Wrong idea about “duty cycle adjustment“
I’d like to remind CIS-E car owners to an essential, but largely unknown, fact that I addressed at the beginning of this thread already.
Still many posts here at Benzworld reveal the belief that the air/fuel mixture is changed when the duty cycle is changed/readjusted … that the engine is running leaner when the duty cycle is fluctuating at a higher range and that it is running richer when it’s fluctuating at a lower range. I also saw documents posted that reveal or support this belief. Some of them looked like original MB documents at first view, but are not. Even the real MB FSM from Stuttgart can mislead to believe that, also the German version. … But it’s a misconception !
… In fact it is the most widespread misconception about the KE-Jetronic … and the also widespread habit of calling the KE-Jetronic’s Lambda duty cycle adjustment “mixture adjustment”
will probably continue to feed it.
For the sake of better diagnosing/troubleshooting I really recommend to leave this misconception behind and to think of the Lambda duty cycle check/adjustment as what it actually is, namely an “EHA operating range check/adjustment
”. That’s the range within which the EHA’s baffle plate fluctuates during Lambda control in relation to its currentless rest position. … That is not to be confused (!
) with “EHA adjustment” via the small hidden adjustment screw on the backside of the EHA, by which the EHA’s, or to be more specific, its baffle plate’s currentless rest position itself is changed, which is an option that should only be taken into consideration in very few specific cases, verified by specific tests … and carried out properly, again verified by specific tests … and not be discussed in this Lambda control thread, please. :wink_2:
I usually do Lambda control checks/adjustments by directly measuring the EHA current instead of by measuring the on/off signal (in ‘volt’, ‘duty cycle’ or ‘dwell angle’) at the X11 diagnostic coupling. Afterwards I check if the CIS-ECU is telling the X11 diagnostic coupling in form of the Lambda on/off signal (which is actually better called off/on signal) the truth about what it is telling the EHA. That can reveal possible ECU problems that would otherwise be overlooked and lead into wrong troubleshooting directions !
… Only when I’m on the road in my (wife‘s) 300CE I occasionally monitor Lambda control via the off/on signal in the ‘ashtray’ (see post 36
), which also allows me to monitor during driving if there is any (intermittent) problem that is represented by one of the CIS-ECU‘s error codes.
Anyway … no matter how often or where you read or hear about “adjusting / changing the mixture“
, I assure you that on the KE-Jetronic you can not adjust or change the air/fuel mixture !
… unless you turn the Lambda adjustment screw cw beyond or (close) to the system’s leaning limit (duty cycle = 0%) or ccw beyond or (close) to its enriching limit (duty cycle = 100%), or Lambda control is not active (warm-up phase, WOT, or due to a problem).
With its Lambda control function the CIS-ECU tries to keep the air/fuel ratio (via EHA control) in the range where it receives O2 sensor voltage fluctuating around 450 mV, which represents “stoichiometric air/fuel ratio”
… provided that there are no other fuel combustion affecting problems !
.. which is why instead of saying “which represents stoichiometric air/fuel ratio”
, it’s more precise to say “which the ECU takes as representative for stoichiometric air/fuel ratio”
.. a mostly overlooked, albeit for CIS-E diagnosis essential, difference I’ll get back to later.
When the duty cycle is fluctuating
… (with amplitude & frequency as described under “Check procedures“
in post #2 !
) … Lambda control is active. No matter whether it fluctuates around a low value or around 50% or around a high value .. if it is fluctuating, the (intact) ECU is receiving O2 sensor voltage fluctuating (in case of a healthy o2 sensor with about +/- 350 mV) around 450 mV.
Thus, provided that the O2 sensor input to the ECU is reliable and that the fuel combustion is not impaired (in any way !
), when the duty cycle is fluctuating, Lambda is fluctuating with about +/- 0.02 around ‘1‘ (‘λ = 1 +/- 0.02’
) … the air/fuel ratio is fluctuating with about +/- 0.3 around ‘14.7’ in case of non-ethanol fuel (or, for instance, around ‘14’ in case of E10, even though a wideband a/f meter would still read 14.7
) … hence, the air/fuel ratio is stoichiometric
That ratio is not changed when the range of the fluctuating duty cycle is changed !
… unless it is changed beyond or (close) to 0% or 100%.
I explained all that and what happens when the Lambda adjustment screw is turned in more detail in post #1 of this thread.
Don‘t ignore the purpose of the KE-Jetronic’s “duty cycle” !
Two of the most commonly made CIS-E-related mistakes are also still largely made here at Benzworld … ignoring
the Lambda off/on ratio (duty cycle) and, IF
it is checked and it deviates from the target value, rashly readjusting
it via Lambda adjustment screw … or even worse, tampering
with the Lambda adjustment screw without caring about the duty cycle.
That‘s why I‘d like to reiterate what I addressed already in posts 2 & 22, namely that the purpose of the duty cycle is to provide comfortably available diagnostic information
… which should neither be ignored nor wiped away by simply turning the Lambda adjustment screw !
… Since it was likely that the latter would often be done by DIYers or incompetent mechanics at workshops, there was a plug put into the Lambda adjustment tower.
