16V Air Temperature Sensor Test

[The Mighty Gryphon]

I the course of working on my K100RS 16V I have noticed that no matter how I set it up the spark plugs are always a bit sooty.  Valve gaps are correct, throttle sensor is set properly, coolant sensor is working correctly, throttle bodies balanced and the CO pot set with an exhaust gas analyzer.  I thought that the aftermarket exhaust that came with the bike could be the cause, but even with the correct OEM exhaust it still runs a touch rich.  The bike runs well, starts easily, but always has a hint of unburned fuel smell in the exhaust. 

I have seen many bikes have problems with the coolant temperature sensor which causes serious whack when it isn't working properly.  It makes me wonder if I could have a problem with the air temperature sensor.  I have looked everywhere and can't find any information on resistance values or tests that can tell if it is bad.

Does anyone know how to check these things?  I have the fairing off now and want to check the sensor before I put the fairing back.

[Laitch]

I have looked everywhere and can't find any information on resistance values or tests that can tell if it is bad.

Can you perform the test mentioned in this thread to get the 1224 diagnostic code indicating an out-of-range air temperature sensor?

[The Mighty Gryphon]

I have no codes on the Motronic, but I suspect the codes only indicate an open or short circuit condition. 

What happens if the sensor says the air temperature is only 5C and the system makes the engine run just a little bit richer based on higher air density?  It would be enough to soot up the plugs and put a lot of carbon in the cylinders but not enough to noticeably screw up the running of the engine.

It would be nice to know what the sensor value should be at given temperature.  Does it have the same transfer function as the coolant temperature sensor?  Research seems to indicate that the Motronic system uses both Positive and Negative Temperature Coefficient air temperature sensors, and possibly different resistance curves. 

[Martin]

On the 2V Bricks a faulty FPR or blocked return line can cause over fuelling.
Regards Martin.

[K1300S]

The air temp sensor should be a standard bosch part,  so it should be simple to test with ohm meter.  The values for correct range should be findable.  If I was home I would look up in my bosch manuals.

Yes, any restriction in the fuel return line will cause over fueling.  Even a slight bend from using normal fuel line rather than the proper  preformed bmw part will restrict flow enough to jack up the fuel pressure to the injectors.

[The Mighty Gryphon]

So I guess there is no information on the air temperature sensor. 

With all the coolant temperature sensors crapping out lately, it would be nice to have some diagnostic information beyond short or open. 

Maybe it just doesn't have enough effect on the mixture to really be worth worrying about even if it went bad.  Has anyone ever found a bad one???

[K1300S]

most Bosch temp sensors have resistances that go from ~3k ohms cold (65F) down to ~300 ohms on a hot day, if thats any use.  ez to test.

14 F - 9.6K
32 F - 6.4K
50F - 4.3K
68F - 3.0K
86F - 2.1K
104F- 1.5K
122F- 1.0K
140F- .79K

yes, they can go bad.

[The Mighty Gryphon]

Thanks Marshall!  I've seen that information posted for the coolant sensor, but wasn't sure if it was correct for the air temperature sensor. 

The reason I am looking for a specification for the one on the bricks is that it seems Bosch engineers have designed several different sensor characteristics to accomodate normally aspirated, turbo and supercharged induction systems as they seem to have different temperature compensation requirements.  Also, some manufacturers specify Positive Temperature coefficient sensors to make them fail safe so as to not lean out the mixture in an open circuit failure. 

My searches have also found threads on other forums discussing sensors that had changes in output from specification, but not total failure.  I can't find that much design information on the air temperature sensors, but since they are sold in their own product category I suspect that they have diferent design criteria from the water temperature sensors.  I am guessing that since they work over a smaller temperature range in a medium with a lower specific heat their design needs to have lower thermal mass for a faster response to temperature changes.  Who knows what kind of failures their design might lead to?

The price makes me reluctant to just buy one to see if it helps correct my rich mixture issue, especially since the mixture is not rich enough to adversely effect performance beyond some carbon build up in the cylinders.

[matjas]

hi Mighty

I am sort of in the same boat as you are as the bike is running great, goes like stink, idles great and starts great - it is a great bike :D in the end, but when I am overtaking in 5th I am caught up with stench of unburnt fuel that somehow follows me in the vortex created by the bike. Also last year with steady 120kmh /75mph that?/ the bike was returning 5l/100km - this year first tank was 6l to 100km but I got somehow heavy on the throttle over the winter :/ this must be said.
I still need to see the sparkplugs /replaced 10kkm ago/ and tell whether this is just the nature of the beast or it is really beginning to run rich. Temps now in Poland are in the 10-15C zone so no drama at all.
I have done things to my thermostat I am going to describe in a separate thread - maybe this?

To YOUR specific issue, IF I might help I could measure the resistance of the air temp sensor for you at a given ambient temp. If some other people did it too - you might get an idea if yours is reading correct? Just a thought.

Cheers
matjas

[Laitch]

Here is data derived from BMW's proprietary electronic system tester but a good multimeter also could be used to determine the resistance. The Motronic test manual from which this data comes is downloadable from the link supplied by volador on this page.