… continuation of post #1
Purpose of the adjustment:
Since, as I explaned in post #1, the air/fuel mixture is unchanged, no matter whether the system is adjusted to a higher or to a lower duty cycle … what is the purpose of the adjustment?
It’s the EHA’s optimal operating range
with regard to:
- Lambda control (keeping ‘λ ~ 1 +/- 0.02’ by fine-tuning the air/fuel mixture)
- the engine’s running behavior.
Regarding Lambda control
the EHA’s optimal operating range is given when its baffle plate oscillates around its center position (currentless rest position) – in other words, when the EHA current fluctuates around ‘0’ mA
, which is represented by a duty cycle fluctuating around 50%
. That way the EHA has its highest dosage accuracy.
Regarding running behavior
, however, a slightly further opened EHA with its current fluctuating around a mean value slightly below ‘0’ mA
at idle, which is represented by a duty cycle fluctuating around a mean value slightly below 50%
, is better. That has i.a. to do with the in post #1 mentioned better leaning than enriching capability of the EHA. A duty cycle at idle fluctuating around a mean value of about 47%
or a little lower is usually a good choice for an intact KE-Jetronic in my experience.
I’d like to add that there’s another advantage of a duty cycle close to 50%, respectively of an operating range of the EHA’s baffle plate close to its currentless rest position. That way the air/fuel mixture is almost unchanged if the KE-Jetronic goes into limp home mode
due to a failure of the ‘E’ in ‘KE-Jetronic’, and during driving at normal operating temperature most drivers would probably not even notice any change.
That, btw, was a major argument in favor of the KE-Jetronic (as an advanced K-Jetronic) for Mercedes, at a time when BMW was already using the fully electronic L-Jetronic. They did not want to see pictures and reports in the media about S-Classes standing on Autobahn brakedown lanes due to injection system problems ever again either, which they did with the fully electronic D-Jetronic they introduced in the early 70s. Instead they prefered the owner to drive quietly to the dealership and tell the friendly people there that somehow the car behaves strangely for a minute or two after starting it in the morning ... lol.
Above target values apply to an intact system
(not only CIS) !
Generally problems relevant for the fuel combustion have an influence on the EHA control, hence on the duty cycle. Depending on the problem(s), the most suitable duty cycle could, for instance, be below 30% or above 70%.
Here are a few examples of relevant problems:
Oil or coolant getting into the combustion chamber(s), wrong or bad spark plugs, bad distributor cap/rotor, worn injectors, leaking cold start valve, contaminated metering slits in the fuel distributor, incorrect fuel pressure, false air leaks, clogged air filter, incorrect ignition timing / faulty vaccum advance, false input from o2 sensor, problems with the ECU, EHA’s baffle plate damaged, EHA’s coil damaged (resistance should be 18–21 Ω), incorrectly adjusted throttle linkage, throttle plate not resting against its idle stop, air sensor plate not centered or its ‘zero position’ incorrect, control plunger sluggish or stuck, problem with EGR valve, bad battery, bad voltage regulator, …
On the other hand, a duty cycle check might help to track such problems. A high duty cycle might be caused, for example, by a false air leak, which of course should be fixed instead of turning the duty cycle lower with the adjustment screw. A low duty cycle might be caused, for example, by a leaking cold start valve, which of course should be fixed instead of turning the duty cycle higher.
Correcting the duty cycle
to the target value is often done too easily, IMHO, and should only be done if the elimination of the reason for its deviation is an option which doesn’t come into consideration, like maybe for instance in case of a problem inside the fuel distributor.
But, checking the duty cycle
is too often neglected as a quicky and easily done diagnostic measure, IMO.
The kind of fuel
being used (non-ethanol / ethanol-containing) has no effect on the validity of the above EHA current / duty cycle target values (see ‘Basics’ in post #1). However, it has of course an effect on the position to which the control plunger has to be set (via adjustment screw) in order to get to these values !
