I bought an abandoned 93 RS for parts the other day. (I bought it legally from a real shop and its abandoned status is properly documented with the Dept. of Licensing.)  It's got 105K miles and has seen some better days but it does have a decent Works Performance shock and Staintune exhaust.

So far:

- The cluster lights up and the fuel pump primes like it should when the ignition is turned on. (With a screwdriver. )

- The inside of the tank is very clean and the gas looks good.

- The clutch cable is broken but when I pull in the clutch lever and hit the start button the starter relay clicks so I know that's working. But that's all it does.

- Aside from the cluster, neither the headlight nor tail light come on when it is turned on.

- I replaced all of the little black relays thinking the load shed might be bad but that didn't help.

- Also swapped in a new starter relay.

- When I tested the voltage on the top of the starter it reads 12V even when the starter button isn't pushed. Shouldn't that only be 12V when the starter relay is engaged?

- I'm pretty sure the transmission is in neutral by shifting the lever but the gear indicator is stuck on 3 so that switch is probably shot.

Given the 12V reading at the starter I'm thinking maybe it needs a new starter.


Since it's a parts bike I don't plan to spend a whole lot more time and effort on it but I'd like to get it running just for the heck of it.

Anybody got any clues on this puzzle bike?

The output terminal on the start relay should only have 12v when the starter button is pressed, if it has that 12v then the next step is the starter itself. Give the starter body a gentle tap with a hammer handle, if it starts the brushes is probably the problem.

Agree with that.

So if you get 12V whether the start button is pressed or not, then the relay contacts are probably welded. I think the clicking noise you are hearing is possibly the LSR. Pull the starter relay cover off and check the contacts. It might just have a small bead of metal arcing across.

But Duck said that he has already replaced the starter relay with a new one, so it eliminates the welded contacts theory (unless the new relay is also rubbish).  I'd suspect the starter motor.  It's easy enough to pull and test on the bench.  Also, put a voltmeter on the starter relay coils to prove your conjecture that the relay is functioning when the clutch is pulled and start button pressed;  it will confirm its function.

Not disputing the suspect starter motor but that doesn't explain the constant 12V output from the relay. But as you say Rob, a voltmeter across the relay coil will tell if its a fault in the starter circuit.
Is it possible that the st. motor is stuck or has an earth short. That would possibly fry the relay contacts pretty quick. Just some thoughts.
Cheers

Holister wrote:Not disputing the suspect starter motor but that doesn't explain the constant 12V output from the relay. But as you say Rob, a voltmeter across the relay coil will tell if its a fault in the starter circuit.
Is it possible that the st. motor is stuck or has an earth short. That would possibly fry the relay contacts pretty quick. Just some thoughts.
Cheers

Suspect No.1 is the starter motor brushes. Since the starter is used to ground the load shed relay, if there is no continuity through it you will get 12V permanently on the input.

aye agree

OK. Back to the schematics to get my head around that. Cheers

Avenger GT is right about having 12 volts at the starter if the starter is bad.  What you have is a series circuit that flows through the load shed relay and the starter.  With the starter not passing any or little current that makes it have a high resistance.  A resistance that is much higher then the load shed relay coil.  In a series circuit the voltage will be proportional to the resistance of the individual components.  Since the starter has the highest resistance it will have the highest voltage.  In this case the resistance is so much higher it will be all across the starter.

The brushes in the starter are not making contact, so the circuit path thru the windings and brushes is open.  So the LSR is not pulled closed and thus the headlights are not working since they get their power thru the load shed relay contacts.

So it sounds like there is consensus that my suspicion about the starter motor being the culprit may be correct.

Although I know the starter does some weird electrical voodoo I must admit that I don't completely understand how that part of K electronics works - or doesn't.

And thanks for pointing out that it could also be the root cause for the lights not working. That hadn't occurred to me but now that you point it out that makes sense - sort of.

Kind of drizzling here today and the bike is still strapped onto my trailer (which serves as a wonderful outdoor motorcycle lift) so it may be a day or so until I can swap in another starter.

