Quoted from Megawatt Man on MB World
The entire information below is not by me, it is from Megawatt Man, and I quote it verbatim. Perhaps something there may help you or lead you to the answer.
"KE Jetronic Diagnosis
Hello all on this forum. I hope to bring to some the benefits of my experience in effecting a remedy to a faulty KE Jetronic system, on a W124 Mercedes 300CE, Australian version. Not that the latter factor makes much difference, seemingly apart from the Aussie version not including a cold air input temperature sensor. It took me a long time to get there and I hope that the following information will be found of help to people with injection system problems.
It can be very daunting coming to terms with a faulty system. You have a great car but it runs disgustingly, if at all. And the solutions are costly and it seems that nobody in the world apart from a few competent service providers knows anything about it. But not only that, it seems that you need special diagnostic tools that cost the earth. When you look through the internet you find dozens of people who have paid large sums to have their systems serviced to no effect.
But here are solutions to several frequent problems that don't involve diagnostic tools much beyond a multimeter.
Before starting anything, please refer to a tutorial about how the system functions, Bosch have an excellent summary readily searched out on the internet. Search for “Bosch KE Jetronic system function” or similar.
It's likely that your symptoms include “won't start when cold or hot; backfiring; cough and splutter on acceleration”.
You will have found from your searching that a most important component is the “Over Voltage Protection Relay” and it is most important!
But there are some tricky things to check out first, before getting into the complicated parts.
First. Note that all the sensors have a return path to ground. Some have the benefit of a wire from the sensor, while others rely on current paths via the engine block and chassis. This latter path can be fraught! The engine block is of course mounted on compound engine mounts which are not electrically conductive. So there is a flexible copper conductor run from the block to the body. On my car this is located between the manifold and a mounting bracket for the ignition coil. It's flexible because there is relative motion between the engine and the body every time the engine speed changes. And because it's not ideally flexible, some reaction occurs on the attachment points at each end and they can become lose. Result? Poor hot or cold starting; coughing and spluttering.... you get the idea. To find these connections you need to take off the air cleaner, find the connecting cable and look. Check each end for tightness, broken cables etc. If they are loose, complete the job by removing the attachment, thoroughly clean the terminals and tighten them up. Then see how your vehicle behaves. In a few cases a miraculous transformation will have occurred. But the probability is not high, especially if the vehicle has a few km or miles on its clock – the electronics do not have an indefinite life!
Now precisely the same effect would occur if at the other end of the chain of supply was affected – that is the battery supply. This goes to the injection control unit, or computer if you like. This is located behind a lovely plastic moulding immediately in front of the battery. Carefully remove it by inserting a finger in between the bodywork and the moulding and you'll work it out from there. Now great care must be used in following the next check. You'll note that there is a large cable entering a plug that in turn is mounted on top of the computer and it's about 150 mm long. To remove the plug it is necessary to deflect the two horns immediately beneath the cable entering the plug towards the computer body. This will release the plug and allow it to be withdrawn. Very carefully lift the plug from its cable end, precisely in line with its attached position. The end of the plug furthest from the cable has a rectangular tongue located in a slot in the computer's body and this is the last part to come free. Of course it is the first part to position when replacing the plug. And it's here that we must observe absolute care. You'll see that the computer has 25 flat pins that each have to enter a connector slot in the plug and these connector slots are fragile. The first connector is say 25 mm from the tongue and as I said above, the plug is about 150 mm long. So in replacing the plug there is a mechanical advantage of about 6 at the point where the first connector pin enters its fragile socket. On my car the first slot had been damaged due to the plug not having been first positioned so the tongue fully engaged its slot and then possibly the plug was pressed into place in such a way that it was not completely parallel with the socket. It took a fair bit of care to realign the connector so that a good contact was effected, using a tiny jewellers screwdriver, but it was done. It did make a difference.
But those things are almost outside the realm of the KE Jetronic system itself and I'll go on to that now.
Fuel Pressures
When the key is turned on, the fuel pump operates for one second, bringing fuel in the system up to operating levels. Upon engine start, the fuel pump is turned on again. Serious running problems can be experienced if fuel pressures are too low. If you are fortunate enough to have access to fuel pressure measuring devices, it's great to prove the pressures right, this can save a lot of time.
If your vehicle will not start at all, a faulty fuel pump or electrical supply can be suspected. It's not much use turning on the key and then crawling under the rear of the vehicle to check whether the pump is operating, because after the first one second, it should have turned off anyway. That is a two person operation, one turning on the key, one listening for pump operation - for one second only!
