Vintage BMW Motorcycle Owners

I am now having a similar problem to Brad in my '75 R90/6 (with sidecar). The bike starts and runs for 5 minutes or so, then the electrical system cuts out, except the headlight which stays on. I checked the ignition switch and it is not the problem. The wiring diagram suggests that everything runs through the light relay. I have a relay in my headlight bucket which I think might be the light relay, but it is 6 pin and the diagram shows a 5 pin relay. My relay has a number 552N and says "Japan" indicating an aftermarket substitute. Also some of the wiring was replaced by a previous owner, without maintaining the BMW color code. The only other relay looking thing in the headlight bucket is the flasher. I can trace out the wires ok, that just takes time and patience.

So Brad, Is the light relay in your /6 in the headlight bucket? It should have numbers 30,85, 86, 87, and 87b.

Before I try to replace this thing I would like to know for sure what it is.

Thanks

Craig

You might check the starter relay next to the master cylinder under the tank. I've had loose spade connections at the relay causing big annoyances on mine

I had a cut-out problem on a 75/7, which turned out to be a multi-pin socket having worn and shorting out (within the fairing). Also, after this time, connections can corrode within their plastic cover (ie you can't see it). Pull all the connections apart, clean and lubricate. Admittedly, this was a bike used 12 months a year and they use plenty of salt in the UK.

Charlie.

Craig….yep, there is a light relay in the /6 bucket. And yes, those are the numbers of the spades.

Thanks Brad. I think that's what it is. Unfortunately both Max BMW and EME show that relay "out of stock". The bike has 153K miles on it so you have to expect a few things to wear out. BTW my Haynes manual suggests that if you have that relay then your bike is wired as a 1975-1976 model. It is not shown in the 1974 wiring diagram.

And Charlie, I have cleaned all the connections, sanding them down where I can - then spraying them down with electronics parts cleaner , but I think you are right. The problem is inside one of the relays or switches. Probably the "light relay". With the miles one the bike, coupled with the years, this is not unexpected. The trick is narrowing it down to which component has gone bad.

But I got my R75/5 running today so it's all good for now.

Thanks again for your help.

The old Bosch relays with metal covers can be opened, look at the bottom of the relay and you’ll see twisted parts of the cover, gently untwist and the cover will come off. Some of the plastic ones are bonded, others have little clips. Oftentimes you can see burned contacts, try cleaning to get by, but it’s time to get a new one, the contacts are wearing out.