Went to start my k100rs and found no fuel is spraying from all 4 injectors. 

Fuel pump is new and audibly primes.

New fuel filter

Sent in to service all 4 injectors professionally and flow rates are good injectors open and close. 

I have fuel pressure at the fuel rail.

Using a multimeter i found 12 volts on both terminals to all 4 injector connectors when grounding to the frame. But when putting both multimeter probes to both terminals on each connector there is no power or pulse when pressing the starter. 

Following Crazy Frogs troubleshooting guide how should i test for a grounding pulse when measuring at the Injection ECU?

On a 2V K100RS Pin 12 of the L-Jetronic sends a ground pulse to fire the injectors. (Yellow/Gray wire.)

Duck, thanks for the quick reply I'm a novice when it comes to electrical and picked up my first multimeter the other day. Could you explain in greater detail exactly how this test can be performed using a multimeter? What values am i looking for? I've also read elsewhere the test can be performed using an LED?

Where are you in WA?

Analog or digital multimeter? Analog multimeters have a needle and might be better for something like this.

Example:

LED: I've never tried it but it might be possible with a 12V LED.

You can also use a 1.5 volt LED and put a 470 ohm resistor in series with it.

Rick G wrote:You can also use a 1.5 volt LED and put a 470 ohm resistor in series with it.

That's old school from back when 12V LEDs weren't as cheap and readily available as they are now.

With the start button pressed the injectors should be getting constant power and a pulsing earth. I think one of these is missing.

With a 12V LED test light clamped to battery positive and probe to one of the injector plug terminals, the light should pulse, that would be the negative terminal. On the other probe (edit: terminal) the light should remain off (positive to positive)

 With clamp to battery negative and probe to the positive terminal, the light should be constantly on. 

I've posted a photo of this showing which is positive and negative, but can't remember how to send it to this thread. The thread is called:  Troubleshooting a 1985 K100 that won't start/stay running
it's on page 2, post 91.

Zacdonov wrote:Went to start my k100rs and found no fuel is spraying from all 4 injectors. 

Fuel pump is new and audibly primes.

New fuel filter

Sent in to service all 4 injectors professionally and flow rates are good injectors open and close. 

I have fuel pressure at the fuel rail.

Using a multimeter i found 12 volts on both terminals to all 4 injector connectors when grounding to the frame. But when putting both multimeter probes to both terminals on each connector there is no power or pulse when pressing the starter. 

Following Crazy Frogs troubleshooting guide how should i test for a grounding pulse when measuring at the Injection ECU?

Isit 2v or 4v RS? Maybe put year and typoe and VIN on your signature.

2V

It says 1985 in the thread title.

duck wrote:2V

It says 1985 in the thread title.

I didnt go back to look up there.

I purchased a 12v LED test light today and after clamping to battery POSITIVE the light was not flashing on either injector connector terminal when pressing the starter. When clamping to battery NEGATIVE i was getting a pulse on both injector connector terminals while pressing the starter.

What it looks like to me, the first test proves you have no earth, you need to fix that or else the injectors will never open.

The second test shows the wiring is seriously messed up. If you have done electrical modifications, check them over, especially any done before this no go situation.

The fuel injection computer sends a pulsing earth signal. You should have constant power from the fuel relay but your getting a pulsing signal, that makes it look like the computer is involved. Have a good look at the plug for the fuel injection computer.

Maybe it's the computer, there's a test in Bert's guide, or if possible check if the symptoms are the same with your computer in another brick.

I have a spare ICU when swapped i get the same conditions. The injection ecu gets signal on pin 1 from pin 8 on the icu. Using a multimeter I measured from both ends and found continuity. Can we think of any other reasons why the bike would not be getting a grounding signal to the injectors? I’ve ordered a new ecu, airflow sensor, temp sensor, injection harness as well as a main wiring harness. I will not rest until I’ve narrowed this issue haha

Note that there are early and late versions of the wiring harnesses. As an 85, your bike probably has the 61111459520 fuel harness and 61111459037 main harness..

Probably better to find the problem first before spending money. I was thinking swap the fuel injection computer rather than the ICU. 

I'm thinking the fuel injection computer can't supply earth if it's not getting earth, I think that's near the injector rail. I think there might be a photo of that as well in the thread I mentioned, if you found that thread. But the second test results were a real doozy.

Zacdonov wrote:I will not rest until I’ve narrowed this issue haha

Especially if you stay on the parts-buying binge. The piper will be paid.  

Are you using the site's troubleshooting guide flowchart with the black print text, the one with the colored text and boxes, or both? Are you using the Level 2 LE Jetronic manual to help sort this out, also?

My 3rd test was to clamp my test light to battery positive and measure pin 12 on the female connector to the injection ecu for a ground pulse and found nothing when pressing the starter. But with my clamp on battery negative i have 12v and a pulse.

Following Crazy frogs guide I'm supposed to find a pulsing negative on pin 12? What does pulsing negative even mean and is my 3rd test another doozy?

Zacdonov wrote:What does pulsing negative even mean . . .

A pulsed negative is a negative ground connection switched on then off repeatedly. This switching is what opens and closes each fuel injector to fuel each cylinder during operation. The fuel injection control unit controls the duration the injector is open.

Maybe watching this video will help. Imagine it's a Brick engine. It could help you fall asleep if nothing else.

Find a photo which shows which side of the plug for the injector is positive. With the injection ECU unplugged and the start button pressed, you should have constant power on the positive terminal.
of the plug for the injector. If so and it becomes pulsing with the computer plugged back in, that's not looking good for the computer. But you still want to check that the computer is getting earth. 

The computer has many inputs, but only one output, pulsing earth to the injectors. Your tests at the plug are good for checking inputs, but I think the reason you didn't get a reading for the output was because the  computer was unplugged. 

The injection ECU should get earth from plug terminals 5 and 13. With test light clip on battery positive and probe to terminal 5, the light should glow. Same for 13.

It's also good to give these connections a clean with the help of some dioxide (Edit: deoxit)

Measured pins 5 and 13 clamped to battery positive and both have ground. My new used Injection wiring harness was the first to show up in the mail and immediately i got to work. The new injection harness immediately provided pulsing ground to each injector connector and i was able to start the bike for the first time in over 6 months! Now what to do with all the extra parts i ordered haha

Bonus, well done.

Just a point about the pulse, even at idle speed, the light is turning on and off so many times, there's not a really big difference between it pulsing and constantly on. The computer should be receiving constant earth and provides a pulsing earth and duration based on its inputs.

In this photo the positive is on the right.



Test light on the negative terminal.