There's a fuel filter in the line, if it is blocked you'll have poor fuel supply and erratic running, so it will need to be changed. There's another component in the fuel line called the Accumulator. This is just a spring loaded reservoir that maintains fuel pressure during times of low fuel volume requirements, when the pump may be turned off, in the same way as at starting. A symptom of a faulty Accumulator is stalling when idling, say at traffic lights. The acid test for this is to take off the air cleaner to expose the air flow sensor plate. Start the vehicle and run it just for a short time, then shut it down. After about one minute, depress the air flow sensor plate. It should offer resistance to your effort, indicating that residual fuel pressure is being held. If the plate moves downward without resistance, you need a new accumulator.
Diagnosis by Codes
The system is provided with a diagnostic socket from which electrical readings are made that show the condition of the various components. On my car there is an 8 pin socket, concealed by a screw cap, located just above the ignition coil on the inner mudguard. Readings for the injection system are taken from pins 2 and 3 of the socket. Other cars have other styles and sizes of sockets, but there seems to be some commonality in that pins 2 and 3 are used for this purpose. You'll need to determine this from on-line searches, specifically for your model.
But before these codes can be read reliably, the condition of the Over Voltage Protection (OVP) Relay needs to be checked. This relay performs the valuable task of making sure that any short term voltage spikes do not burn out sensitive electronic parts throughout the vehicle, particularly in the computers. Reports abound of OVP relays with failed soldered joint on their printed circuit boards and if these are dodgy, the supply voltage to the injection system components can be intermittently interrupted, producing the same symptoms as the faulty ground or battery connections mentioned above. The OVP relay, while protecting the electronics, turns on the fuel pump relay and other components that prepare the injection system for operation. So a faulty supply from this relay can give symptoms of horrible running, while masking other faults that may be present. You might be able to roughly gauge whether the OVP relay is OK by measuring the voltage at the fuel pump relay while giving the OVP relay a bump. A voltage loss is a sure indication of a problem OVP, but such a method is not reliable. If you don't want to start out with a new or known to be good used unit, you can follow the diagnostic steps, but if you receive variable results, change it out.
These electrical readings are codes, each of which indicates something. There are two forms of these codes. They consist of a set of pulses output by the computer, rising in voltage from zero to the injection system voltage (this can be just a bit less that battery voltage, , depending upon engine operating speed) and the number pulses indicates the nature of the fault. If you don't have access to the proper gizmo to measure these pulses, you can make a pulse counter from a light emitting diode with appropriate resistor and a three connecting leads. Making the test leads is covered in many places on the internet, as is using them to get the computer to output the diagnostic codes. Once connected, you instruct the computer to output one code at a time, then count the number of pulses, then look up a table from Mr Benz to identify the faulty part. My vehicle is a 1988 model and I was not able to find the list, but I reckon that almost everybody else's models are covered by information on many sites. There's some good news though. Alternative checks can be made just using a multimeter on pins 2 and 3 of the diagnostic socket. In one form of multimeter test, the average value of the pulse train voltage is measured and in another form, the on/off ratio of the pulse train is measured. In the following, the average voltage method is described.
Test 1
Average voltage method. Turn on the key, but don't start the engine. Measure the DC voltage between pins 2 and 3. This should be 30% of the injection system voltage as measured usually at pin 6 of the diagnostic socket – it's a little less than battery voltage under the conditions of the test, ie whether the engine is running or not) and correspond to Mercedes' specification of a 70% duty cycle or “on/off ratio”. In fact Mercedes uses the total proportion of its off period as its indicator number, but the multimeter respond to the proportion of time the pulses are on, at injection system voltage. Hence the 30% mention above – 100% minus 70% equals 30%. This initial reading doesn't tell you much, only that the computer is capable of putting out diagnostic codes – but that's something! If you don't receive any readings, maybe your computer is past its prime.
Test 2
Same test conditions
Deflect the air flow sensor plate. The duty cycle should decrease to 10%, that is, the multimeter should read about 11 volts. If no change occurs, that is the reading still is about 3 volts, test the throttle valve switch (Job 07.3-121 in Mercedes shop manuals). Please note that the 10% is nominal, 9% to 11% is seemingly OK.
Test 3
Same test conditions
Deflect the throttle fully, the duty cycle should read 20% or about 10 volts, indicating that the Full Load Contact switch is OK, unless it decreases to only 40% or about 7 volts. In this case test the air flow sensor potentiometer (Job 07.3-121 in the Mercedes shop manual). Again 20% or close to it is OK.
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