Rev counter surge but no crank, pump or start 🤯

[DrNocky]

Lads and lassies,
Had I hair to pull out I would have. The sun is splitting the stones here in Ireland but my 85 K100RS won’t come out to play. When I turn the key the rev counter surges and there’s a whine from the alternator area, but that’s it. Has anyone had this happen and if so, how did you fix it? Here’s a quick video of the problem. TIA, Ian
https://youtube.com/shorts/4E1JijcN2lo?feature=share

[rbm]

What's the history on the bike?  Is it new to you?  Has it functioned properly before this issue started?  Was it put away for the winter and now you're just getting it ready for riding?

The wine sounds like the fuel pump.  Open the gas cap and see if the noise is louder inside the tank.  The tach acting up strange but it might point to a damaged circuit card inside the OEM cluster.

[Laitch]

That's a first for me! It sounds like the uillean pipe's regulators are somewhat clogged but the drones are still rhythmic, anyway.     It's a humdinger of a symptom. I'd feel compelled to track down the noise's location, moving or removing any impediments to locating it.  rbm's suggestion to open the cap is an elegant diagnostic strategy. The fuel pump shouldn't run unless the starter button is pressed.

When the key is turned to On, power flows to he ignition control unit, the fuel injection relay, the coils and the instrument cluster.  What is battery voltage reading between positive and negative poles? Weak batteries—or weak voltage drawn off by defects—can produce noise from struggling components. It's the perfect time to use tapping-with-blunt-objects as a diagnostic tool, starting with tapping the side of the relay box beneath the tank with a screwdriver handle.

[Laitch]

I read your short history here, Dr. It seems your Brick had a wiring fire. Did you ever closely inspect all the wiring effected by that fire? This symptom could be caused by wiring problems.

[The Mighty Gryphon]

After checking the fuel pump, lift the tank so you can get into the relay box.  You don't need to remove the tank just lift it a few inches and remove the cover.

Turn the key ad while the noise is happening, touch the relays one at a time.  I suspect that one of them will be buzzing.  Knowing which one it is should give us a hint of where we need to troubleshoot further.

[frankenduck]

First find the source of the noise that shouldn't be there.

The tach signal to the cluster is a BLU/BLK wire that's a signal out of the Ignition Control Unit Pin 14 that goes to both the #1 coil and the tach in the cluster. Maybe it's the ICU that's gone haywire and is making the funny noise.

The ICU is mounted up at the steering head under the front of the tank.


As TMG suggests, you can also pop the lid off of the relay box and see if any of those are the noisy culprit. Note that you can use a screwdriver as a stethoscope of sorts. Put the screwdriver tip to each relay and press the handle up to your ear to figure out which relay it is if the sound is coming from the relay box.

The L-Jetronic under the seat should be checked too.

[daveson]

The first thing I thought, does it start if you pull in the clutch lever and press the start button? As that bypasses the instrument cluster (and eliminates it from the starting circuit) You say it's an 85RS but the cluster looks to me like a more modern one, maybe the contacts aren't too good

If that allows the bike to start, wiggle the multi pin plugs at the back of the cluster while the start button is pressed, or the wires as that might narrow it down as to which of the wires has poor contact.

[Chaos]

Mine did something similar when the starter motor went bad.  Can't remember if the rev counter went up when I turned the key or pushed the starter button, but there was a hum and no response from the starter motor.  Replacing the starter with a used one cured everything. 

[daveson]

Just had another look. You have the clock at 0, the oil pressure light and gear indicator, the other lights don't come on.

Unless it's reflection, it looks to me that while the tacho goes up, the clock dies out. The clock gets constant power from the battery, so it's a poor power supply problem. You want a good battery and clean connections, and for the battery earth strap at both ends.

[DrNocky]

Thank you all for your suggestions and your input. We’re in different time zones so please forgive my late reply. I have the bike since March and it has been 100% until this issue. The bike Laitch mentioned that had the electrical fire was a K75 that I fixed and recently sold. The keen eyed among you spotted that this is a bit of a Frankenstein’s monster, hence its name is Frank 🤷‍♂️. It appears to have been assembled from bits of various bikes. That’s ok if it’s done right and until now I thought it had been. It’s almost midnight here so I will sign off and update once I’ve worked through your suggestions, beginning with the battery. 🤞

[frankenduck]

I second the vote for possible starter motor issues because the starter can mess with the grounding and make many things go haywire. A usual symptom of a wonky starter is turn signal gremlins so turning on the key and seeing if the turn signals operate normally is an easy diagnostic test to try.

[Martin]

+3 for starter issues.

[daveson]

Yep the load shed relay relies on the starter for earth, but I'm thinking the clock should remain on cause it doesn't rely on the load shed for earth. Even the clock reading zero was a bit susso, I thought. And for example you would think that the alternator light would be on, it's got another separate path for earth amongst others.

[DrNocky]

Greetings from a still sunny Ireland. Long story short. I put a spare battery into the K and it started first turn. Praise be! I could have saved myself a lot of headache if I'd started with the basics. What threw me was the battery was displaying as full on the tender, so I assumed it was good. Everyday is a school day when you own an old bike. Thank you all for your input. Motobrick is a great community and its testament to your love of the old bricks that you weighed in with your wisdom the way you all did. TIL Start with the basics.

[Laitch]

 

[Laitch]

Although it's really too early to start the party. 

[daveson]

It's a bonus if it's a quick fix. A smart charge might be good, but before that, if it's a maintenance free battery, sometimes the top comes off and you can check the electrolyte level. If any are low, just top up like normal, I've saved some like that.

[stokester]

Thanks for the followup.
All of us have been through the "check simple things first" scenario.  Some more than once 
A weak or failed battery can show in various ways similar to the ways a poor ground can give seemingly unrelated symptoms.  The only real test for a battery is under load either by test or measure while attempting to start.