On a related topic, after doing some electrical stuff on my 86 K100 I put the recently revived tank back on yesterday only to discover that it has a massive leak caused by the water that sat it.  Curiously, it's not leaking at the weld seam at the bottom as I'd expect but it actually ate a hole  through the aluminum wall (and then paint) about an inch up from the bottom left low point. But once I get the parts K1100 started I can steal the tank from that to use on the K100.  

A Mystic Red tank on a black bike should look interesting but getting it repainted black to match the bike color shouldn't be cost prohibitive.

Take a jumper cable from the positive (+) side of the battery directly to the starter terminal on the motor.  Engine crank?  If so the starter is good, if not the starter is bad.

B4 you do this the engine must be in neutral and the bike should be on the center stand.  When you jump the starter directly like all the safety interlocks are defeated, if the starter works and the bike is on the side stand and in gear the starter will crank the bike right off the stand and fall over.  Bad.

What is the point in jumping a starter terminal that already reads 12V? Besides, getting anything to that starter terminal with a 93 battery holder base is hard because the ignition amp is in the way.

I'm pretty sure that it's in N. It is strapped onto a trailer with four tie-downs.  I just plan to see if the starter motor will turn.  On the off chance that it is in gear, a quick tap on the start button should just make it lurch a little.

If that works and it does happen to be in gear then I'll take off the tie-downs and put it on the center stand and make sure it's in N since the rear wheel will spin by hand on the center stand.

duck wrote:...
 only to discover that it has a massive leak caused by the water that sat it.  Curiously, it's not leaking at the weld seam at the bottom as I'd expect but it actually ate a hole  through the aluminum wall (and then paint) about an inch up from the bottom left low point.

That's what i've found on a couple of my tanks--the seams are fine, but the worst corrosion occurs at the high water line where the fuel and water meet.  On other websticle forums they thought it was polymers and acids forming from the fuel-ethanol-water-aluminum combination and it has the most intense corrosive activity at the water-fuel interface.

Hmmm... The triple-whammy hypothesis does seem to correlate with empirical data.

Even if it is the wrong color, I'm glad that I have an extra tank to use on hand.

I tried fixing a K75 tank many moons ago with some J-B Weld stuff and though it did work it only lasted about a year.

duck wrote:What is the point in jumping a starter terminal that already reads 12V? Besides, getting anything to that starter terminal with a 93 battery holder base is hard because the ignition amp is in the way.


If that works and it does happen to be in gear then I'll take off the tie-downs and put it on the center stand and make sure it's in N since the rear wheel will spin by hand on the center stand.

The 12V at the starter is coming through the coil of the load shed relay, which is only capable of passing a few milliamps of current. The starter needs the bones of a couple of hundred amps to turn it, so the voltage from the L.S.R. will do nothing to it. When everything is working properly the current path through the starter motor is in reality a short circuit to the load shed relay. If you measure the voltage on the starter on a working bike before you press the start button, you will find that it is practically zero. The 12V you are measuring at the starter on the faulty bike is coming from the negative side of the coil in the L.S.R. It is only present because the path to earth through the starter motor is open circuit. I hope the added diagram helps to show what goes on.

SWEET! Installed a different starter motor: headlight and tail light now work and she fired right up and runs like a top. (Once I remembered to hook the Motronic back up.  )

Thanks for the input, guys. I kind of suspected a bad starter motor when it read 12V but it's always nice to get some confirmation.

I just had the tank on my recently purchased K75RT welded and there were 5 holes, one about an inch up where you said.
I think it is because that is the level of the water and the interface line between fuel and water is where the electrolisis takes hold first

Avenger: Thanks for the explanation on the starter electronics. I understand it a little better now.

Likewise. Thanks.

This is a prime example of why I love this forum.

I can halfway read a wiring diagram, but in no way understand what I am seeing enough to diagnose anything other than dicontinuity or crossed connections.

Avenger's post was great!

There is no way I would have been able to look at a schematic and understand what he made plain.

Thanks so much!

Glad to be of service.