Author Topic: '87 K75C No signal to fuel injectors  (Read 211 times)

Offline dielman95

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'87 K75C No signal to fuel injectors
« on: July 28, 2020, 10:06:40 PM »
Sup Y'all, new to the forum after lurking for a while trying to solve this issue

I bought a running 1987 K75 with 60k miles on it almost a year ago, tore it apart to clean/paint it and rebuild it as a cafe. but getting it to fire up again has been a headache. First the starter wouldn't turn over but I fixed that by replacing the starter relay. Now it'll turn over, but wont start. Upon further investigation, the fuel injectors weren't spurting any fuel. I put in a new fuel pump, I have pressure at the rail, and the injectors are replacement ones I bought that had been ultrasonic cleaned that are reading the proper resistance. Using LED fuel injector test lights at the injector electrical connectors, I'm not getting a pulsing to any of the 3. This is where I'm stumped, I've been following that commonly posted troubleshooting guide for K75/K100s, but I'm not exactly sure how to proceed from here. Maybe i broke something in the mass flow sensor or ECU when moving things around?

Also, maybe unrelated, the gauge cluster is reading 4th gear in neutral, and 5th gear in 1st??

Thanks for the help.
  • Boston, MA
  • 1987 K75

Offline Laitch

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Re: '87 K75C No signal to fuel injectors
« Reply #1 on: July 28, 2020, 10:21:23 PM »
Are you indicating there was pulsating negative voltage at pin 12 of the fuel injection control unit before you used the LEDs to check the injectors?
  • Along the Ridley in Vermont.
  • 1995 K75 77,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline dielman95

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Re: '87 K75C No signal to fuel injectors
« Reply #2 on: July 28, 2020, 11:55:22 PM »
Are you indicating there was pulsating negative voltage at pin 12 of the fuel injection control unit before you used the LEDs to check the injectors?

Before I disassembled the bike, yes. After that, no clue. This latest check was the first time
  • Boston, MA
  • 1987 K75

Offline Laitch

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Re: '87 K75C No signal to fuel injectors
« Reply #3 on: July 29, 2020, 08:44:48 AM »
Before I disassembled the bike, yes.
Does that statement indicate the engine wouldn't start before you disassembled the moto and you were trying to determine the problem by using the flow chart's diagnostic steps?

At this stage, you've skipped steps in the flow chart that might narrow the diagnosis of the present problem's source. It's helpful to work the chart in forward sequence but running the fuel injection pin #12 test right now might be informative.
  • Along the Ridley in Vermont.
  • 1995 K75 77,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline dielman95

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  • Posts: 8
Re: '87 K75C No signal to fuel injectors
« Reply #4 on: July 29, 2020, 10:57:37 AM »
Does that statement indicate the engine wouldn't start before you disassembled the moto and you were trying to determine the problem by using the flow chart's diagnostic steps?

At this stage, you've skipped steps in the flow chart that might narrow the diagnosis of the present problem's source. It's helpful to work the chart in forward sequence but running the fuel injection pin #12 test right now might be informative.

Sorry, if i wasn't clear. The engine was perfectly operational before disassembly. As for conducting the pin #12 test; with the positive lead of the voltmeter in pin #12 and the negative lead to a common ground, I should be getting 12 volts correct?
  • Boston, MA
  • 1987 K75

Offline Skunky

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Re: '87 K75C No signal to fuel injectors
« Reply #5 on: July 29, 2020, 12:03:05 PM »

Also, maybe unrelated, the gauge cluster is reading 4th gear in neutral, and 5th gear in 1st??

Thanks for the help.

Hi Dielman  The Gauge Cluster could be the clue to the whole problem. If I learn't one thing with my build is that old K's don't always like being pulled apart and put back together.
The fact that your showing odd gear readings could be a sign of poor grounding. A fault which is common with the K bikes and could be the cause of all of your problems. I would recommend removing the cables on the ground point under the tank, cleaning the terminals thoroughly and refitting. I would also remove clean and refit the ground from the starter (which . The four pin connector to the fuel tank is also another area which caused me to go through the fault flow charts and turned out to be the culprit (even though the pump appeared to be running). Once running it just gets better.
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Offline The Mighty Gryphon

