Well, the last few days have been quite interesting with what I have learned/discovered about K-Jetronic with Lambda systems. I set out with the goal of getting the mixture right on my 1980 450SLand did so by attempting to use a duty cycle meter… only to discover that it is NOT possible! A lot of the information out there talks about ideal duty cycle settings as they apply to KE-Jet systems. Our K-Jet system though cannot be calibrated the same way and using the wrong information is an exercise in futility and frustration !!!!!
The K-Jetronic WITH LAMBDA system is used on a number of vehicles from Porsche, Volvo, to VW, and of course Mercedes. For our purposes, we are talking about the 1980 450SL, and the 1981 thru 1985 380SL.
The system consists of four basic sections:
The K-Jetronic mechanical fuel injection system (similar to that used on the 1977, 78m and 79 450SL)
The "Frequency Valve" addition, which is essentially a modified ELECTRIC FUEL INJECTOR inserted into one of the lines of the K-Jetronic fuel distributor. The Frequency valve is in parallel to the control pressure circuit from the "WUR", the warm up regulator, aka control pressure regulator. The frequency valve adds finer trim to control pressures than is possible through the purely mechanical WUR and fuel distributor.
An oxygen sensor in the exhaust.
4) An ECU - the engine control unit
. This is sometimes called the engine computer, but in fact, it is a purely analog device which takes the voltage from the O2 sensor and turns it into a signal that drives the frequency valve to vary the pressure in the fuel distribution circuit.
For more details on the ECU see this thread, "Inside the ECU".
In later models with KE
-Jetronic like the 560SL, there is a valve called the EHA valve that replaces not only the "frequency valve" but also the WUR. The EHA is built specifically for modifying control pressure in accordance to the KE lambda O2 system.
frequency valve as used in the 1980 to 85 cars is just a modified fuel injector stuck I to the WUR control pressure loop. It's an engineering stop-gap to get cars to meet emissions regulations of the 1980s. In other words, the K-Jetronic Lambda is somewhat cobbled together from existing components, while a truly modern lambda system (KE-Jet) was still in the developmental stages.
Further adding to confusion and frustration with K-Jet adjustment, is that even from the same year, some manufacturers utilize different options with the same Bosch hardware making their adjustment procedures similar yet incompatible with Mercedes 450SL/380SL procedures. For instance, on the Bosch ECU, Mercedes does not use Pin 17, but VW does. Pin 17 is a duty cycle diagnostic signal, which allows VW mechanics to use a duty cycle meter, but Mercedes does not have easy access to that pin (unless you disassemble the ECU). Instead, Mercedes uses only pin 15, the same pin that drives the frequency valve.
But the signal that drives the frequency valve
CAN NOT BE ACCURATELY MEASURED WITH A DUTY CYCLE METER. And it can’t be set with a dwell meter either.
To set it correctly, you need either the special BOSCH tool that is specifically for that purpose, OR an oscilloscope. (Note: I am looking into developing a procedure that could use a standard duty cycle meter, but don’t hold your breath - there are issues that may just make a duty cycle meter pointless in this application).
EDIT: In my second post below, I will show how you can use a Duty Cycle meter by attaching to a test point *inside* the ECM (Pin 17).
SAY IT AIN’T SO, JOE!
Sorry, it IS so. But the good news is that once you set the mixture RIGHT using an oscilloscope, you will be SHOCKED at how smooth your baby will idle, purr, zoom, and roar!
On more modern systems, like the KE used on 560SLs, the signal sent to the EHA valve is a nice clean square wave. Its duty cycle IS the duty cycle of the EHA.
On OUR systems, remember that the frequency valve is just a modified fuel injector. It gets a signal similar to that used for single point (throttle body) injection systems. Single point injection systems have signals that look like this:
This is a complex signal, and the important details of it CAN NOT BE READ WITH A DUTY CYCLE METER
. Apparently the Bosch tool can differentiate between the PWM portion of the signal and the resting portion. And you can figure this out with a scope attached too Pin 3 of the diagnostics connector, but not a dwell or duty cycle meter (A duty cycle meter must be connected to an internal test point inside the ECU to be useable).
SO WHAT NOW?
I am going to describe a slightly involved, but not difficult, mixture adjustment procedure - which I used today. With the mixture *correct*, I am amazed at how well Freya, my 1980 450Sl, runs. It’s sweet. (A variation of this procedure CAN in fact use a duty cycle meter, but you have to open up the ECU and attach to the internal test point, Pin 17)
I am breaking the procedure into three sections. Section one is connecting and setting up the oscilloscope and understanding the readings. Section two is the correct way to manipulate the 3mm adjustment screw. Section three is the analysis of each adjustment, and its indication for further adjustment.
Ill assume that if you have the Bosch tool (meter) that you know how to use it, and you can skip to section TWO.
SECTION ONE - THE OSCILLOSCOPE
The scope I used was my old JDR 35 MHz dual trace, circa 1986. It’s nothing fancy, a hefty CRT type (I bought mine new for $400 in ’86, today you can get one just like it on ebay for $35). I used a cigarette lighter power inverter to power it, so I could drive with it operating (this is important as will be discussed in section three).
