Hi
i am trying to test my airflow sensor. resistance values are close to what is noted in the Troubleshooting sectiobn of this website, but I noticed that the resistance value does not change when I apply heat to the airflow sensor (using a hairdryer)..........is the temperature sensor faulty and is this enough to prevent a bike from starting?
bike is a 1984 bmw k100.. initially there was rough idling which eventually deteriated to a bike which was difficult to start, had to use the Choke lever to prevent the bike from stalling...would occassionally surge when trying to accelerate.
tried to start the bike after 12 days of no riding...thjere was a lot of black smoke and engine was erratic(rough idle)...felt a lot like a fuel starvation situation....
just before i had these problems with the bike, there was a moment when i pushed the starter and there was no response!!! starter was not turning...managed to push start the bike...i was then able to restart the bike without any problems...but it was not idling smoothly.
removed the battery to top up the electrolyte, replaced the battery and the bike will not start.(battery terminals were NOT reversed) starter is fine and the motor turns over, but will not start!!! when i push the starter, the engine turns over and eventually there is a VERY LOUD backfire!!! the backfire accurs every time i try to start the bike....checked the spark plugs..they were wet and oily..cleaned them up....unplugged the fuel line..there is fuel passing into the injector rail...there is also spark..petrol is reaching the spark plugs...pulled them out and noticed they were wet....i removed the backplate and noticed that the bottom halls sensor was worn (the black plastic section) and there was white showing thru the plastic...don't know what caused that!!!! timing needs to be adjusted..bike hasnt been serviced in three years..apart from oil and filter changes....there is spark and fuel..but the bike refuses to start!!
im currently testing the airflow sensor and noticed that there is no fluctuation in the resistance value when i apply warm air...also don't know if the fuel pressure regulator is ok..if it's faulty..would that prevent the bike from starting???
please help me...i am at my wits end with this bike, which up until recently was perfect!!! i want to keep the bike and don't know what else to do..other than to take it to a garage. the control unit has a low failure rate.. so im hoping it's something simple like a relay or fuel pressure regulator....
thanks.

Over which contacts are you measuring the air temp?
And how long ago did you clean all electrical connections, specially the ground ones under the tank and on the gearbox?

Go back to the basics and start eliminating the standard issues.

i attached the multimeter probes to the two cpnnectors on the sensor ( 5 and not certain,,anyway..it was the ones mentioned in the trooubleshooting section of this website...must admit i haven't checked the wiring but the ground wires were cleaned a few years ago. i have never cleaned the connecors..something i might consider doing asap. i suppose i'm grasping at straws messing around with the airflow meter....ive removed the cover and all the connectors appear to be ok...very hesitant to continue messing with the afm....ny ideas what's causing the backfire? thanks for your assistance.

As you have already found an issue with your Hall sensor it might be that the ignition trigger coming from the Hall sensor is not always working. This would mean the bike runs irregular and the fuel will pass through the engine unignited, possibly until hot exhaust gasses from the other cilinders reaches it and then it ignites. That would be in your exhaust. This happens as the spark plug will not spark due to a lacking signal. On your bike your Hall sensor has two pick-ups which gives signal for the units 1 and 4 or for 2 and 3 at the same time. Have you got a mate close by who has a spare or who's one you can borrow? There is a way to test the signal, but I haven't got it handy. BMW used to have a little box with a LED which they used for adjusting the Hall Sensor for correct ignition timing. This has been explained elsewhere. If you have no other options I'll have a look. Good luck!

Here is a page on how to test the Hall sensors. I did not have the time to finish it, and you will have to find the pin out of the k100 Hall sensors plug. Click here to see the page

Bert

i tested the spark plugs..there is definetely spark and fuel. im concerned about the airflow sensor. when i measured the resistance across the air temperature contacts, it was approx 194ohms..i then appplied hot air to the sensor using a hairdryer...the resistance reduced to approx 180 ohms...is this corect? and is this enough to prevent the engine from starting?

I'm having identical trouble to you but have cleaned all my earths. I tested the resistance on the airflow meter today and found 2 shorts so sourced another meter.

gabriel wrote:i tested the spark plugs..there is definitely spark and fuel. im concerned about the airflow sensor. when i measured the resistance across the air temperature contacts, it was approx 194ohms..i then applied hot air to the sensor using a hairdryer...the resistance reduced to approx 180 ohms...is this correct? and is this enough to prevent the engine from starting?

If you are following Bert's troubleshooting guide and if you are testing the air flow temperature sensor between Pins 8 & 9 on the EFI connector, those measurements seem in line with the expected values and there's not reason to suspect the air flow temperature sensor.

You've got wet plugs and spark and it turns over, are you sure you have aeration atomisation of the fuel? Maybe an injector is stuck open, Might be a cause, if you can I'd check the injectors to ensure they work.