Hello to anyone reading this.

My '84 K100RS recently developed a fault and after working through a number of possible causes i'm wondering if anyone can help me with the diagnostics/solution. I've trawled this and a few other forums for advice/info' and have carried out most of the checks recommended but still no nearer identifying the problem/fix.

A couple of weeks ago i rode a good distance to an airport left the bike there for a week (in good weather) came back and on the ride home it wouldn't turn the starter after a fuel refill. Puzzled but bump started it and then at an overnight stop checked it would restart which it did no problem.

Next day on the motorway after about 30 miles the engine died with just a couple of gasps to get me over to the hard shoulder. Checked for obvious (had a full tank of fuel and no signs of a vacuum building in it), fuses good, all fuel supply pipes attached, instruments all OK.

Restarted then about 2-3 miles further died again. Did this a couple of times. (The longer i waited to restart the further it tended to go). Evenutally wouldn't turn the starter, but then worked again after jigging the back wheel backwards in gear. So thought this could be a clogged starter causing earth/ground problems.

Got a breakdown van to get me home and cleaned the starter internals. Looked in reasonable condition.

On test ride died again after about 20 minutes. But let it cool for half an hour or so and it would restart and get me home.

Repalced the fuel filter (with a slightly oversized part available at the time, and the old filter didn't seem blocked, tested by blowing through) and internal tank fuel hoses.

Fuel pump works and looked at the gauze filter at the pick up end of pump. This filter is not the round plastic fitting as shown in the Haynes manual and fitted as a modification to early models but is a gauze pocket which seems to be listed for post'93 models. This might be something that needs renewal/replacement?

Tested the temperature sensor probe which appears to be OK giving reasonable readings of resistance and mid temperature ranges but couldn't test high end temp (the temp probe has been a cause of a previous problem but was replaced).

Removed airbox and visually checked fuel and vacuum pipes to pressure regulator and crankcase breather.

Haven't been able to pressure test the fuel rail / feed pipes (no pressure tester in my possession) and would like to know how i could check that the fuel return line is working?

In the meantime the problem seems to be getting worse as on reassemly after the checks done so far the bike only runs (at idle on choke) for about 5 minutes before it begins to cut-out and die. After a wait it will restart.

Haven't heat tested the Hall Sensor but given that the problem now occurs before the engine is even hot i doubt this is the cause.

So, thanks again if you've read this far and ... ANY IDEAS?!

Regards
George, Dorset, UK

I left my K12RS at Hurn a few weeks back and always wonder if it is going to start ok. Actually, I had your type of problem but remember I am referring to a K12. Some times I would turn the key and get the fuel pump whine, then other times nothing, no pump, no starter. Once the bike got started I could ride for miles. Exactly the same thing happened at Hurn, but once started the bike went all the way up the M27. Stood overnight then would not start next day.

I had two different problems at the same time. For a few months now my aftermarket alarm/immobiliser has not always responded to the key fob at gas stations and airports. I sorted that which was due to strong radio frequency pickup blocking out the receiver. If you have an alarm immobiliser remember it is always in the starting circuit.

The next problem I eventually traced to bad fuses. Your bike may have different fuse numbering to my K12. Find the fuse that runs the ECU and fuel pump (mine is F4). The BMW blade fuses have their fuse wire visible along the top edge. In my case the wire did not look broken, but was actually broken at one end and making intermittent contact with ibes from the bike. I have got rid of all those BMW type fuses and replaced them with the type having the wire protected by the plastic moulding (Halfords).

You need to confirm a few things. When your bike won't start, do you hear the fuel pump at key on? Is the bike cranking or not?

I made a pressure tester using my old plastic quick disconnects, a hose tee piece and a 0-60 psi pressure gauge. Machinemart or Hampshire Hose well sell you the bits and you can even use straight hose tails and hose clips if you have no spare QDs.

A crude test is to turn the key on to run the fuel pump. Turn the key off and leave for 5 minutes. Then carefully disconnect the fuel flow hose over a bucket and push in the center if it is a QD. If fuel stills comes out under pressure, then I think your pump is ok. Mine will do this after 20 minutes!

Have you checked that the fuel tank vents are clear? When you get the bike going, idle it for 10 minutes then lift the gast tank flap. If you hear a whooshing noise as air runs in you have blocked tank vents.

