU is bad or I need a better DVM?

By using the test light instead of the FV, you should be seeing the brightness of the test light change as the various
pins 2/7/12 are individually grounded. That basically gives you a visual of whether the ECU is functioning.
If the brightness doesn't change, the ECU is bad. The test light should be the brightest with pin 2 (O2 sensor)
grounded. What does this test indicate?

[Funracer]

Sent you a PM

[mysocal911]

Good luck, hopefully you'll find your problem.

[Funracer]

Poking around last night discovered the throttle micro switch, the one with brown wire and green wire left of the throttle body was out of adjustment. The throttle linkage rod was too long, meaning the the switch was in the “off idle” position all the time. DC reading for setting the mixture is set at idle of course so this switch telling the ECU the wrong info.

Unfortunately, or maybe fortunately depending on your point of view, we have visitors from out of town for the weekend so it may be a day or two before I can check the effects of this discovery.

[mysocal911]

Originally Posted by Funracer

Poking around last night discovered the throttle micro switch, the one with brown wire and green wire left of the throttle body was out of adjustment.

Which pins on the connector?

[Funracer]

Originally Posted by mysocal911

Which pins on the connector?

It goes directly to the AEC

[mysocal911]

Originally Posted by Funracer

It goes directly to the AEC

If the AEC is disconnected, it won't affect the DC. Yes, once the DC is functioning properly,
the switch needs to be adjusted correctly.

[wosm]

Hi Mysocal911, thank you that you are (also) helping funracer to sort out the problem with the high dwell reading. I started this issue on the forum a few months ago. Brought my 911 to two different so called Porsche specialists in the meantime, but the hampering around 2200 rpm is still present. They only solved the backfire problem, re-set the ignition timing according specs (showed at inside of back lid of the car), put the Co on 0.8% etc. I still hear the FV buzzing like hell (high frequency) and read 88% dwell all the time. I even had a tiny fuel leakage at the hose clamp next to the FV. Maybe because of this continues high dwell of 88%?! No matter if I loosen the O2, 15 degree thermoswitch. I dont know if my dwell gauge is a reliable one. I can see the dwell only drops to ~70% if I put a 1.5V battery on the O2 connector.

Last 2 days I did all the tests you (and others) asked Funracer to do. I didnt find any rootcause or strange deviation. Until today! I bridged the Oxs relay connector (pin 30 ->87a) with a wire, disconnected the ACE and also the 15degree thermoswitch (input to pin 7), added the alligator clip on pin30 (12V) of the light pencil and touched the pen to pin15 with ECU connected. The light showed half bright. When I grounded pin 7 the light pencil didnt change from half bright to full brightness. Can I conclude that my ECU is bad? Shall I open the ECU? What to check?

Your support is welcome Many thanks in advance!

[mysocal911]

Originally Posted by wosm

Last 2 days I did all the tests you (and others) asked Funracer to do. I didnt find any rootcause or strange deviation. Until today! I bridged the Oxs relay connector (pin 30 ->87a) with a wire, disconnected the ACE and also the 15degree thermoswitch (input to pin 7), added the alligator clip on pin30 (12V) of the light pencil and touched the pen to pin15 with ECU connected. The light showed half bright.

Great, you've setup an alternate way of evaluating the duty cycle (DC) without using a meter. First, you need to determine which Lambda ECU
you have to evaluate which pins affect the DC. Here're some of the ones mentioned in this thread;

0 280 800 055
2 - .53, O2 input
6 - 12, no Lambda effect (NLE)
7 - 12, enriches at ground (EAG)
9 - 5, NLE
11 - 12, EAG
12 - 12, EAG

0 280 800 006
2 - .53, O2 input
6 - 12, NLE
7 - 12, EAG
9 - 5, NLE
12 - 12, EAG

0 280 800 037
2 - .53 O2 input
6 - 12, NLE
7 - 12, EAG
9 - 5, NLE
11 - 12, EAG
12 - 12, EAG

Originally Posted by wosm

When I grounded pin 7 the light pencil didnt change from half bright to full brightness. Can I conclude that my ECU is bad? Shall I open the ECU? What to check?

First, you need to make sure none of the control pins (2,7,11,12) is grounded (disconnect the AEC) and the oil temp (pin 7).
Then ground each control pin momentarily to see if the light brightness changes.