[CrewChief]

The CIS for early 80s functions about alike for all the Porsche cars. In other words your car CIS is functionaly identical to the CIS in the 924. I have the supplement at home for details when I get there. The same supplement covers both cars.

FWIW at the moment.

The O2 is only really involved when the car is at idle. The various microswitches tell it when idle is by throttle position. The rest of the time the sensor is not really in use. By all accounts, and my experience, the early systems were not very good and will at best 'hunt' when using the sensor output. The cars do run better without the sensor.

However, the lambda computer IS required to run the frequency valve at all times. The CIS distributor is setup to see the frquency valve modified pressure in the lower chamber(s) and will not run correctly with a non-functional frequency valve. No matter what you do; it's just too far off. There is a table which shows the duty cycle of the frquency valve at various operating conditions. Most of the time it is fixed at 50% (IIRC) with microswitches to create accleration enrichement for a cold engine and full throttle enrichement. The table conditions depend on engine temperature, sensor temperature and throttle positions.

If the frequency valve is completely dead the most likely cause is bad connections and wiring. A quick check to see if there is output to drive the frequency valve is to find the 3 prong round connector that is for testing the signal. The red and brown connections were to power the factory supplied tool. Dont' short them. The black wire is the signal. Use a dwell meter to check the square wave to the frequency valve. Since this is a test coming directly from the computer you will then know if the problem is at or after the computer.

The CIS really functions quite well and has plenty of capacity for the power developed. The only major limitation is that it will not tolerate hot cams to jack up the power.

[William Miller]

John, I have an 83 euro. The CIS is set up like the 79 US with no computer or O2 Sensor.
I know my system inside and out because it was so screwed up when I bought the car.

I have book knowledge about the K-lambda system and understand what you are talking about. I believe I've read that you can actually tune (Adjust Mixture) using the dwell meter on the tests connections.

I haven't read it in a while, but if this is true, almost any of us with a dwell meter can do this initial setting without a Gas anylizer or pating a shop to do it. I'm I right? Have you ever done it this way?

One problem (environmentalists wouldn't agree) with the stock K-Lambda system is that it's designed to maintain the correct mixtures to deliver the right gasses to the converter. This is a little leaner than than tuning for max power.

That's why when the sensor is disconnected it possibly runs better. The default settings (Fail safe) are a little richer than optimal for emissions.

[Jase007]

Or,

Retrofit to the 74-76 years (no Lambda computer, O2 sensor, frequency valve, catalytic converter, emissions, etc...) Set the CO, idle screw, sensor plate, check cold start, AAV and go ...

Of course, you'd have to give away your 3.0 for a 2.7 (an extra I happen to have) and everything will work out fine. As a matter of fact, I can alleviate your burden of disposing of the 3.0 once replaced...

Jase

[CrewChief]

Disclaimer: from memory and I only paid attention to the SC portion of the supplement 'sorta'. However, they do function the same.

Yes, I did. I think it is even a 'book' procedure. At the correct mixure setting the duty cycle hunts around (some number?), 45 or 55% or so. All done at idle as remember this system only really took full control of the mixture at idle. Or at least as much control as was allocated to the lower chamber pressure adjustments. So you really are only adjusting the mixture to a basic setting that gets into the middle of the lambda window at idle. I've also monitored the voltage out of the sensor and compared it to the duty cycle and they did track as you would expect. So it's possible to skip the dwell meter (like who has dwell meters anymore?) and just put a high impedance voltmeter on the O2 sensor. IIRC it should vary from 450 mv or so. This opens up the idea of disconnecting the sensor from the lambda system to get rid of the surging and other effects; but you can check the mixture from monitoring the voltage while you adjust.

Speaking of dwell meters. If you have any kind of automotive or electronic scope you can monitor the duty cycle using the scope. Frequency response is not an issue as the equivalent frequency of almost all auto applications is only low audio range (at most) and well within the capabilities of the cheapest scope ever made. Also, many of the $45 multimeters, such as Sears, may have a duty cycle function. Find one of them for the tool box.

As with the earlier CIS (and I guess all the Euro?) the mixture for the rest of the envelope was then set by the metering piston with the added fixed variations of the lower chamber as adjusted by the frequency valve.

For a warm engine the default duty cycle is 55% (?) with full throttle enrichment to 65% (?). Cold engines conditions use an off-idle microswitch for an acceleration enrichment of 2 seconds or passing a 12degree throttle position enrichment of 2 second; unless the off-idle is already active. That kind of thing. The big point being that a warm engine makes very little use of the O2 sensor. ONLY at idle.

Jase, I was, and am, keeping an eye out for the early fuel distributor for my 924. Though now that I seem to have it running ... knock on wood. Also, it would not take too much to be clever and use the frequency valve to enrichen the mixture to suite for track use. All that needs doing is find a circuit for a squarewave generator with variable duty cycle to drive the frequency valve. I have also thought about that as an alternative to the expensive relacement of the lambda computer.

[CrewChief]

Bill, my apologies. Re-reading I see my verbosity got the better of me.

To answer your question: Yes.

[William Miller]

That's ok my next question was going to be how?