Duty cycle deviation should first induce a search for its cause, not for the adjustment wrench !
duty cycle is an easy to check representative of the EHA current which determines the EHA’s operating range that the ECU has to provide in order to keep receiving o2 sensor voltage fluctuating around 450 mV (which the ECU takes as representative for stoichiometric air/fuel ratio).
duty cycle, while the engine is running, is an error code. With ’ignition on, engine off’ it is either an error code or it shows, as far as it is detectable by the ECU, that the system is in order.
Although this diagnostic information, that the (fluctuating & static) duty cycle provides, can be very valuable for troubleshooting and is so easily available, it is downright unbelievably often neglected. There are CIS-E cars it has never been (properly) checked on. Many unnecessary part replacements could be prevented already by properly checking & interpreting the duty cycle !
Do not simply readjust a deviated duty cycle via Lambda adjustment screw !
Simply readjusting a deviated duty cycle via Lambda adjustment screw without caring about what‘s causing the deviation is a bad idea
and can lead to engine damage !
… The further away from 50% the fluctuating duty cycle deviates, let alone if it‘s static 0% or static 100%, the worse a readjustment via adjustment screw can be for the engine.
Regard the duty cycle as a messenger that tells you whether there is a fuel combustion affecting problem or not. If it tells you (by low, or high, or static readings) that there is a problem, listen to it and follow its message, instead of shutting it up and simply readjusting it via adjustment screw !
… As I mentioned in post 2, besides after switching between non-ethanol & ethanol containing fuel, readjusting the duty cycle should be the last thing
to do after fixing all causes for its deviation
, respectively if fixing the cause(s) deliberately does not come into question. I emphasize “all”
because I often see duty cycle readjustments done after something has been fixed/replaced without first checking for all other possibly still existing causes for deviation.
Most duty cycle readjustments that are done, even if they bring subjective improvement about, only mask the deviation’s actual cause(s) which continue(s) to exist, with negative effects !
Here‘s a simple example (of many):
An intake vacuum leak causes low O2 sensor voltage. The ECU immediately reacts by sending higher current (in positive direction) through the EHA’s coil in order to increase the amount of injected fuel as an attempt to compensate for the vacuum leak and raise the O2 sensor voltage again, which works if the leak is not too big and the Lambda enriching limit via EHA control is not reached or exceeded. ... In that case this higher EHA current is represented by a duty cycle fluctuating in a higher range. It can, of course, be lowered again by raising the position of the FD‘s control plunger in relation to the AFM plate’s position via Lambda adjustment screw … which most DIYers would probably do … with the consequence that during each warm-up phase (before Lambda control is active) the engine will suffer from way too rich a/f mixture. … It would be much better to leave the adjustment screw alone and lower the duty cycle again by fixing the vacuum leak, wouldn’t it !?
The duty cycle (respectively the EHA current) should always be checked after fixing a fuel combustion affecting problem, respectively after replacing a fuel combustion relevant part. If (besides FD / AFM replacement which usually does require duty cycle readjustment) after a repair / replacement the duty cycle is still out of line, there is either a problem with the repair / replacement, or there is another problem, or the adjustment screw has been unprofessionally messed with.
Do not blindly tamper with the Lambda adjustment screw !
Tampering with the Lambda adjustment screw without even checking or caring about the duty cycle (or the EHA current) is an even worse idea
than simply readjusting a deviated duty cycle without caring about what‘s causing the deviation … with even higher risks for the engine’s health !
Plus, done without at least knowing exactly how far in which direction the adjustment screw has been turned, valuable diagnostic information is gone for good
) and everything that has an effect on fuel combustion (see examples under “Please note“
in post #2) has to be checked first before further touching the adjustment screw, in order to reliably restore a proper condition of the system.
Maybe this post helps a little to understand how justified the difficult to remove plug in the Lambda adjustment tower was/is. ... Without that plug, the adjustment screw would have been unprofessionally tampered with a lot more often, making diagnosis more difficult for MB workshops & Bosch service stations from the beginning. That plug may not exist anymore in most CIS-E cars, but the reason for it continues to exist !
So don‘t tamper with the Lambda adjustment screw … and make it a habit to occasionally, or better frequently, check the diagnostic information the duty cycle provides !
In case of engine problems checking the duty cycle should be one of the first things to do !
... That alone can already point to many possible suspects as well as rule out many possible suspects.
Check it under the following conditions:
1) ignition on (engine not running)
2) ignition on (engine not running) &
air sensor plate halfway deflected (throttle closed)
3) ignition on (engine not running) &
throttle completely opened (air sensor plate not deflected)
4) engine warmed up to full operating temperature &
running at idle speed
5) engine warmed up to full operating temperature &
running at ~ 2500 rpm
6) engine warmed up to full operating temperature &
running at idle speed again
In steps 1, 2 & 3 the off/on ratio (duty cycle) is static
. … In steps 4,5 & 6 it is supposed to fluctuate
(with amplitude & frequency as described under “Check procedures“
in post #2) !
For detailed instructions about with what meter & how to check the off/on ratio (duty cycle) see what I said under “Measuring device”
, “Check procedures”
and “Static duty cycle”
in post #2 !
And if the duty cycle deviates from what it should be, interpret the deviation correctly and check for its cause(s) !
… Ask someone who‘s familiar with the KE-Jetronic to help you with that if necessary.