R80RT Electrical advice please- main battery cables getting hot!

[Doug-504]

1989 BMW R80RT
Any advice welcomed, fine with all things mechanical but not so good at chasing electrical gremlins.

Much against my better judgement, which is “if it ain’t broke don’t fix it” I removed the Bosch starter motor over the winter break to give it a clean / overhaul. This was mainly a good clean up following disassembly as brushes still had plenty of wear left. Front cover was removed to access starter retaining bolt.

Bench tested starter before replacing, all ok. All back together and all functioning as per normal. Starter operates fine, all lights ,indicators and instrument lights working as normal.

However, both the negative and positive main battery cables are getting VERY hot even when not operating starter, i.e. even if ignition off and no attempt made to start, the battery positive and negative cables are getting hot, so much so insulation is on point of melting! Suggestions much appreciated.
Doug

[gspd]

both the negative and positive main battery cables are getting VERY hot even when not operating starter, i.e. even if ignition off...

You have a direct short to ground somewhere, disconnect the battery immediately!!
Check the battery to starter (thick) wire and the smaller one from the starter to the diode board. One or the other could be pinched under the starter cover or front cover. If you disconnect the battery negative cable and put a test light between the cable and battery terminal, the light should not go on at all. Disconnect the clock (if you have one) for this test.

[Doug-504]

Thanks, for the advice, I’ll give it a look over the weekend and let you know how it goes. Battery is disconnected, didn’t know I could twirl an Allen key so fast once I spotted a wisp of smoke!

[gspd]

Don't worry about the escaped smoke. You can buy more on ebay once you've fixed the short

[Rob Frankham]

One point to mention. On the 'RS and 'RT fairing, there is a known and acknowledged danger that the main positive battery lead can rub against the lower fairing support and short out. Don't know if that's your problem but certainly somewhere to look.

Whatever it is, it's a very serious short and you should avoid as much as possible connecting up the battery until it's fixed. Fortunately, it won't be too hard to find. It is either going to be in the main positive battery wire or the starter. Given your recent disassembly of the motor, I would remove it straight away and check all of the work. Did you test the motor before you refitted it?

Rob

[Doug-504]

One point to mention. On the 'RS and 'RT fairing, there is a known and acknowledged danger that the main positive battery lead can rub against the lower fairing support and short out. Don't know if that's your problem but certainly somewhere to look.

Thanks for the advice Rob, I’ll check this out over the weekend. I had planned to remove the starter again just to check for any issues. I did bench test it before refitting and all was ok, and in fact it worked when I briefly pushed the start button before noticing the dreaded smoke!

Let you know how I get on, hopefully not having to purchase any more missing loom smoke

[Doug-504]

Problem solved, thanks for the advice.

As you both said relatively easy to track down the short. Issue caused by badly routed starter cable, with cracked insulation which was touching the inside of the starter engine cover. Slight burn mark on the inside of cover provided the evidence.

Doug

[SteveD]

Well found. That could've ended badly!

[Rob Frankham]

Glad it worked out right and without too much difficulty (or expense)...

Rob