R75/5 blowing fuses on starting (prolonged)

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r75boxer
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R75/5 blowing fuses on starting (prolonged)

Postby r75boxer » Sun Nov 10, 2019 7:36 pm

After years of trouble free riding my /5 has started blowing fuses. This is the non-fused /5 to which I added a fuse from the battery to the starter (10 amp). It seems it only happens when starting is a prolonged effort. Once started (with a new fuse) it stops and starts all day without issue. I've checked the wiring and all are snug. Could this be bushing/bearing or brushes on the starter?

Kevin
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Re: R75/5 blowing fuses on starting (prolonged)

Postby Mjolinor » Sun Nov 10, 2019 8:12 pm

I don't reckon a 10 amp fuse is enough. Typically current draw is 8 to 10 amps constant but may be as high as 30 amps on first power up.

That is generally, not BMW specific.

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Re: R75/5 blowing fuses on starting (prolonged)

Postby r75boxer » Sun Nov 10, 2019 8:22 pm

I would agree and will up to a 20 amp fuse. My '74 Eldorado uses a 20 amp for the same purpose.

Having said that though, since I've had no issues to this point the starter may require some attention, wouldn't you think?

Kevin
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Re: R75/5 blowing fuses on starting (prolonged)

Postby Mjolinor » Sun Nov 10, 2019 10:05 pm

I would not think so. Certainly the highest current draw is when it starts working. It may be that it is a little older, more worn, sticky or whatever but if a 20 amp fuse fixes it then I would not (as yet) be at all concerned.

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Re: R75/5 blowing fuses on starting (prolonged)

Postby r75boxer » Sun Nov 10, 2019 10:38 pm

Thanks. I'll give that a go. The bike is on the west coast and I'll be there in about 3 weeks.

Kevin
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Re: R75/5 blowing fuses on starting (prolonged)

Postby Rob Frankhamr » Sun Nov 10, 2019 11:14 pm

Sorry, I'm not getting this... can you explain exactly where the fuse is fitted in the circuit please.

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Re: R75/5 blowing fuses on starting (prolonged)

Postby Mjolinor » Sun Nov 10, 2019 11:17 pm

I assumed the solenoid lead. Certainly a 10 amp fuse in the main starter lead wouldn't last long.

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Re: R75/5 blowing fuses on starting (prolonged)

Postby r75boxer » Sun Nov 10, 2019 11:41 pm

Yes, the solenoid lead.

Kevin
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george baker
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Re: R75/5 blowing fuses on starting (prolonged)

Postby george baker » Mon Nov 11, 2019 10:15 am

Hi
depending where this fuse is in the circuit, could it be you have corrosion in a connector leading to a high resistance joint?

George
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Re: R75/5 blowing fuses on starting (prolonged)

Postby vmx1200 » Mon Nov 11, 2019 11:16 am

I still don't get this, when you say solenoid lead do you mean you have put a fuse between the switched live out on the starter relay (terminal 57 I believe) and the solenoid on the starter motor?

If so why would you do that? BMW never put a fuse there on models with fuses.

Apologies if I am being dim :???:
Last edited by vmx1200 on Mon Nov 11, 2019 11:24 am, edited 1 time in total.
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Re: R75/5 blowing fuses on starting (prolonged)

Postby Rob Frankhamr » Mon Nov 11, 2019 11:21 am

In my mind, there are a few possibilities...
  1. a loose connection or fault in the main lead to the starter might cause current from the solenoid circuit to be diverted through the starter to ground causing an excess current to flow.

    ­
  2. An intermittent fault in the solenoid coil might cause a short circuit to ground.

    ­
  3. An failure of the insulation on the lead between the starter relay and the solenoid might allow an intermittent short.

    ­
  4. A sticking solenoid might cause an increased current to flow, enough to blow a marginally rated fuse.

I wouldn't expect a worn starter nose bush to have any effect on the current at this point unless it was so worn that it was causing the starter pinion to stick.

My advice would be to remove, check and clean the solenoid, then check all of the wires in the relevant circuit. Also check the integrity of the main positive battery lead. Off hand, I don't know the expected solenoid peak current but I would say that 10 amps is pretty marginal so I would consider increasing the value of the fuse.

Rob
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Re: R75/5 blowing fuses on starting (prolonged)

Postby r75boxer » Mon Nov 11, 2019 2:10 pm

Thanks Rob. I will do as you suggested. Unfortunately it won't happen till early December when I'm on the coast. Will update this thread at that time.

Kevin
'71 R75/5
'58 R26
'51 R67

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Re: R75/5 blowing fuses on starting (prolonged)

Postby r75boxer » Mon Nov 11, 2019 4:48 pm

I forgot to reply to vmx1200. The fused wire is the red one that goes to the 'Relay for repeat start prevention'. I incorrectly indicated earlier that it was to the solenoid. I meant to say that it came from the same battery post as the solenoid cable (+). So, you weren't being dim, I was.

Kevin
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