Need advice on Lambda CIS ECU - Duty cycle observations enclosed

schoward · 04-17-2014, 06:46 PM

1981 911SC. Car is stock with all CIS components in place and appear to be operating. First off, car runs and drives well. Cold, hot, in between - very good. No catastrophic events. But read on.

I spent a good part of the winter getting all the Lambda back in order. Previously, techs had just set the mixture rich and unplugged the O2. Installed a fresh O2 sensor, new Bosch O2 relay and refreshed the O2 connector which had crumbled apart. Set the mixture using a CO meter and fine tuned with a good three-wire dwell meter. Pretty pleased with the performance, driveability and improved gas mileage. I have no hitching/pulsing feel and hot idle is very stable after doing the work above.

However, probing the ECU with the dwell meter and testing the various duty cycles/enrichments gave me many anomalies which has me thinking ECU is gone bad in many aspects.

For any Lambda ECU gurus, please see my results below and chime in.

Observed behavior:

1. Cold Start (below 15 C/cold switch closed) - I get a constant stable duty cycle of 55% vs. the spec 65% Bentley specifies. Car starts cold fine and idles up and warms up pretty smooth, but a 10% deviation from the duty cycle spec. is significant. The 55% seems to be enough for decent warmup running though. If I disconnect the thermal switch, the duty cycle drops to 45-50% which tells me ECU is seeing the switch and getting the 'cold' ground, but yet max I get is fixed 55% duty cycle.

2. Hot Closed Loop - After 15C switch closes, dwell will initially drop to low 40s or so then raise up and dither in the 50's, eventually settling in at operating temp. fluctuating tightly around 45 dwell (50% duty cycle) which is I think exactly what should happen. Car runs great.

3. Throttle switch goes past 35 degrees - Dwell flutters and drops to 45-50% duty cycle and remains firm/stable at the that value as I roll on the throttle. If there is WOT enrichment that is supposed to happen, not seeing it. Needle stays firm between 40-45 dwell with no spike up as I would have thought. I would have thought needle should go over to right side of scale indicating enrichment. Wondering if I basically am getting No WOT Enrichment? - or is this normal? I haven't ripped into the throttle switch but I can hear it faintly click at it goes past 35 degrees and the ECU seems to see it with the firm/stable duty cycle at 45%-50%. Letting off throttle returns ECU to dithering mode. When driving/accelerating, car does not show any flat spots. Just goes ok.

4. Cold Enrichment (with engine warming up) - 2/3 degrees throttle/just off idle - No spike in the dwell as expected for 2 seconds or at all (Bentley says 75% duty cycle for 2 seconds).

5. Cold Enrichment (with engine cold/warming up) - 15 degree switch closes and I seem to get a momentary flutter to 60% or so dwell but not 75% duty cycle as manual says and certainly not two seconds.

Looking at the above, none are really catastrophic, so looking at each item above individually, I am not surprised that car runs and drives pretty well. While car runs well, I am thinking this the ECU may be bad and I may be 'leaving something on the table' in terms of performance.

Most notably #1 and #3. A higher bone cold fixed duty cycle could potentially yield a smoother warmup idle (albeit a short timeframe until the 15C switch warms up). #3 WOT Enrichment has me stumped. From what I read, I expected to see a bump in duty cycle but instead it seems throttle goes past 35 degrees and it just goes to the default 45-50% duty cycle and stays there. Am I missing out on some much needed enrichment?

For #4 and #5, if they are not working, they are minor because car can be driven off immediately and runs/drives fine cold - surprisingly well in fact. I remember reading these were just minor band aids for cold driveability. I would like them to work though as they should.

Any Lambda ECU gurus want to weigh-in? I would like this thing to work as it should. ECU gone bad you think? If you look at the above, any time the ECU has to provide a fixed duty cycle or enrichment, something seems off or weak in terms of duty cycle. Regular Hot Closed Loop mode seems perfect.