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Re: '87 K75C No signal to fuel injectors
« Reply #6 on: July 29, 2020, 12:47:50 PM »
I bought a running 1987 K75 with 60k miles on it almost a year ago, tore it apart to clean/paint it and rebuild it as a cafe. but getting it to fire up again has been a headache. First the starter wouldn't turn over but I fixed that by replacing the starter relay. Now it'll turn over, but wont start. Upon further investigation, the fuel injectors weren't spurting any fuel. I put in a new fuel pump, I have pressure at the rail, and the injectors are replacement ones I bought that had been ultrasonic cleaned that are reading the proper resistance. Using LED fuel injector test lights at the injector electrical connectors, I'm not getting a pulsing to any of the 3. This is where I'm stumped, I've been following that commonly posted troubleshooting guide for K75/K100s, but I'm not exactly sure how to proceed from here. Maybe i broke something in the mass flow sensor or ECU when moving things around?

Also, maybe unrelated, the gauge cluster is reading 4th gear in neutral, and 5th gear in 1st??

First off, removing the rear loop of the frame has been known to cause multiple operational problems with the engine.  It's why many of us here strongly recommend against doing cafe conversions.  These problems create a lot of mental stress for the brain trust here.

Having said that and acknowledging the water over the bridge, if there is no pulse signal going out to the injectors, it may be the wiring for the Hall Sensors at the front of the engine.  These sensors not only provide the timing of the spark, but also trigger the circuitry in the ECU that operates the injectors.  This leads to my question, "Do you have spark?".

As far as the odd gear indications.  The gear position switch sends a "digital" signal from three internal switches.  If I recall, neutral requires all three switches to be making contact, and if one of them isn't, you will get a different readout on the indicator.  1st gear also requires a certain combination of switch contact.  Again, if one of them isn't making contact it will also give an incorrect readout.

Bottom line is that the gear position switch probably has gotten water and dirt in it and isn't working properly anymore.  There are threads and information here and on other K bike forums on how to disassemble the gear position switch, clean it and reseal it again.  The bad news on this job is that you need to remove the battery tray and the swing arm to get the switch off the transmission to service it.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
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Offline dielman95

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Re: '87 K75C No signal to fuel injectors
« Reply #7 on: July 29, 2020, 05:10:56 PM »
First off, removing the rear loop of the frame has been known to cause multiple operational problems with the engine.  It's why many of us here strongly recommend against doing cafe conversions.  These problems create a lot of mental stress for the brain trust here.

Having said that and acknowledging the water over the bridge, if there is no pulse signal going out to the injectors, it may be the wiring for the Hall Sensors at the front of the engine.  These sensors not only provide the timing of the spark, but also trigger the circuitry in the ECU that operates the injectors.  This leads to my question, "Do you have spark?".

As far as the odd gear indications.  The gear position switch sends a "digital" signal from three internal switches.  If I recall, neutral requires all three switches to be making contact, and if one of them isn't, you will get a different readout on the indicator.  1st gear also requires a certain combination of switch contact.  Again, if one of them isn't making contact it will also give an incorrect readout.

Bottom line is that the gear position switch probably has gotten water and dirt in it and isn't working properly anymore.  There are threads and information here and on other K bike forums on how to disassemble the gear position switch, clean it and reseal it again.  The bad news on this job is that you need to remove the battery tray and the swing arm to get the switch off the transmission to service it.

I do have spark, or at least I did 3 days ago when I checked last. I'll probably double check again today to be sure. Is there an easy way to test whether my Hall Sensor is good?
  • Boston, MA
  • 1987 K75

Offline dielman95

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  • Posts: 8
Re: '87 K75C No signal to fuel injectors
« Reply #8 on: July 30, 2020, 08:16:24 PM »
Hi Dielman  The Gauge Cluster could be the clue to the whole problem. If I learn't one thing with my build is that old K's don't always like being pulled apart and put back together.
The fact that your showing odd gear readings could be a sign of poor grounding. A fault which is common with the K bikes and could be the cause of all of your problems. I would recommend removing the cables on the ground point under the tank, cleaning the terminals thoroughly and refitting. I would also remove clean and refit the ground from the starter (which . The four pin connector to the fuel tank is also another area which caused me to go through the fault flow charts and turned out to be the culprit (even though the pump appeared to be running). Once running it just gets better.
So I cleaned the ground point below the tank and what do ya know, now I'm getting a signal to my fuel injector! I both love and hate that its that simple. The bike still wont start, however. I hear small little pops when I hit the starter like it wants to start, but still nothing. Any ideas?
  • Boston, MA
  • 1987 K75

