K-Lambda CIS facts or myths?

Won · 10:53 PM

I have come across a couple "facts" about the K-lambda CIS on US spec 83SC and would like to know if they are indeed true. Let's say O2 sensor and everything else is connected as per factory installation.

1. The system runs in open loop even with O2 sensor connected when the throttle is closed - true? Is that what the microswitch is for? If not, what is the function of the switch?

2. The system runs in open loop when throttle input >= 60 or so %, true or false? SCs do have a "throttle position switch" of sorts.

These interest me because with the O2 sensor unplugged my narrow band lambda output shows less than 0.1V at idle and around 0.6 - 0.8V anything above idle depending on how much I tweak the mixture (I'm assuming the lean idle is due to vacuum retard, unless it's indicating a problem that I'm totally unaware of

). Supposedly running the system in closed loop will help with emmisions and, to an extent, fuel economy as it will try to maintain lambda=1. But given my conditions it will try to more dump fuel at idle and lean out the mixture at WOT (neither of which are desired) unless the above 2 statements are true.

SC-targa · 03-12-2008, 05:46 AM

I don't believe it runs open loop at idle because plugging in the O2 sensor affects my idle speed.

Regards,

Jerry Kroeger

ChrisBennet · ChrisBennet's Garage

As I understand it, the microswitch is to detect whether the air meter plate is being lifted so in the event of an accident, the fuel pump doesn't continue to run when the engine is not. (I don't know if this works when the car is upside down though...)
-Chris

Won · 03-12-2008, 08:50 AM

The microswitch I was refering to is the one that's actuated by an extention arm attached to the throttle linkage.

fireant911 · 03-12-2008, 12:42 PM

Won,
I think you are right about the system being in open-loop during idle. According to the Bentley manual, the throttle position switch actually contains two microswitches: "one switch provides a closed throttle input to the oxygen sensor control unit and one switch provides full throttle position". The description adds that "when the idle contact switch is closed, the control unit delays the oxygen feedback cycle to prevent an erratic idle. When the full throttle contact is closed, the oxygen sensor control unit switches to an open-loop, fixed (richer) control mode...".

My car (81 SC) behaves like this description. After warmed up, the car goes to closed-loop control - this is easily seen on my A/F gauge. After coming to a stop, the car is still in closed-loop mode for several seconds and then goes to an open-loop mode. This change can both be heard and seen on the gauge. The gauge 'swinging' ceases and the needle is stationary on a AFR of 14.2. I also looked through two more manuals specifically addressing the BOSCH K-jetronic w/Lambda systems and both spoke of the two microswitch system but did not elaborate further than describing the operation.

Won · 03-12-2008, 05:14 PM

Daryl, thank you for the information. As Jerry mentioned the idle definitely changes (guess I had the answer to my first question already) when the O2 sensor is hooked up, but I haven't paid attention to the "delayed" feedback cycle. I'll listen for it next time I drive the car.

Loren, is the enrichment unit the smaller box that resides next to the lambda brain under the passenger seat? What kind of inputs does it take, the WOT microswitch? Does the system ever go in open loop under less that WOT?

I richened up the mixture to read ~0.6V at idle now but oddly enough the output at non-idle RPM was pretty much the same as before - above 0.7V. I'll keep tinkering with the O2 plugged/unplugged.