K100LT - Running on two cylinders but everything seems to be working fine.

[tedmagnum]

I'm new to bricks. But I'm learning fast. But I need some guidance.

I've been handed a 1989 K100LT to try and sort out.  It's only running on cylinders 2 and 3.  With my temperature gun I can see that 1&4 don't warm up. But 2&3 do.

First thing I did is pull the plugs and check for sparks. They are all sparking.
I removed the injector rail and turned the engine. They are all squirting.
I swapped the coils around and the cylinder for 1-4 happily runs cylinder 2-3. So I think the coils are fine.
I have replaced the HT leads and plugs also.

If I unplug the coil for 1-4 when the bike is running, it makes no difference. But if I unplug the coil for 2-3, the bike completely dies.

So, I'm thinking the Hall effect sensors ??  From wiring diagrams, I see there are two. Are the two sensors one for each coil ??

Would the injectors fire if one was faulty ? I would assume the HES also fire the injectors.

In past experiences from faulty Hall effect, I would think I would get NO SPARK at all..

Can anyone point me in the right direction ? 

[The Mighty Gryphon]

The Hall Sensors are on the front of the engine under a T-shaped cover above the pumps.  It's not unusual for them to fail.  Their connector is under the tank on the right side.

As you mentioned, they control spark and the injectors.  There is a control module under the tank near the head that takes the signals and conditions them for the coils and the engine control unit.  They provide ignition timing information for the spark as well as rpm information for the Engine Control Unit.

Use the search function here to find threads about testing the Hall Effect Sensors.

[tedmagnum]

Using a logic probe, I tested the hall effect sensor at the plug as I've done before on the oil head boxers that I usually work on. The wiring is the same.

You just put the probe into the back of the plug and watch the logic probe flash on each of the pick-up wires. 

I've been switching HT leads between plugs and coils all afternoon and then tried a new plug from a different bike.  I get a better spark with new plugs even though the plugs on the bike are meant to be new.  Although, I'm not sure they are the correct plug.

I've ordered four new plugs and will start again from there. 

[The Mighty Gryphon]

I've been switching HT leads between plugs and coils all afternoon and then tried a new plug from a different bike.  I get a better spark with new plugs even though the plugs on the bike are meant to be new.  Although, I'm not sure they are the correct plug.

I've ordered four new plugs and will start again from there.

Not a bad idea. 

Last winter I was resurrecting a K100 that had been idle for quite some time.  I installed a set of plugs that I thought only had a couple thousand miles on them.  Although they looked okay, at least one of them just wouldn't fire in the cylinder even though they showed a spark when they were out.  When I replaced them with new, the bike fired right up and ran properly.

[Laitch]

I get a better spark with new plugs even though the plugs on the bike are meant to be new.  Although, I'm not sure they are the correct plug.
 

What is the brand and model number of the meant-to-be-new plugs? Did you check the setting of their gaps? Are the HT leads OEM or something else?

[daveson]

Yep I think the injectors would fire if the top sensor was faulty, but I haven't verified that with a test. I think the bottom sensor provides signals to pins 8 (rpm signal to FICU) and 9 (coil 2, cylinders 2 and 3) of the ICU, at least that's what I've drawn on my electrical drawings, don't really remember why.

I think coil 1 and the tacho depends on the top sensor. Tacho working?

Maybe check for loose head bolts and compression check.

[rbm]

Try to swap primary connections on both coils.  If the problem moves with the swap (cylinders 1 & 4 start working), the HES is probably the root cause.  If the problem stays with the coil (cylinders 1 & 4 remain dead), then the problem is the 1-4 coil most probably.