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Old 22nd May 2020, 11:53   #1
Andy24601
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Default electrical failure

Okay,whilst running some rca cables I took the passenger side footwell cover off, only to discover that one of the earth connections (multiple black wires on ring terminals) was seriously corroded - to the point when I wiggled it, it pretty much disintegrated. Further wiggling created sparks! I disconnected the battery earth and cleaned everything up - crimping some new ring terminals on. But, when I reconnected the battery I had no instruments, indicators and error messages on the air con. I reckon I've fried something - but what? There are no obvious shorts and my plenum chambers are free of water. Please help
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Old 22nd May 2020, 12:46   #2
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Sparks eh !!.. I think what you need to be looking for is a blown fuse or fuses.-----
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Old 22nd May 2020, 13:02   #3
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So I'e checked the continuity of the fuses under the bonnet and pulled the instrumentation fuses in the passenger footwell and they're fine. I've disconnected the battery and left it for an hour or so in case it all went back to normal of its own accord but no such luck. I don't know why an earth connection being remade should spark - capacitor discharging maybe? All the wires were connected to the earth terminal before (however tentatively) so that probably rules out a dead short
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Old 22nd May 2020, 20:46   #4
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Default Fried BCU?

I've now pulled all the fuses and checked them for continuity. I've soldered ring terminals on the earth wires and made sure they making good contact. I still have no indicator signal, no rev counter, no speedo and a/c in error mode. I read something about upsetting the BCU if you disconnect it and reconnect when it's on - possibly what I did when I touched the earth wires - come on guys, this must make sense to someone?
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Old 22nd May 2020, 20:54   #5
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With enough load any cable can spark if a dodgy connection is wiggled and makes and breaks - it doesn't matter if it is on the live or the earth side.

I haven't heard of reconnecting the earths causing a problem, so maybe you have missed one out and it is still floating or the rca cables you have installed may be the problem? The symptoms you describe sound like a K-bus fault.

If you want to test the BCU K-bus, with the driver's door open and key to position II, if you have a message centre fitted the door open icon should be showing. If it isn't then could be a fault with the BCU or associated wiring.














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Old 22nd May 2020, 21:57   #6
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It will just be sparking in the same way a battery can when reconnecting. Incidentally there was an issue with lower A panel earths when the cars were new as the bolts were cross threaded at the factory.
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Old 23rd May 2020, 11:03   #7
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Quote:
Originally Posted by Andy24601 View Post
So I'e checked the continuity of the fuses under the bonnet and pulled the instrumentation fuses in the passenger footwell and they're fine. I've disconnected the battery and left it for an hour or so in case it all went back to normal of its own accord but no such luck. I don't know why an earth connection being remade should spark - capacitor discharging maybe? All the wires were connected to the earth terminal before (however tentatively) so that probably rules out a dead short
So the fault has to do with the earth wires you moved and reconnected.
Some earth wires become LIVE with the ignition on. Wriggling about a poor connection can cause it to make and break the current flow.

When it connects there will be no arcing or spike in the current flow. However as the connection breaks that is when a spike is generated. This can be a surge of very high voltage that can upset some of the more delicate electronic components in the cars electrical system.

It always pays, when working on the cars electrical system, to disconnect the battery earth terminal.

I know most if this won't help as much as some of the posts above but might give you a guide into where to look to solve your problem.--
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Old 23rd May 2020, 16:31   #8
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Default electrical

You are absolutely right of course and I should have disconnected the battery. I assumed that I wouldn't need to disturb any connections whilst running RCA cables - unfortunately I succumbed to a 'I wonder what happens when I wiggle this obviously-corroded earth connection?' moment (I'm prone to those ). Thank you for all the suggestions so far - the 'open door' display isn't working and so my next stage is to check the wiring running up to the BCU and then probably buy another BCU (I'll worry about key fobs later)
Andy
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Old 23rd May 2020, 17:42   #9
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Quote:
Originally Posted by Andy24601 View Post
You are absolutely right of course and I should have disconnected the battery. I assumed that I wouldn't need to disturb any connections whilst running RCA cables - unfortunately I succumbed to a 'I wonder what happens when I wiggle this obviously-corroded earth connection?' moment (I'm prone to those ). Thank you for all the suggestions so far - the 'open door' display isn't working and so my next stage is to check the wiring running up to the BCU and then probably buy another BCU (I'll worry about key fobs later)
Andy
Don't bother buying another BCU, you are describing a K-Bus fault, and if you have been installing RCA cables, is this part of a head unit upgrade perhaps??

If so, then make sure if you have an aftermarket head unit in place, the K bus connection to the ICE is disconnected

The cable colour code for the K-Bus is white and red, with yellow dots.

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Old 23rd May 2020, 18:06   #10
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Default electrical

I've had the head unit for a while (three months) and I was replacing the RCA cables because the old ones were cheap and shonky. I haven't touched the head-unit wiring - could this still cause problems? The head unit has the usual adaptor with permanent live, switched live, amp switch and earth going to the head unit (plus speaker wires on the other cable). Help!
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