1991 K100RS 4v - No start, Motronic pin 3 no ground signal to fuel pump relay.

[JoggerFogger]

So the bike will start if I stick a stuck relay in place of the fuel pump relay.  This makes the fuel pump run continuously, even with the ignition off.
I am getting fault codes 1122 and 1133 (hall sensor 1&2), but tested the hall sensor and knowing the bike runs bypassing the fuel pump relay.

I am not getting a ground signal from pin 3 on the Motronic which feeds the ground side of the FPR coil.
I thought the coolant sensor was bad, but it's supplying 2500-2700 ohms when cold to pins 21/32 on the motronic.
I assume there's another sensor feed that the Motronic is not happy with and preventing the fuel pump relay signal to be sent.

I'm out of ideas, and have resorted to just checking every wire, connector, ground, but all seem to be good so far.

I just bought this bike 4 days ago as a non-running project, but I feel so close.  would love any suggestions.

Thanks,
Kevin in Portland, OR, USA

[JoggerFogger]

Just a running list of all the things I've checked so far:
1. Coolant Sensor.  2700-2500 ohms cold, dash light goes out once start button pressed.
2. MCU (Motronic Control Unit) has two fault codes, 1122, 1133, but found these are normal when the motor is not turning.
  a. checked the HES (Hall Effect Sensor) anyway with a DIY kit and a 9v battery, works fine.
  b. checked wires from HES and Coolant temp sensor back to MCU.
3. I have a "stuck closed" relay, that when I put it in the socket it starts the fuel pump, bike starts, runs great, further confirmation that the hall sensor works.
4. Cleaned all ground lugs looking for an easy fix.
5. unplugged inspected, and plugged 3x every connector I could find.
6. Disassembled the MCU looking for something obvious, especially related to pin 3.

Even made a video of my FPR (fuel pump relay) test process:
https://youtu.be/ojCoPbFI34M?si=XiDH0uVQjpqwjw5N

Thanks for your help in advance if you have any ideas what to do next.
-Kevin

[daveson]

The temperature light is on, so the computer has been tricked into believing that the engine has overheated. Therefore it will deny earth to the fuel relay, preventing the fuel pump from starting. It won't allow the engine to run, to prevent engine damage. Maybe it will run with the temperature sensor unplugged (although that might be too cold a reading) Anyway the temperature light is on, that's a problem, you might want to find out why, I'm not familiar with 4V engines though, so maybe it's normal.

One check would be to unplug the temperature sensor wire from both ends and use it as an earth for a test light. If the test light glows, there is a short in the wire. Maybe check the wiring diagrams to see if this is a valid guess.

[JoggerFogger]

That too was my first guess. 

I plugged in a new sensor, same result.
I checked continuity in both wires, all good.
I found the proper resistance for cold = 2500 ohms at 20C and Hot, 0-250 ohms at 100C
Mine was about 2700 ohms at 10C and 2600 at 16C -ish.

Also, that light goes out after you attempt a start.

Then found some videos on youtube of startups and they too have the light on at first.  Some videos it goes out right away.


I also did a fault code reading, and one of the possible codes is 1223 (water temp out of range), and it didn't come up

I seriously thought that was the problem at first, and I feel like I've exhausted all options on that front.

Thank you so much for the idea.  Please give me more.

-Kevin 

[daveson]

Ok that wire has continuity, but does it have a short? I haven't looked at the wiring diagrams for your brick, but if you look at them, that might verify that the test I suggested might be worth trying.

If it seems that the computer is failing to send an output, then you can check to make sure it's getting all it's inputs. If all inputs are good and it's not sending an output, that's not looking good for the computer.

I think on your brick the fuel pump only runs until the fuel gets pressurized. The video doesn't show the situation with the start button pressed, was wondering about that.

What's the go with the extra wires to the starter relay, and the yellow one?

[JoggerFogger]

I found a used MCU on eBay in a lot of K100 stuff.   Made a low-ball offer and they took it.  $50 and it's on the way. 

I have a couple test leads laying there with an LED indicator as a 2nd tester for all my measurements.  There's also the anti-theft connector laying in there. 

I did check those wires.  Against ground, against 12v, and against each other unplugged on both ends.   

You're right.   It's the only thing that keeps the MCU from giving a green light.  AND, everything else seems to work on the MCU.   It doesn't make sense that just one of it's 40 functions fails.  It's more likely that there's an age related corrosion or failed wire insulation. 

Well, until the MCU shows up, I'm going to ground pin 85 on the FPR and ride the bike around and see if there's anything else that needs attention.   Hopefully my diagnosis is right.   

Thank you so much for your thoughts. 

Kevin