The only connection you need are to ground the prob, and connect the signal tip to PIN 15 of the ECU, or to PIN 3 of the X11 diagnostics connector under the hood. As I had already disassembled the ECU to get a better understanding of what it did and how it operated, I just connected to Pin 15. My setup was like this.
The scope was set to 10V/Div vertical, 2ms/Div horizontal, and HF Reject on the trigger. With the engine running, you should see a trace something like this:
At the start of each injection cycle (which occur at approximately 72 Hz) is the 50 Volt main pulse. After the 50 Volt main pulse, there is a shoulder of varying length, followed by the PWM (Pulse Width Modulation) portion. The PWM portion is the part we are most interested in. I should say that I use the term PWM somewhat freely, as the actual pulse widths don’t really change - what changes is the DURATION or number of pulses per injection cycle. For ease of discussion, let’s call it the PWM period.
When the O2 sensor/ECU senses that the mixture is too rich, the ECU shortens the PWM period. If it sees the mixture as too lean, the ECU increases the PWM period.
Our job is to adjust the mixture screw on the fuel distributor such that the amount of “lean” or “rich” due to various driving conditions is always within the operational range of adjustment that the O2 sensor and ECU can provide.
SECTION TWO - TURNING THE SCREW
This is a section because turning the screw that adjusts mixture at the front of the fuel distributor is anything but straight forward. And there are a lot of comments and thoughts on how it should be turned, and in all honesty, some of the more important aspects of turning that sensitive annoying little screw are not addressed in many comments.
On the 450SL, the adjustment screw has a spring loaded “top screw” with a safety stop. I am going to discuss adjustment using this type of screw. Some older cars allow for directly engaging with the air sensor screw - and if you have a car like that my main comment is BE REALLY CAREFUL. The discussion below assumes you have a care with a spring loaded hex socket, that has to be pressed to engage the *actual* adjustment screw internal to the air flow sensor. (The spring loaded hex socket has a limited travel, intended to prevent an adjustment driver from going to farr and damaging the air flow sensor, such as when a monkey slams the hood having forgotten to remove the adjustment screwdriver).
There is one universal rule that I want to echo as loudl;y as possible: NEVER turn the screw more than 90 degrees (¼ turn) at a time. The total adjustment range from “so lean it won’t run” to “so rich is stalls in a plume of black smoke” is LESS THAN ¾ TURN. Seriously, the screw is VERY sensitive. Most of the time I am turning it 1/16th, or at most 1/8th (45 degrees rotation) at a time.
But beyond the ¼ turn max rule, there is a multi-step procedure I have adopted for actually turning this screw. Don’t laugh - it took me YEARS before I found the “secret” to Mercedes mixture screw turning. I offer what I have learned here as a way to ease your pain and suffering in screw turning. I will add a video and/or pictures soon, as I do think this is that important.
Gentle engagement. Take your 3mm Allen key, and lightly place it through the opening in the air cleaner, just so it engages the spring loaded HEX socket. Don’t push yet, just rotate the hex key so that it is fully engaged in the spring loaded socket FIRST.
Now, lightly press down on the hex key until the spring loaded hex socket/extender *just barely* touches the internal adjustment screw.
We don’t want to make an “adjustment” yet. Now just lightly contact the internal adjustment screw, and slowly rotate the hex key until you feel a “click” and you actually ENGAGE the adjustment screw.
Note the exact angle of the top of the hex key. Remember you never want to rotate it more than 1/8th turn or 45 degrees at a time (90 degrees absolute max for an initial adjustment) and 15 to 20 degrees (or less) for final adjustments. It REALLY IS that sensitive!
Rotate the hex key counter-clockwise to make the system LEANER, or clockwise to make the system RICHER. Keep careful track of the direction turned and how far you turned it in this iteration (I suggest keeping a note pad, listing each and every turn made during your adjustment session.
Remove the hex key. Take it OUT of the hole. OUT! OUT OF THE HOLE! TAKE IT OUT!
Now go to section THREE
below, where we “condition” our adjustment and evaluate.
SECTION THREE - CONDITION AND EVALUATE
Well now, you’ve managed to defy the odds and properly turn your mixture screw - congratulations! But we are far from done. We need to evaluate to see if our screw turning expedition helped us or hurt us. So we need to condition and evaluate.
We need to get the engine/O2/ECU system to a “normal” state so that we can correctly evaluate our mixture adjustment.
We need to look at the scope traces under certain driving conditions to see if we are getting to our desired “sweet spot”.
After adjusting the mixture, rev the engine to 2500 RPM for 5 seconds. This is needed to allow the mixture to stabilize in the eyes of the O2 sensor. At idle, when fully warm, we want the mixture to hover around 50%.
: at idle, you should see fluctuation of idle between 45% and 55%. If it is CONSTANT (+- 1%) at 50% that means the O2 sensor is not yet at operating temperature. If it is constant at around 60%, that indicates that the engine temp switch is closed (engine too cold).