Sorry this so long. Thanks in advance.

tirwin · tirwin's Garage

I'm still reading your thread. But pull the part number on your ECU and verify it matches your model year. I have seen several posts where some previous owner or mechanic swapped the wrong part.

Also check out post #37 here from JR (javadog). This has a better breakdown of the duty cycle in various states:
Low power in low RPMs after reconnecting O2 sensor

schoward · 04-17-2014, 08:06 PM

Thanks

Just checked . I have an 0 280 800 055 ecu. I was surprised to see this as I thought the unit for 1981 end in a '037' part number vs '055'. But not sure it is material as I thought they same for 81 - 83.

Thanks for the link. The manuals seem to tell me my values are off. Way off. I have patiently worked the dwell on it and these tell me I may have something messed up with the ecu. But maybe I am jumping to conclusions too soon...

tirwin · tirwin's Garage

Ok, I read the whole thread.

Here is my first thoughts.

#1 - Have you verified your timing?

#2 - It would seem unlikely (but not impossible) that the ECU would have a partial failure. My gut tells me that when these old electronics fail, they probably just completely fail. And as you say the car is running well. Therefore, I'm inclined to believe that something else is going on.

#3 - You might try the same measurements with the O2 sensor disconnected.

#4 - Another possibility is something that Jim Williams helped me realize. It could simply be your dwell meter isn't that accurate. He has compared different meters to an actual oscilloscope measurement and noted quite a bit of variation.

tirwin · tirwin's Garage

Quote:

Originally Posted by schoward

Thanks

Just checked . I have an 0 280 800 055 ecu. I was surprised to see this as I thought the unit for 1981 end in a '037' part number vs '055'. But not sure it is material as I thought they same for 81 - 83.

Thanks for the link. The manuals seem to tell me my values are off. Way off. I have patiently worked the dwell on it and these tell me I may have something messed up with the ecu. But maybe I am jumping to conclusions too soon...

Actually '81 looks to be different from '82-83. The Porsche part number is the same (103 00), but the Bosch part numbers are different. '81 was '037' and '82-'83 are '055'.

I think JR noted the differences in the ECUs somewhere. Lemme see if I can dig it up. Probably not a big difference, but it might explain some of what you're seeing.

schoward · 08:34 PM

Quote:

Originally Posted by tirwin

Ok, I read the whole thread.

Here is my first thoughts.

#1 - Have you verified your timing?

#2 - It would seem unlikely (but not impossible) that the ECU would have a partial failure. My gut tells me that when these old electronics fail, they probably just completely fail. And as you say the car is running well. Therefore, I'm inclined to believe that something else is going on.

#3 - You might try the same measurements with the O2 sensor disconnected.

#4 - Another possibility is something that Jim Williams helped me realize. It could simply be your dwell meter isn't that accurate. He has compared different meters to an actual oscilloscope measurement and noted quite a bit of variation.

Haven't messed with my timing in awhile so no. I don't recall where it was left.

although not the same, I have had many cruise control units with various extents of failure so I don't know. But to your point , Car runs fine and you can even feel when you hit open loop past 35 degree switch when driving it 'feels like' some enrichment is happening although dwell doesn't show it - at all. Could be I'm not getting good readings and if you look my original observation in #1 above, directionally it is doing what it should - just out spec with the expected value.

If timing and fuel pressures will affect the fixed duty cycle signals and preset enrichments from the ecu, I'm just not there yet. But didn't think they would.

Thanks for checking on the Ecu part number. I peered under there expecting to see an '037' part number. But nope '055'....

Thanks

tirwin · tirwin's Garage

OK. I found where JR talked about that. It looks like the only difference in the ECUs is in the 055 the enrichment was kept in place until 25 seconds after the O2 sensor was warm and the duration of the acceleration enrichment is a little longer (2.5s for the 055 versus 2s). So that's probably not explaining a lot. My best guess is that the duty cycle measurement isn't completely trustworthy. If you had the mixture checked with an analyzer and it seems to be running fine, maybe it really is.

