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Donald Plocinski
New Member Expired Membership

1974 BMW R90, 11/73 Build Date, 9/73 Transmission:

I'm interested in how the Neutral Light is illuminated, when the above Motorräd is in neutral, Clutch Hand Lever released, ignition ON ?

Both the Neutral Switches, at the Transmission & Clutch Perch, are Normal Open when in the above situation.

Fused 12V power is supplied to Pin 12 - Neutral Light, in the Instrument Cluster.
A Switched Ground for the Neutral Light, is at Pin 6 - Neutral Light in the Instrument Cluster. I think this is the power source for the bulb.

Simultaneously, Fused 12V power is also supplied to Terminal 86 at the Starter Relay.

The Switched Ground from Pin 6, joins with a Switched Ground from the Transmission Neutral Switch.

They connect to Terminal LLK Cathode Terminal of the Diode, in the head light nacelle.
The Anode Terminal of the Diode, Terminal 85b, splits into two(2) Switched Leads (power ?)

Following the split, one(1) lead then goes to Terminal 50 - Starter Thumb Switch; companion lead goes to the Clutch Neutral Switch.

The other pin of the Starter Thumb Switch, Terminal 85, continues to the Terminal 85 of the Starter Relay; combines with the Fused 12V at Terminal 86, to activate the Starter Relay and energize the Starter Solenoid.

It appears to me, that the Diode, Terminals 85b & LLK, is Reverse Biased; as compared to Forward Biased... anyone ?

I can't figure out how, with both the Clutch & Transmission Neutral Switches open, the Pin 6 - Switched Ground in the Instrument Cluster, gets grounded; to illuminate the Neutral Light !

I presume that when the Transmission is in gear, the near zero resistance of the Switched Ground to Ground at the Transmission Neutral Switch, is an easier conductive path then, the resistance of the Neutral Light; which is why it is not illuminated.


Topic starter Posted : 01/08/2021 04:21
Richard Whatley
Member Moderator

Well, one thing is for sure.... the older schematics leave a lot to be desired as far as clarity. Especially once you get inside the gauge set. And secondly, my eyes ain't what they used to be. I got to find some magnification to even look at this.

[color=blue]Don't hide 'em, Ride 'em !!

Posted : 01/10/2021 06:56
andy murphy
Eminent Member Expired Membership


So... the airhead neutral light electrickery has caught you. Don't feel alone or stupid, it has stumped you, me, and most everyone who has felt the need to understand it's function in detail. You are to be commended for digging into it this thoroughly, citing wire colors, terminal numbers, and correct diode terminology. But your first incorrect assumption is that the transmission neutral switch is open when the transmission is in neutral. Both the neutral switches, transmission and clutch lever, are of the normally open configuration, but a cam in the transmission pushes the switch plunger to the closed position in neutral. This should be your AHA moment, with the rest of the circuit making sense now. 

Many people stumble when trying to understand the diode function. In this circuit it provides a logical OR to the starter relay while preventing the neutral light from illuminating when when the clutch switch is closed (clutch lever pulled in) but the transmission is in gear. I think we all agree that being able to start the engine when in gear is essential, that is the function of the clutch switch. But the neutral light should only illuminate when the transmission is actually in neutral. So the engine can be started when the transmission is in neutral OR the clutch lever is pulled in OR both.

I hope this explanation answers your questions. If not, keep asking. I monitor this site daily. One of my mottos is airheads are almost as much fun to work on as they are to ride. 


1995 R100RT

ABC# 15601


Posted : 01/12/2021 13:40


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