1975 R90/6 wire question

What is this wire (the black one with the rectangular connector)? It exits the main wiring harness with the ground wires right at the coils. I think it was plugged into the left coil (it isn't the one to the points or the power wire on the other coil or the jumper wire between the two). My coil circuit is complete which leaves me stumped as to the purpose of this wire.

I suspect that you are looking at a wiring harness that is "ready" for the electronic tachs that came out in '77. That plug would clip to the driver's right side coil to give the coil collapse/saturate rate which the tach knows how to interpret as RPM.

Edit...I think the electronic tachs came out in 1978.

Have you got a volt/ohm meter to see if it is a "hot" circuit, when, and what triggers it??????...............God bless.......Dennis

Can it reach the starter relay? If so, the other end probably goes to the headlight relay in the bucket. If your headlight is always on, that wire should turn it off when cranking if it's hooked up to the relay.

It does seem redundant, as your 1975 model came with a proper lightswitch with an 'off' position.

20774 said:

I suspect that you are looking at a wiring harness that is "ready" for the electronic tachs that came out in '77. That plug would clip to the driver's right side coil to give the coil collapse/saturate rate which the tach knows how to interpret as RPM.

Click to expand...

You are probably correct. There is an unused lead in the headlight bucket with the end sealed up and set aside. I think it might be black as well. Thanks for the replies.

I think I'm wrong on the year for the electronic tach...I think it start with the '78s.

More importantly, I think you right about the tach. Here is the capped off black wire in the bucket.

kurt1305 said:

More importantly, I think you right about the tach. Here is the capped off black wire in the bucket.

Click to expand...

Note that it is right next to the headlight relay. If your relay has a brown wire on it, you could replace it with that black wire and have the other end attached to the starter relay if I'm to believe the wiring diagram in the Haynes. This should result in an 'always on' headlight, except for when the starter is engaged. I think BMW put it there in anticipation of the 'always on' headlight law here in the US. The Haynes shows the hookup of the black wire as 'US models' and the brown (ground wire) setup as 'UK models.'

I have the same wire on my /6, and I've left both ends disconnected like yours. In the interest of reducing the number of parts in stock, the /6 harness includes some redundant wires. There should also be brakelight switch wire in your headlight bucket, intended to go to the mechanical switch as used on the R60 models which still had a drum brake.