After a switch between fuel types, depending on the o2 sensor’s input, the ECU sends different amperage through the EHA in order to change its operating range, accompanied by a correspondingly different duty cycle, so that it continues to receive o2 sensor input fluctuating around 450 mV (which represents ‘λ ~ 1’). Therefore after a switch between fuel types the duty cycle should be checked, and if necessary readjusted to the target values !
I’d also like to point out, that the duty cycle adjustment does not ensure corresponding optimal results if the adjustment of the EHA’s baffle plate
has been improperly changed !
Changing the EHA’s adjustment can make sense in case of a changed fuel distributor condition. There could, for example, be contamination, or the pressure differential valve’s diaphragms / springs may have been replaced and differ from the original ones, etc.. But bear in mind that by changing the EHA’s adjustment the mechanically predetermined fuel flow rate through the lower chambers of the pressure differential valves in relation to the fuel flow rate through the meetering slits into the upper chambers is changed, which IMO should not be done without adequate know-how and care. If done improperly the EHA current’s / duty cycle’s informative value is gone! … and it’s proper adjustment, as for instance also required for other tests, impossible !
I suggest to either use an analog duty cycle meter or an analog voltmeter. Analog meters offer more comfortable monitoring of the fluctuating readings than digital meters.
Duty cycle meter:
Some duty cycle meters show the percentage of the square wave voltage’s ‘on’-time, and others show the percentage of its ‘off’-time. In case of the KE-Jetronic the duty cycle value refers to the square wave voltage’s ‘off’-time. A meter which shows the ‘on’-time would, for instance, read 53% instead of the relevant 47%.
If you’re not sure which version your meter is: With ignition switched on (engine not running) the duty cycle should be about 70% (California: 85%). If the meter shows about 30% (California: 15%), it’s probably the wrong version. And if, while the engine is running, the fluctuating duty cycle drops when the adjustment screw is turned cw, it’s the right version.
The voltage is converted to duty cycle according to the following formula:
duty cycle [%] = [1 - (Vp3 / Vp6)] * 100
Vp3 = voltage between X11 port 3 & port 2 (or ground)
Vp6 = (battery) voltage between X11 port 6 & port 2 (or ground) during the respective rev !
Example for a measurement at idle:
Vp3 (at idle): 7.1 - 7.6 V
Vp6 (at idle): 13.9 V
duty cycle at 7.1 V = [1 - (7.1 / 13.9)] * 100 = 48.9%
duty cycle at 7.6 V = [1 - (7.6 / 13.9)] * 100 = 45.3%
duty cycle mean value: (48.9% + 45.3%) / 2 = 47.1%
(fluctuating with +/- 1.8%
- In case of California version the ECU may have to be switched over to duty cycle output. Check the service manual for instructions if necessary.
- Warm up the engine to its normal operating temperature. A 10-minute warm-up drive is better than letting the engine idle until it’s warm. Make sure that the engine does not heat up too much during the check / adjustment procedure.
- Pull off the vacuum line between the throttle valve and the regeneration valve of the fuel evaporation system at the regeneration valve and block it.
- Keep the A/C switched off.
- Connect the meter to the diagnostic coupling X11 port 3 and 2 (or ground).
- With ignition switched on (engine not running) the duty cycle should be about 70% (California: 85%).
If you’re using a voltmeter it should read 0.3 * Vp6 (California: 0.15 * Vp6)
If in case of engine M116 / M117 the duty cycle is 100%: The installed ECU version doesn’t feature fault diagnosis via static duty cycle.
- Take off the air filter lid and check two other duty cycle values with ignition switched on (engine not running):
With the throttle closed and the air sensor plate deflected the duty cycle should be about 10%. If it stays at 70% there may be a problem with the ‘closed signal’ of the throttle position sensor.
With the throttle fully opened and the air sensor plate not deflected the duty cycle should be about 20%. If it only drops to 40% there’s a problem with the air flow potentiometer.
Put the air filter lid back on for the duty cycle check with the engine running, which should be done with the air filter installed (and clean)!
