Fuel Injection Fault Finding for bosh motronic, bmw K11 by Frank Warner.

I had high fuel consumption, this is how I found the problem.

My fuel consumption rapidly increased, leading to overly rich running. Adjusting the CO2 pot did reduce the rich running at idle but had not effect on the overall consumption. The motronic unit does store errors, but it will only store gross errors not subtle errors. Gross errors are shorts or open circuits, a sensor that changes its characteristic will not be detected as that would be simply assumed to be a change in the sensed quality.
These notes are for my 1993 K11LT and may have some relevance to other motronic equipped vehicles, but you need to do the thinking to check for the differences. The date of manufacture looks to be indicated within engine serial number mine says xxxxx 2293 xxxx. I think this is the same structure as the tyre dates, 22 is the 22nd week of the year (about end of May?), 93 is 1993.
The following is firstly a description of the motronic brains memory and its readout. Then a description on the test setup. Then on each component used, their characteristics and methods to test them.

First

The motronic brain stores faults within it self, these stored faults should be retrieved and investigated as this could save a lot of work.
There is a blue 3 way connector on the left side of the bike near the battery, you'll need to remove the side cover to gain access. Turn on the ignition and then shorting the number 1 pin on this connector to earth for 5 seconds, then the temperature light near the tachometer will flash. The light will light for about 2.5 seconds, then a give a number of short flashes, count these short flashes. This count is for the first number, if this is a 1 then you have a fault. Another 2.5 seconds of light follows the first count, then follows the second count and this is repeated for the third and fourth count. This repeats until you turn off the ignition, or reshort the pin 1 to earth for 5 seconds to get the next fault.
Code      Fault      
1133Hall effect - no signal No 1 motronic plug connections 8 (signal)
1122Hall effect - no signal No 2 motronic plug connections 26 (signal)
1223Water Temperature motronic plug connections 21 to 32
1224Air Temperature motronic plug connections 22 to 32
1215Throttle Position Sensor motronic plug connections 25 to 31; 13 to 31 and 10 to 31
1111CO2 pot motronic plug connections 25 to 31; 25 to 28 and 28 to 31
4444No Fault
0000No Fault
3333Not a fault - a fan test and memory erasure.

Hall effect sensors can fail due to failure of the power on motronic plug connections 5 (power ground) and 29 (power >9 volts). These connections should be checked if both hall effect devices fail. Check that 5 is connected to ground. Then check for a voltage >9 volts on pin 29 when the ignition is on and the motronic is connected.
The unit can store more than one fault, once you have read out one fault, repeat the shorting for a further 5 seconds and check for another fault. Once you get code 0000 or 4444 then you have all the faults. You may note that no fault codes exist for the brain it self, this may be because that most brain faults may cause the brain to ceases functioning at all.
The easiest way to remove the storage of fault codes from the brain maybe to just remove the connector from the brain for a period of time, while your waiting how about checking the battery? The time take to check the battery will ensure that the brain memory is erased.
To ensure correct start up once the brain is reconnected to the system, the first time you turn on the ignition rotate the throttle slowly over its full range. Then turn off the ignition and then all should be ready.

Test Set Up

You will need a ohm meter (resistance meter, most Digital Voltmeter [DVMs] have them included these days) to do these tests. You will need to remove the electrical plug from the motronic brain located on top of the Battery. You need to do this as part of battery maintenance anyway. Once the brain is disconnected it will lose all stored faults, so this should be tested before disconnection.
The electrical plug of the motronic brain showing sensor connections.
All the connector numbers refer to the motronic connector above, unless specified other wise. This is done so that there is a common test connector, includes following wires and connectors. This makes the test simple. If a test fails then you should test the interconnecting wires and connectors. It may be worth noting that the procedure here does not check for shorts to the chassis, you should do these while flexing any and all wires from this connector to the item being tested.

CO2 Potentiometer (Pot)

Located on the left side of the fuse/relay box is this Pot, it has a slotted screw driver adjustment. This is a ten turn Pot meaning that the adjustment range takes about 10 turns (complete revolutions) to go (travel) from one extreme to the other.
Measure the resistance from 31 to 25, this should be 1k ohms +/- 10% and is the value of the pots resistance.
Measure the resistance from 28 to 25 and is the slider to bottom resistance. Measure the resistance from 28 to 31 and is the slider to top resistance. The sum of the slider resistance should approximately add up to the value of the pots resistance. If either or both of the slider resistance is well above the pots resistance then it is faults - get a new one.
If you record the slider resistances then you can always return to this setting. If the bike is running rich at idle then you may like to turn the adjustment say 2 turns clockwise to try to lean the mixture off. But as you as testing the fuel injection system now then I would continue with the tests.

Air Temperature Sensor

Located somewhere in the inlet tract, this is a fragile component! This is a Negative Temperature Coefficient resistor. That means as the temperature rises the resistors resistance falls.
Measure the resistance from 22 to 32. BMW say it measures 2k5 ohms at 20 degrees C. As your air temperature will not be 20 degrees your measured value will be different, but should not be less than 10 ohms or more then 200k ohms.
The real test comes with the next sensor measurement, provided your water temperature is at ambient temperature (eg if you bike has not been operated for some hours).

Temperature      Resistance;
Celisicus         Ohms
05500
202500
401250
60600
80320
100190

Water (Engine) Temperature Sensor

Located on the left side of the water inlet riser in a most difficult to get at location! This is a Negative Temperature Coefficient resistor. That means as the temperature rises the resistors resistance falls.
Measure the resistance from 32 to 21. BMW say it measures 2k5 ohms at 20 degrees C. As your water temperature will not be 20 degrees your measured value will be different, but should not be less than 10 ohms or more then 200k ohms.
Compare the measured valve of the Air sensor and the water sensor, they should be within 10% of each other provided the engine is 'cold'. If not then one of them is faulty. To determine which one it is probably easiest to start the engine and get it a little warm then remeasure the water sensor s resistance, if that has not significantly reduced then the water temperature sensor is at fault.
I measured 2k66 for the air and 2k52 for the water, a difference of 6%. A mild warming of the engine decreased the water sensor reading to 1k69.

Throttle Position Sensor (TPS)

Located on the left side at the end of the throttle body. This is two inter connected potentiometers.
Measure the resistance from 31 to 25. BMW say it measures between 500 and 1000 ohms, I get about 700ohms.
Measure the resistance from 31 to 13. BMW say it measures between 600 and 2400 ohms. Measure the resistance from 31 to 10. BMW say it measures between 600 and 2400 ohms.
Throttle      Resistance      Resistance      
Position         From 31 to 13         From 31 to 10      
Idle995880
1/416101286
1/215941586
3/415791883
Full15801742

The TPS internals, at idle. The TPS internals, at full throttle.
The photos show the internals of the TPS. They show that the slider consist of 4 contacts, the upper pair are connected together, as are the lower pair. The four tracks (the black carbon semi circles) are inter connected off to the TPS connectors. I measured the following values inside the TPS. The horizontal track under the 071 is about 870 ohms. The top arc is about 10 ohms, the next one 800, the next 1000 and 20 ohms the lower track. Pin 2 of the TPS connects to the left hand side of the middle 2 tracks by a 70 ohm resistance. Pin 3 of the TPS connects to the middle of the bottom track by a 400 ohm resistance.
My comment: I think this unit has been constructed to ease the computations of the motronic unit in determining the fuels to inject and the ignition advance. I think a simple sensor would be worth the added computational complexity! the simpler sensor would be easier to test, cheaper and easier to replace and easier carry as a spare or obtain a replacement.

 

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Back to my home page     Dated 20 March 2002.

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