Alternator Wiring Problem

[4129R]

When connecting my battery, I noticed a mild spark when I connected the earth, even though the ignition was off.

Then I noticed the alternator was hot. It is a 17ACR.

The brown and purple wiring on the alternator is connected to the main thick + wire at the starter solenoid connected to the battery, so that brown and purple wire is permanently live.

Can someone please explain why you need a permanently live wire to the alternator connected direct to the battery, and unfused, which is making the alternator hot, and flattening the battery.

[jbcollier]

The "mild spark" is not unusual depending on what you have.  There can be electronics that charge capacitors, etc when first hooked to power.  I use an ammeter to check the actual draw.  When I hook it up, it's normal for it to jump to 1 or 2 amps, slowly fall and then stabilize.  It's fine as long as it stabilizes at 80 milliamps or less.

The alt getting hot indicates a diode has failed inside the alt.  Unfortunately, this not unusual for Lucas alternators.  They have under-spec'd diodes that do not like to be run at full capacity and will pop if you do for anything but a brief period.  That's why they come with a warning to fully charge the battery before running a replacement alt.

Whole alts and parts are readily and cheaply available but I would recommend replacing it with something more robust.

Alternators use diodes to rectify their AC output.  They do not need to be disconnected when off and run most efficiently when hooked directly to the battery.  Repair or replace your alt and it will be fine.

[4129R]

The alternator was brand new, and the engine has not even run yet.

[RafM]

I would double check all of the connections are correct plus make sure there is not a short circuit somewhere, back of the ammeter perhaps?

[jbcollier]

If the alternator is getting hot, then it is wired incorrectly, or the alternator has a problem.  Can you post a picture of how your alt is wired?  The small terminal is for the charging warning light.  If that was hooked to 12v, it might be causing the issues you are experiencing.

https://redpowerphotos.s3.us-east-2.amazonaws.com/monthly_2020_08/1000951245_LucasAlternatorwiring.png.c3e1b61309901ec18d0686c672fe657f.png

https://redpowerphotos.s3.us-east-2.amazonaws.com/monthly_2020_08/lucas-17acr-alternator-wiring-diagram-7.gif.ed668b27d0f34af08f67a026b50ebd0d.gif

[4129R]

I would double check all of the connections are correct plus make sure there is not a short circuit somewhere, back of the ammeter perhaps?

There are only two wire connections, the thick brown wire to the ammeter, and the brown and purple wire from the starter solenoid. There is no connection for the ignition warning light. I have even run a dedicated earth wire from the alternator to the chassis.

Why would a brand new alternator get hot, when the engine has not even run?

[jbcollier]

Can you post a picture of your connections?

[jbcollier]

There are three connections:

NP - to main battery connection at the starter solenoid
NW - to ammeter
NY - to charge light

NP and NW connect to the large connectors (3/8"?)
NY to the small connector (1/4")

[4129R]

There is no connection on the back of the alternator for a second small connection.

The two large male connections are joined together.

[jbcollier]

You can see it right there.  The silver connector takes the NY wire.  It also provides the initial "excitement" for the field on start up.  The NP and the NW go to the two large terminals.  Yes, they are joined, that's the way it's supposed to be.

[4129R]

You can see it right there.  The silver connector takes the NY wire.  It also provides the initial "excitement" for the field on start up.  The NP and the NW go to the two large terminals.  Yes, they are joined, that's the way it's supposed to be.

Thanks for that. Banks seems to have told me incorrect information when I specifically asked him about this.

[jbcollier]

Don't know Richard that well but most mechanics are simply awful on wiring. Not as bad as bodymen though, never ever let a body hook up wires!

[4129R]

Like this?

At least now the ignition warning light comes on.

[jbcollier]

Yes.

You can get a connector kit:

https://www.autoelectricsupplies.co.uk/product/47

Make sure any connectors are properly attached.  Insulated crimp connectors like the middle yellow one are notorious for developing poor contact over time.

[4129R]

Thanks for that, the connection is right up against the luggage box so a close right angle connector is needed.