- Start the engine, let it idle and wait until the reading starts to fluctuate (it takes a moment until the o2-sensor reaches its operating temperature). If it doesn’t start to fluctuate after a while, the meter may be displaying a static error code (see ‘Static duty cycle’ further down).
- Increase the engine’s speed and monitor the meter while you keep the speed at about 2500 rpm. The reading should fluctuate! Record the values between which it fluctuates – it should be a range not much bigger than 4%, for instance: valley = 42%, peak = 46% (mean value = 44%). The fluctuation frequency (1 cycle = from ‘valley’ to ‘peak’ and back to ‘valley’) should be about 1 Hz (1 cycle per second).
- Then check the reading at idle. Again it should fluctuate, and again record the values between which it fluctuates. The fluctuation frequency should be about 0.5 Hz (1 cycle per 2 seconds).
- The mean value at idle should not differ by more than +/- 10% from the mean value at 2500 rpm.
In case of engine M116 / M117 of model years ’86 & ‘87 the mean value at idle should be 5-15% higher than the mean value at 2500 rpm.
Static duty cycle:
- Remove the plug from the adjustment tower (if it’s still in there), so that the Allen wrench can be inserted. You can put a drop of oil into the adjustment tower if you like.
- Then start the engine, let it idle and wait until the reading starts to fluctuate again.
- Please note: adjustments are always done at idle (not at the higher rev)!
- Then insert a 3 mm Allen wrench into the spring-loaded adjustment pin in the adjustment tower and carefully push it down. Don’t put too much pressure on it, otherwise the air sensor plate’s lever below the adjustment pin might be pushed down, which can easily stall the engine. With the Allen wrench engaged, turn the adjustment pin a little to and fro in order to let it snap into the actual adjustment srew, which is located in the air sensor plate’s lever.
- Turn the adjustment srew in small steps. Even tiny turns can change the duty cycle by several percent.
Cw turns lower the duty cycle … ccw turns raise the duty cycle.
- After each step briefly rev the engine and let it settle for about 10 seconds before taking readings.
- I recommend to record the total adjustment angle. If you turn the adjustment srew too far the engine will stall. And if you can not remember how far and in which direction you have turned it, you may not get the engine restarted. Then the KE-Jetronic needs to be reset in order to get the engine started again, which is not very difficult, but unnecessary labor.
- After the adjustment to the desired value at idle, check the duty cycle at 2500 rpm and then again at idle.
Readjust if the mean value difference between both engine speeds exceeds the above-named allowance.
A static (not fluctuating) duty cycle value with the engine running and the o2-sensor at operating temperature, indicates a problem according to the following list:
- 0%: problem with the meter
or diagnostic coupling (X11)
or too rich setting (beyond the EHA’s ‘leaning limit’)
- 10%: TPS (throttle position sensor), throttle fully closed signal
or (if at 2.000 rpm) no/false supply voltage to POT (air flow potentiometer)
- 20%: TPS, ‘throttle fully open’ signal
- 30%: CTS (coolant temperature sensor)
- 40%: no/false output voltage from POT
- 50%: o2 sensor (aside from not having reached its operating temperature yet)
- 60%: car speed signal (displayed during driving or engine still running after driving)
- 70%: CPS (crankshaft position sensor)
or EZL (electronic ignition module)
- 80%: IATS (intake air temperature sensor)
- 95%: micro switch of throttle linkage (4- and 6-cylinder engines)
- 100%: problem with the meter
or diagnostic coupling (X11)
or ECU ‘N3’ (missing connection to voltage supply or to ground)
or OVP (overvoltage protection relay)
or o2 sensor signal (short to ground)
or too lean setting (beyond the EHA’s ‘enriching limit’)
Consider that in case of a static duty cycle reading, there may be just a problem with the connection of a component (loose / broken cable, damaged plug) instead the component itself.
Depending on the running behavior after duty cycle readjustment an additional EHA check which includes overrun cut-off and acceleration enrichment and lower chamber fuel pressure tests under various engine operating modes may be recommendable.
Don’t forget to reconnect the vacuum line of the fuel evaporation system!