K75S engine cutting out

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Rosbif
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Posts: 8
Joined: Sun May 20, 2007 2:32 pm
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Location: La belle France

K75S engine cutting out

Post by Rosbif »

A friend has an old K75S which has started to give him problems.

He was pulling away from the supermarket the other day and had to brake to let a car past and the engine cut out. He started it up and proceeded to the exit where he had to brake for traffic and the engine cut out again. He restarted and managed to ride home without using the brakes (we both live in rural France where traffic is very light!!) and proceeded to do some checks. The cutting out was happening when either the front or rear brake is applied and additionally, when pressing the horn button. He then found the bike ran OK for a short while (even when using the brakes), but the problem has returned so it's clearly an intermittant fault.

He has a second K75S as a spare bike, so he's been swopping various relays over to check for obvious faults. Checking out the various BMW forums, it seems that problems often arise through worn starter brushes (bad earthing) but he's cleaned out the starter motor and measured the brushes which are almost as new. The load shed relay is another potential culprit, but it seems to be working OK. He's checked the clutch switch and sidestand switch, both working. Battery is good. Fuel pump is working.

This morning, he's had the bike on the stand and the engine running and it doesn't cut out when the brakes are applied. However, the stop light bulb only glows weakly. A test lamp applied to the relevant output terminal on the bulb monitoring unit fully lights up.

There's probably a simple explanation as to why using the brakes/horn is causing the engine to cut out, but we're puzzled? Any ideas?

Thanks in advance
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