Discovery - Alternator dead short?

[hotponyshoes]

I'm sure I've been through this on a car before but can't remember exactly what I did to diagnosis is...

My good lady has a LR D4, she started the car, drove for a couple of mins then had a "charging system fault" message that (apparently) disappeared after a couple of miles.
Completely ignored it and drove home fine.

About an hour later went to go out again and it wouldn't unlock.
I popped home and had a quick look. Battery voltage was right down to 3v and the battery was hot.
I disconnected the battery and went back to work.

Came home about 3hrs later. Battery had self-recovered to about 5v.

Did a bit of poking around this evening.
There are 2 main red cables going to the battery +.
One runs into the main fuse box and the other goes to the starter & alternator.
Separated and probed them. The fuse box one has some continuity to ground but I'd expect that.
The starter/alternator one was zero resistance to ground.

I removed the alternator and the wire was then reading no continuity to ground.

Car did start and drive (only briefly as I needed to move it) with plenty of errors showing!

So, with the alternator on the bench, There is one 8mm stud terminal (B+) and a single-pin small connection.

Putting the meter across the main stud and the alternator case shows zero resistance.
I swapped the probes over and get zero on both ways.

I'm thinking that's not normal and the alternator has a dead short which would have caused the rapid power drain and the hot battery?

However, just to confuse myself, I found another alternator the same (different D4) that I had previously replaced. This one was removed as it had a "charging system fault" permanently on. But that car was running fine for a couple of days in between battery charges. I measured that one and also get very little resistance between the stud and the case?

Anything further I should check before I cough up the cash for a new alternator?

 

[m_c]

Alternators going short to ground isn't unusual.
Usual give away is the alternator is hot to touch if you connect it up.

Officially you'd check it by measuring current with something capable of measuring high current (clamp on meter would be my choice), but you can usually tell by connecting a fully charged battery and feeling if the alternator is warming up without the vehicle running.

 

[sako243]

Battery had self-recovered to about 5v.

Dead, anything below 10.8V open circuit is a dead lead acid. You might recover it but I wouldn't trust it on a D4.

However, just to confuse myself, I found another alternator the same (different D4) that I had previously replaced. This one was removed as it had a "charging system fault" permanently on. But that car was running fine for a couple of days in between battery charges. I measured that one and also get very little resistance between the stud and the case?

Try spinning the alternator by hand whilst measuring the resistance and see if it increases.

I'd expect you to see a short one way because of the diodes (depending on the type of meter to a certain extent) but shouldn't the other way. It's not uncommon for diodes to fail short circuit so it may just need the diode pack swapping out.

 

[hotponyshoes]

Alternators going short to ground isn't unusual.
Usual give away is the alternator is hot to touch if you connect it up.

Officially you'd check it by measuring current with something capable of measuring high current (clamp on meter would be my choice), but you can usually tell by connecting a fully charged battery and feeling if the alternator is warming up without the vehicle running.

I didn't see it until after she found she couldn't unlock it. Couldn't get my hand onto the alternator without removing some other bits and didn't have time as I was in-between jobs at work. The battery was warm to touch though. Bearing in mind it was down to 3v by this stage so I don't think it had much power left in it!

I have got an amp clamp but didn't use it (due to time) just disconnected the battery and went back to work.

 

[hotponyshoes]

Dead, anything below 10.8V open circuit is a dead lead acid. You might recover it but I wouldn't trust it on a D4.

Try spinning the alternator by hand whilst measuring the resistance and see if it increases.

I'd expect you to see a short one way because of the diodes (depending on the type of meter to a certain extent) but shouldn't the other way. It's not uncommon for diodes to fail short circuit so it may just need the diode pack swapping out.

I put it on charge when I got home and by the time I had removed the alternator it was capable of starting the car.
It's on charge now and I'll see what it's like in the morning.
I have got a battery analyser so can check the condition. But I know what you mean about trusting it on a D4.
It's only about 18mths old as the first thing I did when she got the car was put a new decent battery on it!
Plus I'd rather not buy a battery and an alternator at the same time if possible!!

No change when spinning it by hand but it's a bit tricky to hold the probes and spin it properly. I could have another go tomorrow and find some test lead clips out..

There is a decent auto-electrical place near me but the chap that did all the testing/rebuilds left and I don't think they have replaced him yet..

 

[Trevorleach]

New alt and battery required, the alt is told what to do via a pwm signal from the engine ecu.
I have had less than 18 mths from a D3 battery.
All the modern LRs require a perfect battery.

 

[zzr1200]

I run a D2, 1000cca battery minimum, same for D3/4 I would presume, a battery would last 3 or 4 years if your lucky. My old diesel P38 would eat a battery every 2 years.