I've been working on a K100LT and have run into a now non-firing situation. At some time in the past a PO has seriously bodged the shielded cable that goes from the ignition module to the hall sensor assembly (bodged as in cut the outer sheathing off, removed the shielding and then wrapped the shielding spiral-wise back along the length of the cable...). This looked like a simple-enough repair as in cut the original bodged cable out from the ignition module to the black plug, then using the correct crimp pins at the IM end and the black plug end, fit a new section of OEM shielded cable. So far, so simple.

Well, maybe. The bike won't start. Continuity from socket to plug is correct. There is also continuity and power down to the HE sensors. Fuel is being pumped (ask my jeans how I know...). The spark plugs are dry, suggesting that there is no signal from the ignition module to the FICU.

Using an analogue multimeter I checked the brown (signal) to red (positive) and cranked the engine. The meter flicked about 1.25v as expected. The same test on the orange (other signal) left the meter indicating infinity - no movement at all. That suggests that just one Hall sensor has gone awol. Reasonable assumption but... if just one HE has gone awol, why am I not getting fuel through to the engine and sparks on two out of the four plugs?

Question asked by Puzzled of Tunbridge Wells.

just a thought without checking ....but could the hall effects become reversed as in top - bottom going into the ecu at the wrong connection at that plug and time in relation to piston timing ?  I have a feeling that the timing is generated by one of the hall effects with additional information from the second

Possible... I can't check directly because the bike is now about 200 miles and a big splash away from here. I'll pull the cover off mine and look tomorrow - not so daft because I have a good mental picture of the plate on the LT.

[Edit] It's not that. I just had the sense to check the manual!

if just one HE has gone awol, why am I not getting fuel through to the engine and sparks on two out of the four plugs?

Because the 2-3 sensor controls the fuel injection timing. All the 1-4 sensor does is fire the plugs.

RicK G wrote:Because the 2-3 sensor controls the fuel injection timing. All the 1-4 sensor does is fire the plugs.

Something is sort-of making sense here but my ignorance is still showing. Taken at face value, this suggests that (depending on which HES has failed), you'd either get sparks or you'd get wet plugs. So, trying to add some logic to this, if the 2-3 HES controls the fuel injection, would a faulty 2-3 HES prevent the 1-4 HES from actually firing the plugs (as in some logic inside the ignition module)?

From what I can work out they should still fire but then again we are talking about a BMW here

Thanks Rick. We've got a s/h HES plate on the way from Motorworks and I have a spare ICU and FICU in the cupboard, just in case.

Dai wrote:....
Using an analogue multimeter I checked the brown (signal) to red (positive) and cranked the engine. The meter flicked about 1.25v as expected. The same test on the orange (other signal) left the meter indicating infinity - no movement at all. That suggests that just one Hall sensor has gone awol. Reasonable assumption but... if just one HE has gone awol, why am I not getting fuel through to the engine and sparks on two out of the four plugs?

Question asked by Puzzled of Tunbridge Wells.

Dear Puzzled
The info I've read on the Jetronic says that it receives signals from both hall sensors, 4 timing signals per 720° (1 engine cycle) and then divides the signals by 2 and shapes them into 2 pulses for the injectors.

If one HES fails I would suspect the FICU would not have the required number of timing signals to form any injection pulse so no fuel to the injectors. You'd also get only one coil firing.

Do you know what I really hate? It's when you do a really simple job and something related to that job (but not part of it and has not been touched) decides to give you the finger for no apparent reason. What was it that RickG said?

RicK G wrote:(...) but then again we are talking about a BMW here

I'm now trying to fix this problem by remote control from about 250 miles away - I'll keep you informed.

Just had a phonecall confirming it was the HES and the bike is running again . The owner may just be appearing here at some point.