I would still verify that the timing is set properly for peace of mind.

mysocal911 · 09:18 PM

Quote:

Originally Posted by schoward

Thanks

Just checked . I have an 0 280 800 055 ecu. I was surprised to see this as I thought the unit for 1981 end in a '037' part number vs '055'. But not sure it is material as I thought they same for 81 - 83.

Thanks for the link. The manuals seem to tell me my values are off. Way off. I have patiently worked the dwell on it and these tell me I may have something messed up with the ecu. But maybe I am jumping to conclusions too soon...

The dwell measurement using a meter may not provide an accurate reading.
The FV is an inductive element like an ignition coil which causes 'ringing'
(an inductive voltage rise above the 12 volt supply) when switched 'off'.
This will cause problematic readings for some meters. One really needs a scope
or a meter which filters the ringing to read the proper dwell (duty cycle - DC).

The duty cycle should be 50% with the temp switch open, no O2 connected,
and no enrichment. The dwell time referenced HERE is the time the FV is grounded.
As the dwell time increases, the DC decreases (the FV is grounded for a longer time)
providing more enrichment, e.g. cold start, acceleration. The DC HERE is defined
as the time the FV is NOT grounded or it's the time pin 15 is at 12 volts.

The O2 input (pin 2) will cause the DC to vary around 50% if the mixture is set
properly and the temp switch is open (pin 7) and there's no enrichment. Obviously
the WUR must have reached its stable value (no more control pressure changes).
One can always check that the Lambda ECU is capable of responding to the O2 sensor
by grounding pin 2 (lean condition). This will reduce the DC causing the FV
to stay 'on' longer enriching the mixture, i.e. an increased dwell time.

One really needs a measurement device that more or less integrates the FV signal
like the Fluke Automotive DVM or a scope to accurately monitor the DC/dwell.

tirwin · tirwin's Garage

Quote:

Originally Posted by mysocal911

The duty cycle should be 50% with the temp switch open, no O2 connected,
and no enrichment. The dwell time referenced HERE is the time the FV is grounded.
As the dwell time increases, the DC decreases (the FV is grounded for a longer time)
providing more enrichment, e.g. cold start, acceleration. The DC HERE is defined
as the time the FV is NOT grounded or it's the time pin 15 is at 12 volts.

I think this is a really critical point. Dwell time and duty cycle are confusing. And what you're saying is a matter of perspective.

Simple example. If I tell you that the door was open 25% of the time in an hour and you say that that the door is shut 75% in the same hour, we are saying the same thing, just differently. So the first thing to understand is what exactly you're measuring.

Here is a picture Jim Williams provided.

Look at the square wave on the bottom of the picture. Each vertical grid line on the scope represents a unit of 10%. What this shows is a 65% dwell time (6.5 units) meaning the FV was grounded (ON) 65% of the time. This is what the meter is reading.

Going back to what Dave said - "As the dwell time increases, the duty cycle decreases (the FV is grounded for a longer time) providing more enrichment, e.g. cold start, acceleration."

The duty cycle is the amount of time the FV is not grounded.

So a dwell time of 25% and a duty cycle of 75% is saying the same thing.

To make it more confusing, on page 6-32 of the Probst CIS book, Figure 7-3 says:

"If the needle swings high on the scale when the system is operating in closed-loop, the mixture is set too lean; enrich the mixture."

The corresponding picture shows a dwell in the 54-63 range (on a 4-cyl scale) which is a duty cycle of 60-70%.

And Figure 7-4 says:

"A low swinging needle (duty cycle) means the mixture is set too rich; turn the screw counterclockwise to lean the mixture."

Here the picture shows a dwell in the 26-34 range (a duty cycle of 28-36%).

Quote:

Originally Posted by mysocal911

One really needs a measurement device that more or less integrates the FV signal like the Fluke Automotive DVM or a scope to accurately monitor the DC/dwell.