GENERAL NOTES ON DUTY CYCLE (Updated Nov 26th 2014)
50% (steady within 1%)
- the 50/50 duty cycle is the default when the O2 sensor is not yet at operating temperature, but the engine temp switch is open, meaning the engine is above 60 degrees F (16 C).
60% (steady within 1%)
- the 60/40 duty cycle is set when either the temp switch is 60 degrees F or less (16C), and/or the throttle switch is closed for full acceleration.
44% to 56% (fluctuating)
- Normal fluctuation with throttle at idle for a correctly operating system.
40% to 60% (fluctuating)
- Manuals report this as Normal fluctuation with throttle NOT at idle, and no engine load, for a correctly operating system. Using a DMM, it can be difficult to gauge as the "flutter" is over +- 10% - so at 2500 RPM you may see values from 20% to 40%, and at 1000 it may range from 35% to 55% - a big factor is the condition of your fuel distributor. As such, I hesitate to state "exact figures" for proper operation at any particular RPM load or no load. You'll have to find how your car behaves under various conditions.
20% to 85% (fluctuating)
- based on my own road tests, Ideally keep the system in this range for most driving conditions when fully warm. You'll find that freeway speed deceleration downhill will cause "enrich" up to the max 95%, and certain partial throttle acceleration uphill may result in max lean at 12.5%.
- Approximate "MAX leaning" limit, the system is leaning out the mixture as much as possible to correct for a rich condition.
- Approximate "MAX Enrich" limit, the system is enriching the mixture as much as possible to correct for a lean condition.
Greater than 90% (steady)
- this indicates a short in the O2 sensor. Note that in the video attached after warm engine start, the scope traces slowly drifts to hard left - this sensor had a short in the connector.
Greater than 90% (fluctuating)
- This can indicate an O2 sensor that is contaminated with silicone or other o2 sensor poisons.
Because of the advanced age of these cars, IMO Lambda operation is best evaluated UNDER LOAD
and various driving conditions.
This means - yes - you need actual driving to both condition
Fortunately, there are service stations or other suitable locations near most freeway on/off ramps - plan your adjustment session accordingly.
So what I am suggesting is make an adjustment, then get on the freeway and drive to the next freeway off-ramp (watching scope trace), and adjust some more. If you are methodical about it, you should be able to be done in under an hour. You’ll find that the Lambda system is so sensitive that you really can’t evaluate the adjustments without actual engine-under-load conditions.
To watch the scope trace while driving, I record it on video so I can keep my eyes on the road, then evaluate the video once stopped, prior to the next adjustment.
I have some video samples that covers the “look” of the various traces on the oscilloscope here, which some explanatory graphics:
1980 450SL Lambda ECU operation - Live Oscilloscope While Driving
the first few seconds of the video (warm start) shows the trace move left from 50% to 94%. This is because the oxygen sensor was SHORTED
Under normal operation, in warm start, the trace should be at 50% static until the O2 sensor warms up.
The slow drift is the idle response time responding to the shorted (I.e. 0 volts) signal, instead of the OPEN circuit signal of a cold O2 sensor.
I didn't realize this at the time I made the video, so the video does not indicate that the warm start trace is actually what you'd expect to see with a shorted or poisoned sensor. The short was intermittent and weak, so did not affect the normal operation once the sensor was at operating temperature, so the remainder of the video is valid.
This is a trace where mixture is too rich, and Lambda can’t fully lean it out:
This is a trace where mixture is too lean (NOTE: this can also mean that the O2 sensor is not ready yet, so don't adjust richer until you drive for a few minutes):
The difference between these two traces is about ⅜ turn. However the difference in the center (most sensitive) between 45% and 55% is 1/16th turn or less.
You’ll see that freeway driving will cause the ECU to lean things out a bit (move the PWM to the right), and that coasting and idling will cause the ECU to richen things up (expand the PWM to the left). What you are looking for is freeway cruise and acceleration, plus coasting, stop and go, and idle, to all fit within the “adjustment range” for the Lambda system.
The one other tip I’ll add is: When you FLOOR it, the throttle switch forces the ECU into open-loop mode with a 60% cycle. If you adjust your mixture so that freeway cruise is favoring the right side a bit (i.e. around 40%), then when you floor it, you’ll go richer and get that boost of power. I have reason to believe this is the intended setting scenario.
Now, though all of this until this last sentence, I’ve avoided the term “duty cycle” - you’d see though (watch the video) that there is a duty cycle of the amount of the wave form that is PWM, and the amount that isn’t. Unfortunately, you can’t read this directly using a regular duty-cycle meter - you need an oscilloscope, OR the Bosch tool that is designed specifically for converting the PWM portion TO a duty cycle for diagnosis.
NOTE: I am looking into some methods of adjustment that could allow the use of a regular duty cycle or dwell meter, but have yet to asses them for accuracy.
This concludes the overview and adjustment of 1980 thru 1985 K-Jetronic WITH Lambda fuel injection systems. I hope you found this helpful.