I have an Air/fuel meter on my car that is bassically a volt meter powered by a narrow band O2 sensor. I have used this for a tuning referance although many say it's not accurate except very close to lambda.
There is a new device on the market that is sold with a wide band sensor which both records the air/fuel mixture over tima but it also can be displayed with a meter but for your pourposes can be used to feed the stock computer. It can be programmed to give this output in a scale that will cause the duty cycle to be set for a richer or max power setting.

Charlie knows all the details. We were going to try a group buy but my money got dried up with my restoration.

The details are here on the board somewhere. They are now selling it on pelican parts. Take a look when you get a chance.

My reason for asking was I don't throw anything away and still have my dwell meter. Now it has a second use, tuning K-lambda CIS!

Anyone want to be the ginipig?

[CrewChief]

Interesting. I don't know how well it would work just using the stock computer as it was only designed to tweak the mixture for emissions as opposed to running the motor. As you stated, the stock O2 sensor is narrow band and is useful only close to center. So also the computer. But that's the only function it had. I don't know if it would react correctly, or quickly enough, to use lambda in a feedback loop. That is distinctly different than just useing the computer as a squarewave generator to statically change adjust the mixture on the track.

I'll try tonight to publish the table of conditions and outputs for the SC. If I have not; that should give a good insight into how the system was intended.

[William Miller]

Take a look at what you can find on this board and on PP. I don't know the exact details But I think it will work for you.
I'd love to see your reasearch and analysis. It seems like that's what your into.

A table of conditions and outputs is the most important tool I can think of in understanding how these systems work. I've seen many bit's and pieces in the litrature that I have read, but not a comprehensive sequence of events. Including time and tempature during warm up.

As far as I know this will be information that is not published in this form anywhere.

Although my car has a much simpler CIS system all the parts are there to make it perform well. It's just not made to perform well AND keep emmission low.

[CrewChief]

To Ken Robinsons original question. It turns out that the manual I have is (a copy first) probably from about 1979 or 80. "Porsche Engines with Oxygen Sensor". Covers the " 924/924 Turbo and 911SC" and honorable mention of the 928 somewhere. The table of conditions I have is only for the 924/924T and is in the yearly supplement for that model for 1980.

The "P Engines w O2 Sensrs" does have a wiring diagram for the SC. Also a trouble shooting logic tree. However the fuse numbers do not match those you all quoted.

I would suggest a test light or voltmeter test at the frequency valve first. It's not buzzing; but why? There should be 12 volts at one of the two connections of the FV, as the power is fed to the FV and the computer switches the ground side. The diagram in the pamphlet shows the frequency valve wired to a 6 pin multiple plug. Pin 4 of that plug should be the 'hot' lead of the frequency valve and connects to 87b or the 'Power Supply Relay'. 87 of the same relay supplies the O2 Control Unit pin 8. 87b and 87 of the PSR are both supplied from 30 of the same relay which connects in turn to "Fuse box in trunk - fuse 1)" If you gots power to all these places then its time to check the (squarewave) output of the brain.

Check that either by the book at the test connector (which connects to pin 17 of the O2 Cntrl Unit) or directly at the FV using the ole dwell meter.

Note that the functioning (or not) of the sensor and other junk is not really important to getting the frequency valve running. In fact, as previsouly mentioned, I would disconnect the sensor and forget about it. With a good cat I bet it would pass emissions without.

Grounds are problematic with these things. I see two to check. The O2 Cntrl Unit pin 16 and Power Supply Relay 85 both show connected to the same engine ground. O2 Ctl U pin 5 goes through the 6 pin multi plug pin 2 to a body ground.

Hope this helps.

Tomorrows a track day. Good night.

[Charlie Stylianos]

I'll chime in here as I helped Ken with this today.

There should be 12 volts at one of the two connections of the FV, as the power is fed to the FV and the computer switches the ground side.

This checked out fine. We had 12v to the FV from the lambda relay side. When we tested the computer side, we read a constant +.5v. It could have been switching fast enough that the multimeter wasnt keeping up.

The diagram in the pamphlet shows the frequency valve wired to a 6 pin multiple plug.

Roger. The Acceleration Enrichment Unit.

Pin 4 of that plug should be the 'hot' lead of the frequency valve and connects to 87b or the 'Power Supply Relay'.

This is where we tested and found the +12v.

87 of the same relay supplies the O2 Control Unit pin 8.

Did not check voltage to pin 8 at the computer, should be ok since relay is new. Probably good to check anyway.

87b and 87 of the PSR are both supplied from 30 of the same relay which connects in turn to "Fuse box in trunk - fuse 1)"

+12v here also.

Check that either by the book at the test connector (which connects to pin 17 of the O2 Cntrl Unit) or directly at the FV using the ole dwell meter.

Nope. Still no dwell activity

Grounds are problematic with these things. I see two to check. The O2 Cntrl Unit pin 16 and Power Supply Relay 85 both show connected to the same engine ground. O2 Ctl U pin 5 goes through the 6 pin multi plug pin 2 to a body ground.

Pin 16 and relay 85 checked out ok. Need to check pin 5 on the brain.

If you gots power to all these places then its time to check the (squarewave) output of the brain.

That was our next step.....swap out the brain and see what happens.


Thanks John. That was great stuff!!