You have to try and isolate the issues and fault find when the fault is there. A starter motor not cranking is completely different to a fuel supply problem and completely different again to an ignition/lack of spark problem. Work in reverse. When the bike will not start or stops running, pull a plug and check for spark - eliminate the ignition. Then move on to the fuel system - check for voltage at the fuel pump connector when in the fault state and see if the fuel rail has remained pressurized or not. Check your interlock and cut out circuit.



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Thanks very much for your response Voxmagna. Just running some more tests today. Will update as soon as.

Interested to hear you park at Hurn with the bike. Do they charge? They do for everything else even a drop-off!

Looked at three components today:

1. Hall Sensor - the backing plate looked quite corroded and sprayed the lot with contact cleaner, then refitted.

2. All elec connnections off the coils and sprayed. Tested primary and secondary coils for resistance and got good readings.

3. Took-off Throttle Postion Sensor (TPS) but gave-up trying to break into it.

About to test resistance on injectors but as it was proving a bit fiddly i gave the engine a start and it sprung into life, after having coughed, stumbled and spat for the last few days.

So my money's on the Hall Sensor or slightly failing coils.

Still having some problems with maintaining a low idle.It used to sit comfortably at 900 rpm when warmed up but now needs at least 1000rpm. That's OK, but fully releasing both choke and throttle or opening the throttle once warm is now causing it to die. Which may well point to a bad Hall Sensor (HS)

I've fiddled with the positioning of the TPS but until i'm certain the Hall Sensor is letting me down i can't be certain that the TPS position is at fault causing problems with the idle.

Will heat test the HS and look for a replacement. Have already ordered a set of used coils.

In the course of all these investigations i've gained a better understanding of a number of parts/systems.

One small but niggling surprise was to find that the large rubber hose (90degree bend) that joins the airbox to the plenum chamber has a gap evident into the airbox despite the large jubilee clip securing it to the metal airflow housing.

Surely this is allowing unfiltered air in? Is this fitting meant to be closed completely? Would be pleased to hear from anyone.

OK, will follow-up on any suggestions and post any results i have.
Cheers to all for creating this wealth of knowledge (and encouragement).

OK - i think i have a result.

Heat tested the Hall Sensor (HS) today and that produced the problem.

Cooled the HS, problem gone. Repeated several times with same result.

Got on the phone to Motorworks for a used HS unit.

Have read somewhere that it's not actually the sensor that's likely to fail but the wiring with insufficient or degraded heat insulation. Does anyone have any knowledge of whether this can be rectified at reasonable cost (and skill)?

As far as I know some have replaced the "pick ups" on the earlier bikes HS with Honeywell brand. Cheap also...! And as far as I know they are the same HS over a long time of K models.Different part numbers over the years but more to do with the plug than anything else.

From something I read recently Honeywell dosn't make them anymore but there may be an alternative. Different brand....Hum where did I see that?I'll try to remember but what you should do is look around and maybe Google in the K100-K1100 models, Hall sensor repairs and see what comes up.

Check out Bob Fleischer's site: BMW Motorcycle Tech for good info on Hall sensors and replacements.

Yes that but I think he is still talking Honeywell brand and those could be N/A???? But that refreshes my memory that's what they were discussing, alternative brand to his recommendations. Now I know where to look considering the price of them HSs for the K1200RS. Not usually known to fail but some do fail.

BTW I also found a little corrosion inside/around my HS, looks like nobody noticed the missing screw from before I bought that bike, and neither did I notice for a while.

I don't have any part numbers to go on and don't want to read Snowbum today...

Here is what I found but that's for the airheads, mind you that's a start if just to look at alternatives from Siemens:

Siemens HKZ101S replacement Hall effect vane sensor for the (obsolete) Honeywell 2AV54

Honeywell 2AV54 comes up here again....worth looking at if only for the method of replacement.

http://advwisdom.hogranch.com/Wisdom...ll_sensors.pdf

More part numbers there:

http://www.k100-forum.com/t287-hall-sensor-replacement

Thanks for the input on the Hall Sensors. The replacement part did the trick.

Yes, the Honeywell sensors are no longer available but there is Chinese made replacement. If i can find the link i will post it.

However i'd like to test if it's the sensor at fault or the wiring. If i make progress on that i will update the post.

Thanks again for everyone's contribution.