Not starting, weird injector behavior

[joshb]

Hey there y'all,

Shelter-in-place seemed like a great time to get down to some maintenance on the brick (1992 K75S). I went through the following procedures and all went smoothly:
- Renewed Coolant
- Cleaned commutator on starter
- Input/Output Spline Lube
- Replaced the clutch cap (23131338731)
- Fork Oil
- Valve clearances

Additionally, I got my original injectors cleaned by Mr. Injector a few months back, and they've been working great since. I haven't futzed with them since reinstalling them after cleaning.

Rolled it out to sync up the throttle bodies and was met with the sad sad sound of just my starter trying it's damnedest but nothing else.

Current behavior:
1. Fuel pump has power and is running
2. Fuel rail has pressure
3. Starter motor runs
4. Battery was fully charged when the issue started

Diagnostics
I've been running through Bert's troubleshooting guide, and found that the fuel injectors are not pulsing. When I first depress the starter switch I'll get a quick spurt and then they stop, no dripping, no spray, just silent. I hooked my oscilloscope up to pin 12 on the Jetronic and only get a constant 12V signal while the starter is depressed.

Airflow Meter Measurements

Vane Closed: 63 Ohms
Vane Open: 870 Ohms
Constant Pins: 362 Ohms

The open values are out of spec, and I'm wondering if that's the cause of the injectors not firing. It seems strange that the airflow meter would just fail suddenly.

Jetronic measurements

Pin 1: 15 mv briefly when starter is first pressed
Pin 2: 11.8v with throttle off and starter pressed
Pin 3: 11.8v with throttle open and starter pressed
Pin 4: 8.8v with starter pressed
Pin 5: 1.2 Ohm
Pin 7 to Pin 5: 63 Ohm (closed)
Pin 7 to Pin 8: 135 Ohm (closed)
Pin 8 to Pin 9: 135 Ohm (closed)
Pin 9: 11.8 v with starter pressed
Pin 10: 2.5k Ohm
Pin 12: To Pin 9 -> 5.6 Ohm
Pin 13: 1.2 Ohm
Fuel Injector Ground Pin to Pin 12 -> 0.2 Ohm
Pin 12 With Starter: Constant 12v on scope, injectors only spray fuel right as the starter button is depressed

Ignition Module Measurements

Pin 1 - 0.2 Ohm
Pin 2 - 11.8 V (ignition on)
Pin 3 - 11.8 V
Pin 4 - No path to ground....
Pin 6 - 10.2 V (starter switch depressed)
Pin 7 - 11.8 V (ignition on)
Pin 8 - not sure how to measure this w/ ignition module unplugged
Pin 9 - not sure how to measure this w/ ignition module unplugged
Pin 10 - 11.8 V (ignition on)
Pin 11 - 10.2 V (starter switch depressed)

I tested the fuel injector relay, it's working fine and switches at 12V with 0 Ohm resistance. I disconnected the temperature sensor and the throttle position sensor, neither have an impact.

I also noticed that I'm not getting any spark either.

I found this thread, which seemed like a similar issue: http://www.motobrick.com/index.php/topic,9325.50.html

To follow along the same lines, I've got a new airflow meter, jetronic, and ignition module on the way that should show up next week.

Does anything else spring to mind for anyone?

[daveson]

"...not sure how to measure 9 with ICU unplugged..."
Previously I've done it with a computer safe test light while the ICU was plugged in with clip on battery positive and probe to pins 9 and 14 (ignition off)  maybe consider a multimeter too.

Spose you've done basics like cleaning the multiplugs,  stuff like that. Strange so many faults at the same time.  If you have constant earth at the injectors, fuel would come out the headers while cranking, or a strong fuel smell, if positive and earth really are good.  The ICU wouldn't supply all outputs if it wasn't getting input from the hall sensors,  probably too early to jump to guesses though.

Check for power and earth at the coils.

[The Mighty Gryphon]

I also noticed that I'm not getting any spark either.

The Hall Effect Sensors at the front of the engine have two functions. 

First, they provide the timing command for the ignition, and second, their signals coordinate the injector pulses to the rotation of the crankshaft.  If you aren't getting both ignition or injection the Hall Sensors is where I would suggest checking first.

I would check the connector from the sensors under the tank.  It may have been disturbed when you did some of the other jobs.

[joshb]

Going about checking the hall sensors out today. I'm afraid something far more sinister is at play here, but wanted to check my assumptions:

When I'm looking at the magnets that trigger the hall effect sensors, should I be seeing them rotate when the starter motor is running? Because they ain't.

[The Mighty Gryphon]

Absolutely.  There is a cup attached to the output shaft that should turn when the engine is cranking.  There are openings in the cup that trigger the ignition.

What kind of noise are you hearing when the starter is running?  Do you hear the popping of valves opening and closing and compression, or just the whirring of the motor.

If no compression noise, your sprag clutch is stuck and not engaging.  You need to roll the bike backwards in 5th gear and pop the clutch to free it up so it will turn the engine.  It's commonly referred to around here as the backspin.

If that is the problem, it means you need to get some engine cleaner in there like Seafoam to clean out the crap that is making it stick.

[joshb]

Oooo. Good to know. I'm getting the same behavior after doing the backspin, it sounds like I can briefly hear the fuel pump turn on right after I pop the clutch.

The only sounds I'm hearing are the fuel pump and the starter motor when I try to start it. Is there a possibility that I horked something while pulling the transmission for the spline lubes or while reinstalling the starter?

[The Mighty Gryphon]

The starter motor is turning, correct?  What I don't know is if it is turning the engine over.  If the sprag doesn't engage, the engine won't turn over. 

Were you pushing the bike backwards when you popped the clutch?  Sometimes you need to do it a couple times to make it engage.  If the backspin doesn't fix it, you need to do a search on "sprag".  Lots of people have had problems with it, but mostly on early models.

[joshb]

The starter motor is turning, and the crankshaft is not rotating.

I pushed the bike backwards, popped the clutch, and continued pushing. Basically the same operation as when the starter motor wouldn't fire up due cruft on the commutator. I did this about ten times at varying speeds, but no love.

Thanks for pointing out the sprag clutch, I'll do some research and report back.

[Martin]

The sprag starter clutch can sometimes be freed up by running a high detergent oil for a couple of tankful's.  You may have to push start it. Never run moly or friction additives these will effect the sprag clutch. There are lots of posts online on fixes. Try here for a bit more information. https://bmwmotorcycletech.info/kstartersprag.htm
Regards Martin.

[joshb]

Whelp. I found a thread on the BMWMOA forums with someone reporting similar issues, also right after cleaning the commutator on their starter.

Pulled mine out, threw it on the bench, and sure enough - it's running backwards.

[Paul_Waterloo]

Whelp. I found a thread on the BMWMOA forums with someone reporting similar issues, also right after cleaning the commutator on their starter.

Pulled mine out, threw it on the bench, and sure enough - it's running backwards.

Well that could be an issue.....hope all works out!

[The Mighty Gryphon]

That's good news!  That reverse running Is the result of getting the motor put together backwards.  It's the reason the first step is to mark all the parts to get them back together correctly. 

Sounds like you're only about an hour away from having it running.

[BlitzenGruv]

You haven't been to England lately, have you?
Perhaps the bike got the idea that it was supposed to be positive Earth?