R90S Starter operable when engine running

[davidpritchardph@gmail.com]

I have a new to me R90S, vintage 1975, and have a problem that the starter will operate when the engine is running and the gear shift is in neutral. When starting the starter won't operate unless the gearshift is in neutral so that is ok - but when the engine is running, with the gearshift still in neutral, the starter will operate if the button is accidentally pushed. Does anybody else have this problem?
I checked the starter relay - it looks fairly new and at a glance the connections look ok but maybe a new one is indicated.

[schrader7032]

Welcome! I was wondering if the switch associated with the clutch lever is working. You are not supposed to be able to start the bike while in gear. To be able to do that, one must pull in the clutch lever then the bike can be started. This primarily is a situation where you say stalled the bike at an intersection and rather than struggle to find neutral, you can just pull the clutch hand lever and the bike can be started. Does this function work on your bike?

[davidpritchardph@gmail.com]

Thanks for the suggestion Kurt. I was not aware of that functionality - I have the bike in bits right now but when I get it back together I'll look into it.

[srankin]

In order to figure out what is going on, I would suggest looking at a wiring schematic for that bike. It is not beyond belief that I is possible that the issue you have is just "normal".

You should have the starter engage when in neutral. As Kurt mentions it should engage when the clutch is pulled in and you are in gear. That is when you are trying to start from a dead engine. As to protecting the starter once the engine is running form being engaged, well, there is a possibility BMW figured no one would push the starter button while the engine is running and didn't bother to put in a safeguard to prevent it.

LOL, I am assuming that the starter only engages when you push the starter button. If I am wrong and the starter is engaging without the button being pushed, forgive me. That is another issue altogether and is dire. The former not so much.

I won't swear to it but I think my two bikes's starter would engage while the engine is running if the starter button is hit. Forgive me if I don't try it. Good luck, St.

[schrader7032]

St -

I would sure hope there is some protection against inadvertent starting. Consider all the people who use tank bags...if there weren't some kind of override, an awful lot of starters would prematurely engage.

[srankin]

So out of curiosity I just did an experiment with my RT. Engine running in neutral, the starter will engage when the starter button is pushed. This confirms my belief that this is common on all BMW bikes as I recall very infrequently pushing the starter button while my other bikes were running. No, I didn't try it with the bike in gear and the clutch lever pulled.

So, yeah, there is no protection against this happening, BMW must have thought it was so rare that too add another bit of wiring wasn't worth it.

If I were not lazy, I suppose I could study the schematic and trace out the system to prove what I am saying or disprove. St.

[schrader7032]

Looking forward to the analysis!

[srankin]

I gave the schematics I have for R90S and my RT and can find no device or interlock to prevent the starter from engaging while the bike is running in neutral or even while in gear. St.

[Jim D 5112]

I think that the starter being able to engage by pushing the button is pretty common on not just motorcycles but also cars from that period of time. Not sure about the latest models offered today. It’s really not a big issue just don’t push the button. I have mistakenly done it already myself. The bendix drive only works in one direction. When your engine starts you can get the same effect by not leaving off of the button or ignition switch on a car. A course the automobile switch has a spring loaded switch to assist you when it starts kinda like your starter button.

[schrader7032]

I've thought more about the function of the starter etc in various modes. I'm not really that good at reading the diagrams but thought more along the lines of how some of the circuits are known to work.

I think it's pretty clear that, unknown normal circumstances and no wiring anomalies, the only way the starter turns is when the handlebar button is pushed. This activates the starter relay internal coil which pulls the internal contacts together which further sends voltage to the starter itself where another set of relays are which allows power direct from the battery to energize the starter motor. It looks the two terminals on the starter relay 85 and 86 represent the internal coil.

So how I think things work...

IGNITION ON, ENGINE NOT RUNNING

1) In neutral, push starter button and the starter relay is energized which triggers the starter solenoid and the starter motor.

2) Not in neutral, starter won't engage...not real sure how the diagram shows that, but we've all experienced this. By pulling in the clutch lever, which disconnects the engine from the drive train, the starter button can be pushed to start the engine. The clutch lever switch allows power to reach the starter relay.

IGNITION ON, ENGINE RUNNING

3) So you're running at speed, would the starter button activate the starter? I don't think so because of 2) above. The bike is not in neutral so wouldn't the combination of the transmission neutral switch and clutch lever switch seems to suggest the starter won't work? I suppose you could pull in the clutch lever and the starter would but this is something none of us ever do.

4) You're pushing the bike up to a pump or sitting astride the bike. I'm thinking that if you pushed the starter button, the starter would engage, grinding against the flywheel teeth because of 1) above. Can't say as I've done this...I think I'd remember!

So maybe there are no interlocks to prevent inadvertent starting in certain situations. Seems strange that BMW would not design in some safeguards to prevent that. Maybe they felt that the scenarios wouldn't likely come up.

As a final point, it's important to have the right neutral switch for the bike and transmission. Early neutral switches had one wire going to them and they grounded internally to the transmission. The later ones have two wires, one carrying power the other going to ground. The switch closes and passes power through which in term lights the neutral light in the instruments.