Offline Martin

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Re: '87 K75C No signal to fuel injectors
« Reply #9 on: July 30, 2020, 08:58:29 PM »
Pull the plugs, are they wet or dry?
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline dielman95

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Re: '87 K75C No signal to fuel injectors
« Reply #10 on: August 01, 2020, 10:39:04 AM »
Pull the plugs, are they wet or dry?
Regards Martin.
The plugs are not wet

I read somewhere else that the ignition coil wires are easy to mix up the order, I saw that was true in my case. I fixed the order and it fired up for like 3 seconds, then died. Now I cant get it to catch. I hear a lot of popping and some gnarly 'clacking" coming from either the starter or the alternator. Any thought?
  • Boston, MA
  • 1987 K75

Offline Laitch

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Re: '87 K75C No signal to fuel injectors
« Reply #11 on: August 01, 2020, 11:44:42 AM »
The plugs are not wet
If you're checking them after multiple starting attempts during which the engine wouldn't fire, then they should be wet if the injection system, Hall sensor and fuel injection relay are working correctly. There should be pressure at the fuel rail and pulsating fuel spray from the injectors. That will take inspection again. There is a test for Hall sensor function in the popular troubleshooting guide you have been using.

  • Along the Ridley in Vermont.
  • 1995 K75 77,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline dielman95

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  • Posts: 8
Re: '87 K75C No signal to fuel injectors
« Reply #12 on: August 01, 2020, 05:24:11 PM »
If you're checking them after multiple starting attempts during which the engine wouldn't fire, then they should be wet if the injection system, Hall sensor and fuel injection relay are working correctly. There should be pressure at the fuel rail and pulsating fuel spray from the injectors. That will take inspection again. There is a test for Hall sensor function in the popular troubleshooting guide you have been using.

Yeah, I'm an idiot. They're wet. And very black, too. On my last attempt to start, the air flow sensor got blasted off the bike by a backfire.
  • Boston, MA
  • 1987 K75

Offline Laitch

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Re: '87 K75C No signal to fuel injectors
« Reply #13 on: August 01, 2020, 05:56:53 PM »
. . .the air flow sensor got blasted off the bike by a backfire.
That is an important component of fuel mixture calculation. It also has a delicate air temperature sensor mounted in it. Clean it with air flow sensor cleaner only then remount it. It's vane might have been stuck and the blast concussion might have freed it, or broken it and the wiper element. If it freed it, the moto might start and run without further work if the AFS was at the root of this no-start condition.

You still need to inspect your system to determine if the injectors are operating correctly—LED on injector blinking,  injector ticking or fuel stream pulsating—or stuck open.
  • Along the Ridley in Vermont.
  • 1995 K75 77,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline dielman95

  • Curious
  • Posts: 8
Re: '87 K75C No signal to fuel injectors
« Reply #14 on: August 02, 2020, 07:58:01 PM »
That is an important component of fuel mixture calculation. It also has a delicate air temperature sensor mounted in it. Clean it with air flow sensor cleaner only then remount it. It's vane might have been stuck and the blast concussion might have freed it, or broken it and the wiper element. If it freed it, the moto might start and run without further work if the AFS was at the root of this no-start condition.

You still need to inspect your system to determine if the injectors are operating correctly—LED on injector blinking,  injector ticking or fuel stream pulsating—or stuck open.
The injectors seem to be working fine. I got a puling LED light during testing, and they all spray gas as well. I had a spare, operational AFS laying around I clamped on, and still the same symptoms. I can get the bike started 1/10 times by blipping the throttle, but it idles extremely rough and backfires aggressively; it'll die if i don't keep giving it throttle. I pull the plugs after each attempt, they're wet and extremely black. Could the Hall Sensor be the issue here?
  • Boston, MA
  • 1987 K75

Offline daveson

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Re: '87 K75C No signal to fuel injectors
« Reply #15 on: August 02, 2020, 08:11:04 PM »
Finally someone who might have the same strange problem I once had.

Look at the recent thread "BMW K75 - wont start with fuel pump connected"

When you do get it going are you able to ride it a short distance and get it to say 60mph?
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline Martin

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Re: '87 K75C No signal to fuel injectors
« Reply #16 on: August 02, 2020, 08:25:36 PM »
Check the values of the temperature sensor. Check the connector to the sensor is it clean and hve a good connection. If the plugs are wet try pulling the #6 fuse Kraffstoppe (fuel pump) you should be able to start it on the residual fuel in the combustion chamber.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

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