Charging problem

[funkyjoe]

The other day, I was driving home and my lights went dim. Eventually the battery ran out of juice and I stopped. No Charging voltage at the battery. I replaced the alternator and the battery. Checked continuity between the charging post and the battery +ve terminal, all ok. But still no charging. I detected a 12 volts between the -ve terminal and earth with the -ve terminal disconnected, so there is a drain to earth somewhere.
Will this stop the alternator charging the battery?
Or.. does anyone know what might be stopping the charging?

[Martinaston]

"I detected a 12 volts between the -ve terminal and earth with the -ve terminal disconnected, so there is a drain to earth somewhere."
I don't quite get what your saying there I think that may be just the 12v that the clock is drawing.

You could try taking the earth strap between the engine and chassie off and give it a good scrub to make sure you have a good connection. That normaly buggers the charging circuit.

[funkyjoe]

I have discovered that the D+ terminal is draining to earth. I assume this would cause the problem, anyone know what route this wire takes?

[Martinaston]

By how many amps is it draining the battery ?

[funkyjoe]

It isn't unless I start it, then the systems drain it. No charge voltage at battery. The D+ should be gicving 12V I thought, but I detect a few ohms only between D+ and earth. Earthe lead from engine to body is good

[Martinaston]

Whats D+ ?

[Martinaston]

Right got it, the blue wire are you checking it at the alternator or pin 1 on the diagnostic socket ?

[funkyjoe]

I am checking it at the alternator. Been googling, D+ runs the ingition light, which works and goes out when I start the car. B+, the main charging output (The 13mm nut) only gives me 12.5 volts when the engine is running measured at the B+ terminal. Could it be that this reconned alternator is broke too?
I'd have expected the charging voltage at the alternator for sure whatever I read at the battery.

[Martinaston]

Its normaly the brushes on the alternator that fail, its the voltage regulator that bolts into the back of the alternator that may need changing.
Pull the regulator out and check the springs move ok under the brushes and see how long they are.(the brushes not the springs)

[funkyjoe]

Tbe alternor is a new (refurbished) one, arrived today to replace the old one (on the floor beside me).
So could be just a duff one

[Martinaston]

Whats the voltage across the battery with the engine running ?

[funkyjoe]

12.5, or 12.1 with the lights on, no recovery

[Martinaston]

Is the battery new as well ? if not check the acid level.

[funkyjoe]

A working alternator would drive that to 14V in both cases. Going to visit GSF tomorrow. They have one in, got my new one from GSF online so some shennanigans to get them to change it for me, and take it off and fit new one at the GSF shop.
I really cannot think of any other reason why there would be battery voltage only at the B+ charging terminal when engine is running, ignition light working except for a faulty regulator/alternator (I suspect the regulator).

[funkyjoe]

the battery is new today too

[Martinaston]

You could pull the regulator out to see if its new and make sure the brushes are fully extended so they touch the slip rings.

[funkyjoe]

It looks new, but that doesn't guarantee it works. The ignition light wouldn't go off if the brushes weren't in contact with the slip ring. B+ has it's own set of diodes separate to D+, so it's probably working inside, but not outputting to the B+ to charge.
I reckon trying another new one up the shop will tell me, also they can sort me out a known good one. Only takes 20 minutes to get the thing out and replace, plus losses due to being disabled and the cold weather, brrr

[Martinaston]

When did the original one start playing up ?

[funkyjoe]

Monday night. The ignition light wasn't working eaither, that may be brushes worn out, 200K on clock. It turns very freely, but the charging was fine until Monday. I think I've been missing the ignition light for a bit longer.

[Martinaston]

200k's not bad going, M20 or M40 ?

[funkyjoe]

M20 alternator.
Since there is no feedback loop that causes the alternator not to generate electricity, then it has to be broke.

[Brianmoooore]

Since there is no feedback loop that causes the alternator not to generate electricity, .

There is a feedback loop - it's through the ignition lamp from battery + to the D terminal on the alternator.
Ignition lamp is an essential part of the charging circuit, although there's supposed to be a resistor across the lamp to still provide a reference voltage when the lamp is blown.

[funkyjoe]

The new alternator had rubber mounts, and the old one earthed via the mounts. Problem solved by earth wire from alternator to body