Battery and alternator issue

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Model you own: w123
Location: Central Coast of NSW

Battery and alternator issue

Post by GREENPEDRO » Mon 06 May, 2019 9:11 pm

Hi all,
I did something really stupid recently so it is my bad.
Swapping a newer battery into my 300TD I was distracted and had to leave the process for a couple of hours to attend to something else.
When returning to the battery swap I stupidly was not paying attention and connected the battery leads back to front.
After a short time I noticed smoke coming from the engine bay and (of course) a burning smell.
Quickly disconnected the neg lead when I realized my error.
The wiring going into the back of the alternator had begun to melt and fuse the connections.
Pulled the wiring apart and re-insulated it as far as I could see any damage, I could not see inside the alternator.

The problem still exists in that when I hook up the battery,(correctly) - 1) the alternator gets really hot within about a minute and 2) the battery light on the dash is on from the moment the battery is hooked up without the car being turned on at the ignition.
Have I burnt out the alternator wiring within it ?
Any suggestions would be appreciated to see if I can recover my stupidity.

Thanks in advance.

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Re: Battery and alternator issue

Post by CraigB » Mon 06 May, 2019 9:43 pm

I'm not an auto electrician, but if it hasn't burnt out the windings then you may just need to replace the relatively inexpensive brush assembly and I can imagine that melting and still bringing a postive wire in contact with the earthed body. Or perhaps easier just to swap in another alternator.
Craig Baulderstone
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Christo C
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Region: New South Wales
Location: Sydney [Beaconsfield]

Re: Battery and alternator issue

Post by Christo C » Tue 07 May, 2019 1:55 am

You have most likely melted the alternator’s internal rectifiers and maybe the voltage regulator at a minimum, possibility some of the windings or compromised the insulation layers.
I’d take the Alternator to an auto electrician and see if it can be rescued.... otherwise just replace it.
If you didn’t attempt to start the car with the battery reversed it’s unlikely you damaged much more than the Alternator and it’s wiring to the battery.
1986 230E W124.023 M102.982 Thistle Green Saloon "Janis"
Beaconsfield, SYDNEY

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Re: Battery and alternator issue

Post by mathuisella » Tue 07 May, 2019 10:10 am

You've killed the alternator.

I had a friend do it to his car about 5 years ago.

new alternator time i'm afraid.

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Posts: 97
Joined: Mon 31 Aug, 2015 1:01 pm
Model you own: w123
Location: Central Coast of NSW

Re: Battery and alternator issue

Post by GREENPEDRO » Tue 07 May, 2019 7:35 pm

Thanks all for your replies, it confirms what I was suspecting.
Note to self and lesson to others, pay attention to what you are doing even with the simplest of things.
Cheers and thanks.

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