Flack
Member Since: 06 Sep 2006
Location: Preston Lancashire
Posts: 6232
  
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I had a customer who mtried to start his car when it was in gear and it blew this fuse, some early ones dont even have a fuse so just bridge it to get you going if your stuck.
Flack 
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 30th Dec 2017 7:05 pm |
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WW88
Member Since: 19 Aug 2017
Location: Northants
Posts: 250
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fhb22266 wrote:Just did a little calculation, 400a at 12v is equivalent to 20a at 240v, so would 20a fuse wire work the same? Or are my calculations worse than a 5 year olds? 
You could give it a go, calculations are about right so I can’t see why it won’t work for a test.
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| 30th Dec 2017 7:07 pm |
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fhb22266
Member Since: 11 Jul 2014
Location: Oban
Posts: 36
 
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Thanks Flack and WW88 I'll try fuse wire and see what happens. Fingers crossed!
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| 30th Dec 2017 7:12 pm |
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L319
Member Since: 14 Dec 2013
Location: Herefordshire
Posts: 2080
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fhb22266 wrote:Just did a little calculation, 400a at 12v is equivalent to 20a at 240v, so would 20a fuse wire work the same? Or are my calculations worse than a 5 year olds?
400A is 400A , what you are calculating is power, not the same thing.
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| 30th Dec 2017 7:28 pm |
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fhb22266
Member Since: 11 Jul 2014
Location: Oban
Posts: 36
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Im no expert, but according to Ohms law voltage is inverse of amperage. Thats how high voltage high tension lines carry so much power on relatively thin cable then gets transformed down to low voltage high amperage. So 20 amp fuse wire at 240v should carry 400a at 12v. I'm just not sure whether a.c. to d.c. makes any difference, but don't think it should.
Anyway, if 20a fuse wire won't work I'll try a bypass as Flack and Mike suggested, so will find out tomorrow as my inspection light as now out of amps 
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| 30th Dec 2017 9:28 pm |
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Pete K
Member Since: 15 Jan 2016
Location: GL
Posts: 10360
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You look at the size of your 20A fuse wire
And then the size of the battery cable.
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| 30th Dec 2017 9:32 pm |
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fhb22266
Member Since: 11 Jul 2014
Location: Oban
Posts: 36
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I must agree the size of the cable going to the starter and alternator is huge, but then it has to be a lot bigger so that the fuse is the weekmpoint and not the cable. It's the same on mains cable, they are much bigger than the fuse wire, if you don't have mcbs.
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| 30th Dec 2017 9:39 pm |
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fhb22266
Member Since: 11 Jul 2014
Location: Oban
Posts: 36
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Ok just tried bypass of fuse, still not cranking, just solenoid click. So is the general consensus it's the starter motor? Will be over 130k old now. Is there any way to test it without taking it out?
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| 30th Dec 2017 10:16 pm |
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Globetrotter448
Member Since: 21 Mar 2017
Location: Londonderry NSW
Posts: 1780
 
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The alternator is what normally blows that fuse, so make sure the cable on the alternator is disconnected. Make sure battery has good voltage then put power to the solenoid to see if starter moves
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| 30th Dec 2017 10:38 pm |
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fhb22266
Member Since: 11 Jul 2014
Location: Oban
Posts: 36
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Thanks Globetrotter. I reconnected the alternator before I bypassed fuse, so will take it off and try again tomorrow
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| 30th Dec 2017 10:49 pm |
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Disco_Mikey
Member Since: 29 May 2007
Location: Dundee, Scotland
Posts: 20727
 
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The alternator would only draw a huge current, there would be lots of sparks when reconnecting the bypass or battery terminal
If there is still no crank, it's either a faulty starter motor, poor earth or seized crank. If the engine turns by hand, it's not a crank issue.
Earth can be bypassed with the jump lead again, battery negative terminal to the engine. Assuming that's ok, it only leaves the starter motor My D3 Build Thread
TDV8 Retrofit Build Thread
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| 30th Dec 2017 11:54 pm |
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anglefire
Member Since: 09 Mar 2010
Location: In the Club House
Posts: 4180
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fhb22266 wrote:Just did a little calculation, 400a at 12v is equivalent to 20a at 240v, so would 20a fuse wire work the same? Or are my calculations worse than a 5 year olds?
No.
A 20A fuse is a 20A fuse it matters not what the voltage is.
You are thinking more along the lines of power - 400A X 12V = 4800W = 20A x 240v Mark.
2006, D3 SE Auto - gone but not forgotten.
2014 BMW 530d M Sport Tourer.
1977 Triumph Spitfire 1500
_________________________________________________
Disco Picture Website Here
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| 31st Dec 2017 9:26 am |
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fhb22266
Member Since: 11 Jul 2014
Location: Oban
Posts: 36
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Disconnected alternator main cable, tried 30a fuse wire in fuse bridge at battery terminal. That immediately burnt away. So tried heavy duty jump lead instead. Huge sparks, smoke, cable started heating really quick, quickly disconnected it again. Checking restance from cable to chassis shows high restance value.
So, either a short circuit on the cable or through the starter motor? My money is on starter as it did start inititailly the other day but immediately died.
Next step, under car, take off cable from starter and test for earth leak. I'm going to take the plunge and order a new denso starter anyway now as I need the car back up and running by next weekend. Also ordered 2 x 12v 400a fuses.
Apparently I need a lot of swearing to get this starter out? 
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| 31st Dec 2017 2:57 pm |
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Disco_Mikey
Member Since: 29 May 2007
Location: Dundee, Scotland
Posts: 20727
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I have seen a few cables chafe against the diff casing, which causes a dead short, blowing the fuse instantly...
I wouldn't expect the fuse to blow immediately, without attempting to crank 
Starter is not difficult, but is fiddly... My D3 Build Thread
TDV8 Retrofit Build Thread
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| 31st Dec 2017 3:23 pm |
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