Agreed.

mysocal911 · 07:32 AM

Quote:

Originally Posted by tirwin

I think this is a really critical point. Dwell time and duty cycle are confusing. And what you're saying is a matter of perspective.

Simple example. If I tell you that the door was open 25% of the time in an hour and you say that that the door is shut 75% in the same hour, we are saying the same thing, just differently. So the first thing to understand is what exactly you're measuring.

Here is a picture Jim Williams provided.

Look at the square wave on the bottom of the picture. Each vertical grid line on the scope represents a unit of 10%. What this shows is a 65% dwell time (6.5 units) meaning the FV was grounded (ON) 65% of the time. This is what the meter is reading.

Going back to what Dave said - "As the dwell time increases, the duty cycle decreases (the FV is grounded for a longer time) providing more enrichment, e.g. cold start, acceleration."

The duty cycle is the amount of time the FV is not grounded.

So a dwell time of 25% and a duty cycle of 75% is saying the same thing.

On page 6-32 of the Probst CIS book, Figure 7-3 says:

"If the needle swings high on the scale when the system is operating in closed-loop, the mixture is set too lean; enrich the mixture."

The corresponding picture shows a dwell in the 54-63 range (on a 4-cyl scale) which is a duty cycle of 60-70%.

And Figure 7-4 says:

"A low swinging needle (duty cycle) means the mixture is set too rich; turn the screw counterclockwise to lean the mixture."

Here the picture shows a dwell in the 26-34 range (a duty cycle of 28-36%).

My meter is measuring the dwell time (but the function on the meter says duty cycle, which is really confusing).

Agreed.

Your providing the scope trace helps provide insight into the 'spike' (ringing/overshoot)
referenced in the previous post. That's what can be problematic for some meters
when attempting to measure dwell. The 'spike' basically decreases the dwell time
because the area of the 'spike' adds to positive portion of the square wave potentially
offsetting the meter's reading and distorting the actual dwell time measurement.

psalt · 08:10 AM

I have patiently worked the dwell on it and these tell me I may have something messed up with the ecu. But maybe I am jumping to conclusions too soon...

Schoward,

My advice would be to use a Wide band O2 sensor and actually measure the AFR at WOT. After all, that is what really matters, not duty cycle or dwell. Closed loop dwell is a proxy for AFR, a high number tells you the basic setting is too lean and the system is trying to correct it by richening up the mixture. A high open loop number is not directly comparable, it is only an assumption that the mixture is rich. A steady 50% default setting is not comparable to the system dithering the duty cycle slowly back and forth over 50%. You want the open loop WOT mixture to be around 13:1 AFR under load. Most of the time you need to set the closed loop dwell a bit on the rich side, 30-40, to get the open loop mixture correct. When you adjust the mixture with the FD screw, you are really only changing the open loop and cold running mixture, the system corrects the average mixture back to stoich when in closed loop. Usually if the WOT mixture is too lean, these engines will ping like crazy on pump fuel under high load. If you get the WOT mixture around 13, you usually can run a few more degrees of timing, especially if you get the hot breather air/oil out the the intake. MBT ignition timing is closer to 35 BTDC, the factory detuned these engines to 25 BTDC max to meet the 87 CLC octane fuel spec.

schoward · 04-25-2014, 12:54 PM

Guys - thanks. Just back from vaca so catching up.

Interesting effect illustrated here on the ringing/ distortion that can affect readings with non-professional equipment. I wonder if that is why I see things in some cases that are directionally correct but not dead on spec( #1 above). As an additional possible mishap example of using home grade equipment, I accidentally left the rear defogger on during one of my tests and it seemed to drop the dwell readings about 5%. I get the concept of closed loop and open loop readings not being congruent and the inverse relationship of duty cycle and dwell.

I need to study up more and perhaps get into air fuel mixtures as psalt suggests. Luckily things are running fine so I can take my time and learn on this one before flying off the handle and tearing into things with no direction.

Again, thanks.